21.1 Sending Data with GET and Displaying the Resultant Page

CONTENTS

Chapter 20. JavaServer Pages

  •  20.1 JSP Overview
  •  20.2 Advantages of JSP
  •  20.3 JSP Scripting Elements
  •  20.4 The JSP page Directive
  •  20.5 Including Files and Applets in JSP Documents
  •  20.6 Using JavaBeans with JSP
  •  20.7 Defining Custom JSP Tags
  •  20.8 Integrating Servlets and JSP
  •  20.9 Summary

Topics in This Chapter

  • The benefits of JSP

  • JSP expressions, scriptlets, and declarations

  • Controlling the structure of the servlet that results from a JSP page

  • Including files and applets in JSP documents

  • Using JavaBeans with JSP

  • Creating custom JSP tag libraries

  • Combining servlets and JSP: the Model View Controller (Model 2) architecture

20.1 JSP Overview

JavaServer Pages (JSP) technology enables you to mix regular, static HTML with dynamically generated content from servlets. You simply write the regular HTML in the normal manner, using familiar Web-page-building tools. You then enclose the code for the dynamic parts in special tags, most of which start with <% and end with %>. For example, here is a section of a JSP page that results in "Thanks for ordering Core Web Programming" for a URL of http://host/OrderConfirmation.jsp?title=Core+Web+Programming:

Thanks for ordering <I><%= request.getParameter("title") %></I> 

Separating the static HTML from the dynamic content provides a number of benefits over servlets alone, and the approach used in JavaServer Pages offers several advantages over competing technologies such as ASP, PHP, or ColdFusion. Section 20.2 gives some details on these advantages, but they basically boil down to two facts: that JSP is widely supported and thus doesn't lock you into a particular operating system or Web server and that JSP gives you full access to Java servlet technology for the dynamic part, rather than requiring you to use an unfamiliar and weaker special-purpose language.

The process of making JavaServer Pages accessible on the Web is much simpler than that for servlets. Assuming you have a Web server that supports JSP, you give your file a .jsp extension and simply install it in any of the designated JSP locations (which, on many servers, is any place you could put a normal Web page): no compiling, no packages, and no user CLASSPATH settings. However, although your personal environment doesn't need any special settings, the server still has to be set up with access to the servlet and JSP class files and the Java compiler. For details, see your server's documentation or Section 19.2 (Server Installation and Setup).

Although what you write often looks more like a regular HTML file than a servlet, behind the scenes the JSP page is automatically converted to a normal servlet, with the static HTML simply being printed to the output stream associated with the servlet. This translation is normally done the first time the page is requested. To ensure that the first real user doesn't get a momentary delay when the JSP page is translated into a servlet and compiled, developers can simply request the page themselves after first installing it.

One warning about the automatic translation process is in order. If you make an error in the dynamic portion of your JSP page, the system may not be able to properly translate it into a servlet. If your page has such a fatal translation-time error, the server will present an HTML error page describing the problem to the client. Internet Explorer 5, however, typically replaces server-generated error messages with a canned page that it considers friendlier. You will need to turn off this "feature" when debugging JSP pages. To do so with Internet Explorer 5, go to the Tools menu, select Internet Options, choose the Advanced tab, and make sure "Show friendly HTTP error messages" box is not checked.

Core Approach

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When debugging JSP pages, be sure to turn off Internet Explorer's "friendly" HTTP error messages.

Aside from the regular HTML, there are three main types of JSP constructs that you embed in a page: scripting elements, directives, and actions. Scripting elements let you specify Java code that will become part of the resultant servlet, directives let you control the overall structure of the servlet, and actions let you specify existing components that should be used and otherwise control the behavior of the JSP engine. To simplify the scripting elements, you have access to a number of predefined variables, such as request in the code snippet just shown.

This chapter covers versions 1.0 and 1.1 of the JavaServer Pages specification. JSP changed dramatically from version 0.92 to version 1.0, and although these changes are very much for the better, you should note that newer JSP pages are almost totally incompatible with the early 0.92 JSP engines and that older JSP pages are equally incompatible with 1.0 JSP engines. The changes from version 1.0 to 1.1 are much less dramatic: the main additions in version 1.1 are the ability to portably define new tags and the use of the servlet 2.2 specification for the underlying servlets. JSP 1.1 pages that do not use custom tags or explicitly call 2.2-specific statements are compatible with JSP 1.0 engines; JSP 1.0 pages are totally upward compatible with JSP 1.1 engines.

20.2 Advantages of JSP

JSP has a number of advantages over many of its alternatives. Here are a few of them.

Versus Active Server Pages (ASP) or ColdFusion

ASP is a competing technology from Microsoft. The advantages of JSP are twofold. First, the dynamic part is written in Java, not VBScript or another ASP-specific language, so it is more powerful and better suited to complex applications that require reusable components. Second, JSP is portable to other operating systems and Web servers; you aren't locked into Windows NT/2000 and IIS. You could make the same argument when comparing JSP to ColdFusion; with JSP you can use Java for the "real code" and are not tied to a particular server product.

Versus PHP

PHP is a free, open-source HTML-embedded scripting language that is somewhat similar to both ASP and JSP. One advantage of JSP is that the dynamic part is written in Java, which already has an extensive API for networking, database access, distributed objects, and the like, whereas PHP requires learning an entirely new, less widely used language. A second advantage is that JSP is much more widely supported by tool and server vendors than is PHP.

Versus Pure Servlets

JSP doesn't provide any capabilities that couldn't, in principle, be accomplished with a servlet. In fact, JSP documents are automatically translated into servlets behind the scenes. But it is more convenient to write (and to modify!) regular HTML than to have a zillion println statements that generate the HTML. Plus, by separating the presentation from the content, you can put different people on different tasks: your Web page design experts can build the HTML using familiar tools and leave places for your servlet programmers to insert the dynamic content.

Does this mean that you can just learn JSP and forget about servlets? By no means! JSP developers need to know servlets for four reasons:

  1. JSP pages get translated into servlets. You can't understand how JSP works without understanding servlets.

  2. JSP consists of static HTML, special-purpose JSP tags, and Java code. What kind of Java code? Servlet code!

  3. Some tasks are better accomplished by servlets than by JSP. JSP is good at generating pages that consist of large sections of fairly well structured HTML or other character data. Servlets are better for generating binary data, building pages with highly variable structure, and performing tasks (such as redirection) that involve little or no output.

  4. Some tasks are better accomplished by a combination of servlets and JSP than by either servlets or JSP alone. See Section 20.8 (Integrating Servlets and JSP) for details.

Versus Server-Side Includes (SSI)

SSI is a widely supported technology for inserting externally defined pieces into a static Web page. JSP is better because you have a richer set of tools for building that external piece and have more options regarding the stage of the HTTP response at which the piece actually gets inserted. Besides, SSI is really intended only for simple inclusions, not for "real" programs that use form data, make database connections, and the like.

Versus JavaScript

JavaScript, which is completely distinct from the Java programming language, is normally used to generate HTML dynamically on the client, building parts of the Web page as the browser loads the document. This is a useful capability and does not normally overlap with the capabilities of JSP (which runs only on the server). Although JavaScript can also be used on the server, most notably on Netscape, IIS, and BroadVision, Java is more powerful, flexible, reliable, and portable.

20.3 JSP Scripting Elements

JSP scripting elements let you insert code into the servlet that will be generated from the JSP page. There are three forms:

  1. Expressions of the form <%= expression %>, which are evaluated and inserted into the servlet's output

  2. Scriptlets of the form <% code %>, which are inserted into the servlet's _jspService method (called by service)

  3. Declarations of the form <%! code %>, which are inserted into the body of the servlet class, outside of any existing methods

Each of these scripting elements is described in more detail in the following sections.

In many cases, a large percentage of your JSP page just consists of static HTML, known as template text. In almost all respects, this HTML looks just like normal HTML, follows all the same syntax rules, and is simply "passed through" to the client by the servlet created to handle the page. Not only does the HTML look normal, it can be created by whatever tools you already are using for building Web pages. For example, we used Allaire's HomeSite for most of the JSP pages in this book.

There are two minor exceptions to the "template text is passed straight through" rule. First, if you want to have <% in the output, you need to put <\% in the template text. Second, if you want a comment to appear in the JSP page but not in the resultant document, use

<%-- JSP Comment --%> 

HTML comments of the form

<!-- HTML Comment --> 

are passed through to the resultant HTML normally.

Expressions

A JSP expression is used to insert values directly into the output. It has the following form:

<%= Java Expression %> 

The expression is evaluated, converted to a string, and inserted in the page. That is, this evaluation is performed at run time (when the page is requested) and thus has full access to information about the request. For example, the following shows the date/time that the page was requested:

Current time: <%= new java.util.Date() %> 

It would result in code in the _jspService method (called by service in servlets that result from JSP pages) similar to the following.

out.print("Current time: "); out.println(new Java.util.Date()); 
Predefined Variables

To simplify these expressions, you can use a number of predefined variables. These implicit objects are discussed in more detail later in this section, but for the purpose of expressions, the most important ones are:

  • request, the HttpServletRequest

  • response, the HttpServletResponse

  • session, the HttpSession associated with the request (unless disabled with the session attribute of the page directive see Section 20.4)

  • out, the PrintWriter (a buffered version called JspWriter) used to send output to the client

Here is an example:

Your hostname: <%= request.getRemoteHost() %> 
XML Syntax for Expressions

On some servers, XML authors can use the following alternative syntax for JSP expressions:

<jsp:expression>Java Expression</jsp:expression> 

However, in JSP 1.1 and earlier, servers are not required to support this alternative syntax, and in practice few do. In JSP 1.2, servers are required to support this syntax as long as authors don't mix the XML version and the ASP-like version (<%= ... %>) in the same page. Note that XML elements, unlike HTML ones, are case sensitive, so be sure to use jsp:expression in lower case.

Installing JSP Pages

Servlets require you to set your CLASSPATH, use packages to avoid name conflicts, install the class files in servlet-specific locations, and use special-purpose URLs. Not so with JSP pages. JSP pages can be placed in the same directories as normal HTML pages, images, and style sheets; they can also be accessed through URLs of the same form as those HTML pages, images, and style sheets. Here are a few examples of installation locations and associated URLs:

  • Tomcat default installation directory

    install_dir\webapps\ROOT install_dir\webapps\ROOT\anyDir 
  • Tomcat URL

    http://host/filename.jsp http://host/anyDir/filename.jsp 
  • JRun installation directory

    install_dir\servers\default\default-app install_dir\servers\default\default-app\anyDir 
  • JRun URL

    http://host/filename.jsp http://host/anyDir/filename.jsp 

Note that, although JSP pages themselves need no special installation directories, any Java classes called from JSP pages still need to go in the standard locations used by servlet classes (e.g., .../WEB-INF/classes).

Example: JSP Expressions

Listing 20.1 gives an example JSP page and Figure 20-1 shows the result. With Tomcat we installed the page in install_dir\webapps\ROOT\cwp\Expressions.jsp and used a URL of http://host/cwp/Expressions.jsp. Notice that we included META tags and a style sheet link in the HEAD section of the HTML page. It is good practice to include these elements, but there are two reasons why they are often omitted from pages generated by normal servlets. First, with servlets, it is tedious to generate the required println statements. With JSP, however, the format is simpler and you can make use of the code reuse options in your usual HTML building tool. Second, servlets cannot use the simplest form of relative URLs (ones that refer to files in the same directory as the current page) since the servlet directories are not mapped to URLs in the same manner as are URLs for normal Web pages. JSP pages, on the other hand, are installed in the normal Web page hierarchy on the server, and relative URLs are resolved properly. Thus, style sheets and JSP pages can be kept together in the same directory. The source code for the style sheet, like all code shown or referenced in the book, can be downloaded from http://www.corewebprogramming.com/.

Figure 20-1. Typical result of Expressions.jsp.

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Listing 20.1 Expressions.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>JSP Expressions</TITLE> <META id="keywords"       CONTENT="JSP,expressions,JavaServer,Pages,servlets"> <META id="description"       CONTENT="A quick example of JSP expressions."> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H2>JSP Expressions</H2> <UL>   <LI>Current time: <%= new java.util.Date() %>   <LI>Your hostname: <%= request.getRemoteHost() %>   <LI>Your session ID: <%= session.getId() %>   <LI>The <CODE>testParam</CODE> form parameter:       <%= request.getParameter("testParam") %> </UL> </BODY> </HTML> 

Scriptlets

If you want to do something more complex than insert a simple expression, JSP scriptlets let you insert arbitrary code into the servlet's _jspService method (which is called by service). Scriptlets have the following form:

<% Java Code %> 

Scriptlets have access to the same automatically defined variables as expressions (request, response, session, out, etc.). So, for example, if you want output to appear in the resultant page, you would use the out variable, as in the following example.

<% String queryData = request.getQueryString(); out.println("Attached GET data: " + queryData); %> 

In this particular instance, you could have accomplished the same effect more easily by using the following JSP expression:

Attached GET data: <%= request.getQueryString() %> 

In general, however, scriptlets can perform a number of tasks that cannot be accomplished with expressions alone. These tasks include setting response headers and status codes, invoking side effects such as writing to the server log or updating a database, or executing code that contains loops, conditionals, or other complex constructs. For instance, the following snippet specifies that the current page is sent to the client as plain text, not as HTML (which is the default).

<% response.setContentType("text/plain"); %> 

It is important to note that you can set response headers or status codes at various places within a JSP page, even though this capability appears to violate the rule that this type of response data needs to be specified before any document content is sent to the client. Setting headers and status codes is permitted because servlets that result from JSP pages use a special variety of Writer (of type JspWriter) that partially buffers the document. This buffering behavior can be changed, however; see Section 20.4 for a discussion of the buffer and autoflush attributes of the page directive.

As an example of executing code that is too complex for a JSP expression, Listing 20.2 presents a JSP page that uses the bgColor request parameter to set the background color of the page. Some results are shown in Figures 20-2 and 20-3.

Figure 20-2. Default result of BGColor.jsp.

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Figure 20-3. Result of BGColor.jsp when accessed with a bgColor parameter having the RGB value C0C0C0.

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Listing 20.2 BGColor.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD>   <TITLE>Color Testing</TITLE> </HEAD> <% String bgColor = request.getParameter("bgColor"); boolean hasExplicitColor = true; if (bgColor == null) {   hasExplicitColor = false;   bgColor = "WHITE"; } %> <BODY BGCOLOR="<%= bgColor %>"> <H2 ALIGN="CENTER">Color Testing</H2> <% if (hasExplicitColor) {   out.println("You supplied an explicit background color of " +               bgColor + "."); } else {   out.println("Using default background color of WHITE. " +               "Supply the bgColor request attribute to try " +               "a standard color or RRGGBB value, or to see " +               "if your browser supports X11 color names."); } %> </BODY> </HTML> 
Using Scriptlets to Make Parts of the JSP File Conditional

Another use of scriptlets is to conditionally include standard HTML and JSP constructs. The key to this approach is the fact that code inside a scriptlet gets inserted into the resultant servlet's _jspService method (called by service) exactly as written and that any static HTML (template text) before or after a scriptlet gets converted to print statements. This means that scriptlets need not contain complete Java statements and that blocks left open can affect the static HTML or JSP outside of the scriptlets. For example, consider the following JSP fragment containing mixed template text and scriptlets.

<% if (Math.random() < 0.5) { %> Have a <B>nice</B> day! <% } else { %> Have a <B>lousy</B> day! <% } %> 

When converted to a servlet by the JSP engine, this fragment will result in something similar to the following.

if (Math.random() < 0.5) {   out.println("Have a <B>nice</B> day!"); } else {   out.println("Have a <B>lousy</B> day!"); } 
Special Scriptlet Syntax

There are two special constructs you should take note of. First, if you want to use the characters %> inside a scriptlet, enter %\> instead. Second, the XML equivalent of <% Java Code %> is

<jsp:scriptlet>Java Code</jsp:scriptlet> 

The two forms are treated identically by some JSP engines, but jsp:scriptlet is not required to be supported until JSP 1.2.

Declarations

A JSP declaration lets you define methods or fields that get inserted into the main body of the servlet class (outside of the _jspService method that is called by service to process the request). A declaration has the following form:

<%! Java Code %> 

Since declarations do not generate any output, they are normally used in conjunction with JSP expressions or scriptlets. The declarations define methods or fields that are later used by expressions or scriptlets. One caution is warranted however: do not use JSP declarations to override the standard servlet lifecycle methods (service, doGet, init, etc.). The servlet into which the JSP page gets translated already makes use of these methods. There is no need for declarations to gain access to service, doGet, or doPost, since calls to service are automatically dispatched to _jspService, which is where code resulting from expressions and scriptlets gets put. However, for initialization and cleanup, you can use jspInit and jspDestroy the standard init and destroy methods are guaranteed to call these two methods when in servlets that come from JSP.

Core Note

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For initialization and cleanup in JSP pages, use JSP declarations to override jspInit and/or jspDestroy.

For example, here is a JSP fragment that prints the number of times the current page has been requested since the server was booted (or the servlet class was changed and reloaded). Recall that multiple client requests to the same servlet result only in multiple threads calling the service method of a single servlet instance. They do not result in the creation of multiple servlet instances except possibly when the servlet implements SingleThreadModel. For a discussion of SingleThreadModel, see the isThreadSafe attribute of the page directive (Section 20.4) and Section 19.4 (The Servlet Life Cycle). Thus, instance variables (fields) of a servlet are shared by multiple requests and accessCount does not have to be declared static below.

<%! private int accessCount = 0; %> Accesses to page since server reboot: <%= ++accessCount %> 

Listing 20.3 shows the full JSP page; Figure 20-4 shows a representative result.

Figure 20-4. Visiting AccessCounts.jsp after it has been requested nine previous times by the same or different clients.

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Listing 20.3 AccessCounts.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>JSP Declarations</TITLE> <META id="keywords"       CONTENT="JSP,declarations,JavaServer,Pages,servlets"> <META id="description"       CONTENT="A quick example of JSP declarations."> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>JSP Declarations</H1> <%! private int accessCount = 0; %> <H2>Accesses to page since server reboot: <%= ++accessCount %></H2> </BODY> </HTML> 
Special Declaration Syntax

As with scriptlets, if you want to use the characters %>, enter %\> instead. Finally, note that the XML equivalent of <%! Java Code %> is

<jsp:declaration>Java Code</jsp:declaration> 

Predefined Variables

To simplify code in JSP expressions and scriptlets, you are supplied with eight automatically defined variables, sometimes called implicit objects. Since JSP declarations result in code that appears outside of the _jspService method, these variables are not accessible in declarations. The available variables are request, response, out, session, application, config, pageContext, and page. Details for each are given below.

request This variable is the HttpServletRequest associated with the request; it gives you access to the request parameters, the request type (e.g., GET or POST), and the incoming HTTP headers (e.g., cookies).

response This variable is the HttpServletResponse associated with the response to the client. Note that since the output stream (see out) is normally buffered, it is legal to set HTTP status codes and response headers in JSP pages, even though the setting of headers or status codes is not permitted in servlets once any output has been sent to the client.

out This variable is the PrintWriter used to send output to the client. However, to make the response object useful, this is a buffered version of PrintWriter called JspWriter. You can adjust the buffer size through use of the buffer attribute of the page directive. The out variable is used almost exclusively in scriptlets, since JSP expressions are automatically placed in the output stream and thus rarely need to refer to out explicitly.

session This variable is the HttpSession object associated with the request. Recall that sessions are created automatically, so this variable is bound even if there is no incoming session reference. The one exception is if you use the session attribute of the page directive to turn sessions off. In that case, attempts to reference the session variable cause errors at the time the JSP page is translated into a servlet.

application This variable is the ServletContext as obtained by getServletContext. Servlets and JSP pages can store persistent data in the ServletContext object rather than in instance variables. ServletContext has setAttribute and getAttribute methods that let you store arbitrary data associated with specified keys. The difference between storing data in instance variables and storing it in the ServletContext is that the ServletContext is shared by all servlets in the servlet engine (or in the Web application).

config This variable is the ServletConfig object for this page.

pageContext JSP introduced a new class called PageContext to give a single point of access to many of the page attributes and to provide a convenient place to store shared data. The pageContext variable stores the value of the PageContext object associated with the current page.

page This variable is simply a synonym for this and is not very useful. It was created as a placeholder for the time when the scripting language could be something other than Java.

20.4 The JSP page Directive

A JSP directive affects the overall structure of the servlet that results from the JSP page. The following templates show the two possible forms for directives. Single quotes can be substituted for the double quotes around the attribute values, but the quotation marks cannot be omitted altogether. To obtain quote marks within an attribute value, precede them with a back slash, using \'; for ' and \" for ".

<%@ directive attribute="value" %> <%@ directive attribute1="value1"               attribute2="value2"               ...               attributeN="valueN" %> 

In JSP, there are three types of directives: page, include, and taglib. The page directive lets you control the structure of the servlet by importing classes, customizing the servlet superclass, setting the content type, and the like. A page directive can be placed anywhere within the document; its use is the topic of this section. The second directive, include, lets you insert a file into the servlet class at the time the JSP file is translated into a servlet. An include directive should be placed in the document at the point at which you want the file to be inserted; it is discussed in Section 20.5. JSP 1.1 introduces a third directive, taglib, which can be used to define custom markup tags; it is discussed in Section 20.7.

The page directive lets you define one or more of the following case-sensitive attributes: import, contentType, isThreadSafe, session, buffer, autoflush, extends, info, errorPage, isErrorPage, and language. These attributes are explained in the following subsections.

The import Attribute

The import attribute of the page directive lets you specify the packages that should be imported by the servlet into which the JSP page gets translated. If you don't explicitly specify any classes to import, the servlet imports java.lang.*, javax.servlet.*, javax.servlet.jsp.*, javax.servlet.http.*, and possibly some number of server-specific entries. Never write JSP code that relies on any server-specific classes being imported automatically. Use of the import attribute takes one of the following two forms:

<%@ page import="package.class" %> <%@ page import="package.class1,...,package.classN" %> 

For example, the following directive signifies that all classes in the java.util package should be available to use without explicit package identifiers.

     <%@ page import="java.util.*" %> 

The import attribute is the only page attribute that is allowed to appear multiple times within the same document. Although page directives can appear anywhere within the document, it is traditional to place import statements either near the top of the document or just before the first place that the referenced package is used.

Note that some servers have different rules about where to put different types of class files. For example, the Java Web Server 2.0 lets you put the actual servlet classes in the servlets directory but requires you to put classes used by servlets or JSP pages in the classes directory. The JSWDK and Tomcat have no such restrictions; check your server's documentation for definitive guidance.

For example, Listing 20.4 presents a page that uses two classes not in the standard JSP import list: java.util.Date, and cwp.ServletUtilities (see Listing 19.21). To simplify references to these classes, the JSP page uses

<%@ page import="java.util.*,cwp.*" %> 

Figures 20-5 and 20-6 show some typical results.

Figure 20-5. ImportAttribute.jsp when first accessed.

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Figure 20-6. ImportAttribute.jsp when accessed in a subsequent request.

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Listing 20.4 ImportAttribute.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>The import Attribute</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H2>The import Attribute</H2> <%-- JSP page directive --%> <%@ page import="java.util.*,cwp.*" %> <%-- JSP Declaration --%> <%! private String randomID() {   int num = (int)(Math.random()*10000000.0);   return("id" + num); } private final String NO_VALUE = "<I>No Value</I>"; %> <%-- JSP Scriptlet --%> <% Cookie[] cookies = request.getCookies(); String oldID =   ServletUtilities.getCookieValue(cookies, "userID", NO_VALUE); if (oldID.equals(NO_VALUE)) {   String newID = randomID();   Cookie cookie = new Cookie("userID", newID);   response.addCookie(cookie); } %> <%-- JSP Expressions --%> This page was accessed on <%= new Date() %> with a userID cookie of <%= oldID %>. </BODY> </HTML> 

The contentType Attribute

The contentType attribute sets the Content-Type response header, indicating the MIME type of the document being sent to the client. For more information on MIME types, see Table 19.1 (Common MIME Types) in Section 19.10 (The Server Response: HTTP Response Headers).

Use of the contentType attribute takes one of the following two forms:

<%@ page contentType="MIME-Type" %> <%@ page contentType="MIME-Type; charset=Character-Set" %> 

For example, the directive

<%@ page contentType="application/vnd.ms-excel" %> 

has the same effect as the scriptlet

<% response.setContentType("application/vnd.ms-excel"); %> 

Unlike regular servlets, where the default MIME type is text/plain, the default for JSP pages is text/html (with a default character set of ISO-8859-1).

Listing 20.5 shows a JSP page that generates tab-separated Excel output. Figure 20-7 shows the result in Internet Explorer on a system that has Microsoft Office installed.

Figure 20-7. Excel document (Excel.jsp) in Internet Explorer.

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Listing 20.5 Excel.jsp
First   Last    Email Address Marty   Hall    hall@corewebprogramming.com Larry   Brown   brown@corewebprogramming.com Bill    Gates   gates@sun.com Larry   Ellison ellison@microsoft.com <%@ page contentType="application/vnd.ms-excel" %> <%-- There are tabs, not spaces, between columns. --%> 

The isThreadSafe Attribute

The isThreadSafe attribute controls whether or not the servlet that results from the JSP page will implement the SingleThreadModel interface. Use of the isThreadSafe attribute takes one of the following two forms:

<%@ page isThreadSafe="true" %> <%-- Default --%> <%@ page isThreadSafe="false" %> 

With normal servlets, simultaneous user requests result in multiple threads concurrently accessing the service method of the same servlet instance. This behavior assumes that the servlet is thread safe; that is, that the servlet synchronizes access to data in its fields so that inconsistent values will not result from an unexpected ordering of thread execution. In some cases (such as page access counts), you may not care if two visitors occasionally get the same value, but in other cases (such as user IDs), identical values can spell disaster. For example, the following snippet is not thread safe since a thread could be preempted after reading idNum but before updating it, yielding two users with the same user ID.

<%! private int idNum = 0; %> <% String userID = "userID" + idNum; out.println("Your ID is " + userID + "."); idNum = idNum + 1; %> 

The code should have used a synchronized block. This construct is written

synchronized(someObject) { ... } 

and means that once a thread enters the block of code, no other thread can enter the same block (or any other block marked with the same object reference) until the first thread exits. So, the previous snippet should have been written in the following manner.

<%! private int idNum = 0; %> <% synchronized(this) {   String userID = "userID" + idNum;   out.println("Your ID is " + userID + ".");   idNum = idNum + 1; } %> 

That's the normal servlet behavior: multiple simultaneous requests are dispatched to multiple threads concurrently accessing the same servlet instance. However, if a servlet implements the SingleThreadModel interface, the system guarantees that there will not be simultaneous access to the same servlet instance. The system can satisfy this guarantee either by queuing up all requests and passing them to the same servlet instance or by creating a pool of instances, each of which handles a single request at a time.

You use <%@ page isThreadSafe="false" %> to indicate that your code is not thread safe and thus that the resulting servlet should implement SingleThreadModel. The default value is true, which means that the system assumes you made your code thread safe and it can consequently use the higher-performance approach of multiple simultaneous threads accessing a single servlet instance.

The session Attribute

The session attribute controls whether or not the page participates in HTTP sessions. Use of this attribute takes one of the following two forms:

<%@ page session="true" %> <%-- Default --%> <%@ page session="false" %> 

A value of true (the default) indicates that the predefined variable session (of type HttpSession) should be bound to the existing session if one exists; otherwise, a new session should be created and bound to session. A value of false means that no sessions will be used automatically and attempts to access the variable session will result in errors at the time the JSP page is translated into a servlet. Turning off session tracking may save significant amounts of server memory on high traffic sites. Just remember that sessions are user-specific, not page-specific. Thus, it doesn't do any good to turn off session tracking for one page unless you also turn it off for related pages that are likely to be visited in the same client session.

The buffer Attribute

The buffer attribute specifies the size of the buffer used by the out variable, which is of type JspWriter (a subclass of PrintWriter). Use of this attribute takes one of two forms:

<%@ page buffer="sizekb" %> <%@ page buffer="none" %> 

Servers can use a larger buffer than you specify, but not a smaller one. For example, <%@ page buffer="32kb" %> means the document content should be buffered and not sent to the client until at least 32 kilobytes have been accumulated or the page is completed. The default buffer size is server specific, but must be at least 8 kilobytes. Be cautious about turning off buffering; doing so requires JSP entries that set headers or status codes to appear at the top of the file, before any HTML content.

The autoflush Attribute

The autoflush attribute controls whether the output buffer should be automatically flushed when it is full or whether an exception should be raised when the buffer overflows. Use of this attribute takes one of the following two forms:

<%@ page autoflush="true" %> <%-- Default --%> <%@ page autoflush="false" %> 

A value of false is illegal when buffer="none" is also used.

The extends Attribute

The extends attribute designates the superclass of the servlet that will be generated for the JSP page and takes the following form:

<%@ page extends="package.class" %> 

Use this attribute with extreme caution since the server will almost certainly be using a custom superclass already.

The info Attribute

The info attribute defines a string that can be retrieved from the servlet by means of the getServletInfo method. Use of info takes the following form:

<%@ page info="Some Message" %> 

The errorPage Attribute

The errorPage attribute specifies a JSP page that should process any exceptions (i.e., something of type Throwable) thrown but not caught in the current page. It is used as follows:

<%@ page errorPage="Relative URL" %> 

The exception thrown will be automatically available to the designated error page by means of the exception variable.

The isErrorPage Attribute

The isErrorPage attribute indicates whether or not the current page can act as the error page for another JSP page. Use of isErrorPage takes one of the following two forms:

<%@ page isErrorPage="true" %> <%@ page isErrorPage="false" %> <%-- Default --%> 

The language Attribute

At some point, the language attribute is intended to specify the underlying programming language being used, as below.

<%@ page language="cobol" %> 

For now, don't bother with this attribute since java is both the default and the only legal choice.

XML Syntax for Directives

Some servers permit you to use an alternative XML-compatible syntax for directives. These constructs take the following form:

<jsp:directive.directiveType attribute="value" /> 

For example, the XML equivalent of

<%@ page import="java.util.*" %> 

is

<jsp:directive.page import="java.util.*" /> 

20.5 Including Files and Applets in JSP Documents

JSP has three main capabilities for including external pieces into a JSP document.

  1. The include directive. The construct lets you insert JSP code into the main page before that main page is translated into a servlet. The included code can contain JSP constructs such as field definitions and content-type settings that affect the main page as a whole. This capability is discussed in the first following subsection.

  2. The jsp:include action. Although reusing chunks of JSP code is a powerful capability, most times you would rather sacrifice a small amount of power for the convenience of being able to change the included documents without updating the main JSP page. The jsp:include action lets you include the output of a page at request time. Note that jsp:include only lets you include the result of the secondary page, not the code itself as with the include directive. Consequently, the secondary page cannot use any JSP constructs that affect the main page as a whole. Use of jsp:include is discussed in the second subsection.

  3. The jsp:plugin action. Although this chapter is primarily about server-side Java, client-side Java in the form of Web-embedded applets continues to play a role, especially within corporate intranets. The jsp:plugin element is used to insert applets that use the Java Plug-In into JSP pages. This capability is discussed in the third subsection.

The include Directive: Including Files at Page Translation Time

You use the include directive to include a file in the main JSP document at the time the document is translated into a servlet (which is typically the first time it is accessed). The syntax is as follows:

<%@ include file="Relative URL" %> 

There are two ramifications of the fact that the included file is inserted at page translation time, not at request time as with jsp:include (see the next subsection).

First, the included file is permitted to contain JSP code such as response header settings and field definitions that affect the main page. For example, suppose snippet.jsp contained the following code:

<%! int accessCount = 0; %> 

In such a case, you could do the following:

<%@ include file="snippet.jsp" %> <%-- Defines accessCount --%> <%= accessCount++ %>              <%-- Uses accessCount --%> 

Second, if the included file changes, all the JSP files that use it need to be updated. Unfortunately, although servers are allowed to support a mechanism for detecting when an included file has changed (and then recompiling the servlet), they are not required to do so. In practice, few servers support this capability. JSP 1.1 lets you supply a jsp_precompile request parameter to instruct the JSP engine to translate the JSP page into a servlet. However, this fails in JSP 1.0. So, the simplest and most portable approach is to update the modification date of the JSP page. Some operating systems have commands that update the modification date without your actually editing the file (e.g., the Unix touch command), but a simple portable alternative is to include a JSP comment in the top-level page. Update the comment whenever the included file changes. For example, you might put the modification date of the included file in the comment, as below.

<%-- Navbar.jsp modified 3/1/00 --%> <%@ include file="Navbar.jsp" %> 

Core Warning

graphics/corenote.gif
 

If you change an included JSP file, you must update the modification dates of all JSP files that use it.

For example, Listing 20.6 shows a page fragment that gives corporate contact information and some per-page access statistics appropriate to be included at the bottom of multiple pages within a site. Listing 20.7 shows a page that makes use of it, and Figure 20-8 shows the result.

Figure 20-8. Ninth access to SomeRandomPage.jsp.

graphics/20fig08.gif
 

Listing 20.6 ContactSection.jsp
<%@ page import="java.util.Date" %> <%-- The following become fields in each servlet that      results from a JSP page that includes this file. --%> <%! private int accessCount = 0; private Date accessDate = new Date(); private String accessHost = "<I>No previous access</I>"; %> <P> <HR> This page &copy; 2001 <A HREF="http//www.my-company.com/">my-company.com</A>. This page has been accessed <%= ++accessCount %> times since server reboot. It was last accessed from <%= accessHost %> at <%= accessDate %>. <% accessHost = request.getRemoteHost(); %> <% accessDate = new Date(); %> 
Listing 20.7 SomeRandomPage.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Some Random Page</TITLE> <META id="author" CONTENT="J. Random Hacker"> <META id="keywords"       CONTENT="foo,bar,baz,quux"> <META id="description"       CONTENT="Some random Web page."> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       Some Random Page</TABLE> <P> Information about our products and services. <P> Blah, blah, blah. <P> Yadda, yadda, yadda. <%@ include file="ContactSection.jsp" %> </BODY> </HTML> 
XML Syntax for the include Directive

The XML-compatible equivalent of

<%@ include file="..." %> 

is

<jsp:directive.include file="..." /> 

Including Files at Request Time

The include directive (see the previous subsection) lets you include actual JSP code into multiple different pages. Including the code itself is sometimes a useful capability, but the include directive requires you to update the modification date of the page whenever the included file changes. This is a significant inconvenience. The jsp:include action includes the output of a secondary page at the time the main page is requested. Thus, jsp:include does not require you to update the main file when an included file changes. On the other hand, the page has already been translated into a servlet by request time, so the included pages cannot contain JSP that affects the main page as a whole. This is a minor restriction, and jsp:include is almost always preferred.

Core Approach

graphics/corenote.gif
 

For file inclusion, use jsp:include whenever possible. Reserve the include directive for cases when the included file defines fields or methods that the main page uses or when the included file sets response headers of the main page.

Although the output of the included files cannot contain JSP, they can be the result of resources that use JSP to create the output. That is, the URL that refers to the included resource is interpreted in the normal manner by the server and thus can be a servlet or JSP page. This is precisely the behavior of the include method of the RequestDispatcher class, which is what servlets use if they want to do this type of file inclusion.

The jsp:include element has two required attributes, as shown in the sample below: page (a relative URL referencing the file to be included) and flush (which can be omitted in JSP 1.1, and must have the value true if present).

<jsp:include page="Relative URL" flush="true" /> 

As an example, consider the simple news summary page shown in Listing 20.8. Page developers can change the news items in the files Item1.html through Item3.html (Listings 20.9 through 20.11) without having to update the main news page. Figure 20-9 shows the result.

Figure 20-9. Including files at request time makes it easier to update the individual files.

graphics/20fig09.gif
 

Listing 20.8 WhatsNew.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>What's New at JspNews.com</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       What's New at JspNews.com</TABLE> <P> Here is a summary of our three most recent news stories: <OL>   <LI><jsp:include page="news/Item1.html" flush="true" />   <LI><jsp:include page="news/Item2.html" flush="true" />   <LI><jsp:include page="news/Item3.html" flush="true" /> </OL> </BODY> </HTML> 
Listing 20.9 Item1.html
<B>Bill Gates acts humble.</B> In a startling and unexpected development, Microsoft big wig Bill Gates put on an open act of humility yesterday. <A HREF="http://www.microsoft.com/Never.html">More details...</A> 
Listing 20.10 Item2.html
<B>Scott McNealy acts serious.</B> In an unexpected twist, wisecracking Sun head Scott McNealy was sober and subdued at yesterday's meeting. <A HREF="http://www.sun.com/Imposter.html">More details...</A> 
Listing 20.11 Item3.html
<B>Larry Ellison acts conciliatory.</B> Catching his competitors off guard yesterday, Oracle prez Larry Ellison referred to his rivals in friendly and respectful terms. <A HREF="http://www.oracle.com/Mistake.html">More details...</A> 

Including Applets for the Java Plug-In

With JSP, you don't need any special syntax to include ordinary applets: just use the normal HTML APPLET tag. However, except for intranets that use Netscape 6 exclusively, these applets must use JDK 1.1 or JDK 1.02 since neither Netscape 4.x nor Internet Explorer 5.x supports the Java 2 platform (i.e., JDK 1.2 or 1.3). This lack of support imposes several restrictions on applets:

  • In order to use Swing, you must send the Swing files over the network. This process is time consuming and fails in Internet Explorer 3 and Netscape 3.x and 4.01-4.05 (which only support JDK 1.02), since Swing depends on JDK 1.1.

  • You cannot use Java 2D.

  • You cannot use the Java 2 collections package.

  • Your code runs more slowly, since most compilers for the Java 2 platform are significantly improved over their 1.1 predecessors.

To address these problems, Sun developed a browser plug-in for Netscape and Internet Explorer that lets you use the Java 2 platform in a variety of browsers. This plug-in is available at http://java.sun.com/products/plugin/ and also comes bundled with JDK 1.2.2 and later. Since the plug-in is quite large (several megabytes), it is not reasonable to expect users on the WWW at large to download and install it just to run your applets. On the other hand, it is a reasonable alternative for fast corporate intranets, especially since applets can automatically prompt browsers that lack the plug-in to download it.

Unfortunately, however, the normal APPLET tag will not work with the plug-in, since browsers are specifically designed to use only their built-in virtual machine when they see APPLET. Instead, you have to use a long and messy OBJECT tag for Internet Explorer and an equally long EMBED tag for Netscape. Furthermore, since you typically don't know which browser type will be accessing your page, you have to either include both OBJECT and EMBED (placing the EMBED within the COMMENT section of OBJECT) or identify the browser type at the time of the request and conditionally build the right tag. This process is straightforward but tedious and time consuming.

The jsp:plugin element instructs the server to build a tag appropriate for applets that use the plug-in. Servers are permitted some leeway in exactly how they implement this support, but most simply include both OBJECT and EMBED.

The jsp:plugin Element

The simplest way to use jsp:plugin is to supply four attributes: type, code, width, and height. You supply a value of applet for the type attribute and use the other three attributes in exactly the same way as with the APPLET element, with two exceptions: the attribute names are case sensitive, and single or double quotes are always required around the attribute values. So, for example, you could replace

<APPLET CODE="MyApplet.class"         WIDTH=475 HEIGHT=350> </APPLET> 

with

<jsp:plugin type="applet"             code="MyApplet.class"             width="475" height="350"> </jsp:plugin> 

The jsp:plugin element has a number of other optional attributes. Most, but not all, parallel attributes of the APPLET element. Here is a full list.

  • type For applets, this attribute should have a value of applet. However, the Java Plug-In also permits you to embed JavaBeans elements in Web pages. Use a value of bean in such a case.

  • code This attribute is used identically to the CODE attribute of APPLET, specifying the top-level applet class file that extends Applet or JApplet.

  • width This attribute is used identically to the WIDTH attribute of APPLET, specifying the width in pixels to be reserved for the applet.

  • height This attribute is used identically to the HEIGHT attribute of APPLET, specifying the height in pixels to be reserved for the applet.

  • codebase This attribute is used identically to the CODEBASE attribute of APPLET, specifying the base directory for the applets. The code attribute is interpreted relative to this directory. As with the APPLET element, if you omit this attribute, the directory of the current page is used as the default. In the case of JSP, this default location is the directory where the original JSP file resided, not the system-specific location of the servlet that results from the JSP file.

  • align This attribute is used identically to the ALIGN attribute of APPLET and IMG, specifying the alignment of the applet within the Web page. Legal values are left, right, top, bottom, and middle.

  • hspace This attribute is used identically to the HSPACE attribute of APPLET, specifying empty space in pixels reserved on the left and right of the applet.

  • vspace This attribute is used identically to the VSPACE attribute of APPLET, specifying empty space in pixels reserved on the top and bottom of the applet.

  • archive This attribute is used identically to the ARCHIVE attribute of APPLET, specifying a JAR file from which classes and images should be loaded.

  • name This attribute is used identically to the NAME attribute of APPLET, specifying a name to use for inter-applet communication or for identifying the applet to scripting languages like JavaScript.

  • title This attribute is used identically to the very rarely used TITLE attribute of APPLET (and virtually all other HTML elements in HTML 4.0), specifying a title that could be used for a tool-tip or for indexing.

  • jreversion This attribute identifies the version of the Java Runtime Environment (JRE) that is required. The default is 1.1.

  • iepluginurl This attribute designates a URL from which the plug-in for Internet Explorer can be downloaded. Users who don't already have the plug-in installed will be prompted to download it from this location. The default value will direct the user to the Sun site, but for intranet use you might want to direct the user to a local copy.

  • nspluginurl This attribute designates a URL from which the plug-in for Netscape can be downloaded. The default value will direct the user to the Sun site, but for intranet use you might want to direct the user to a local copy.

The jsp:param and jsp:params Elements

The jsp:param element is used with jsp:plugin in a manner similar to the way that PARAM is used with APPLET, specifying a name and value that are accessed from within the applet by getParameter. There are two main differences, however. First, since jsp:param follows XML syntax, attribute names must be lower case, attribute values must be enclosed in single or double quotes, and the element must end with />, not just >. Second, all jsp:param entries must be enclosed within a jsp:params element.

So, for example, you would replace

<APPLET CODE="MyApplet.class"         WIDTH=475 HEIGHT=350>   <PARAM id="PARAM1" VALUE="VALUE1">   <PARAM id="PARAM2" VALUE="VALUE2"> </APPLET> 

with

<jsp:plugin type="applet"             code="MyApplet.class"             width="475" height="350">   <jsp:params>     <jsp:param id="PARAM1" value="VALUE1" />     <jsp:param id="PARAM2" value="VALUE2" />   </jsp:params> </jsp:plugin> 

The jsp:fallback Element

The jsp:fallback element provides alternative text to browsers that do not support OBJECT or EMBED. You use this element in almost the same way as you would use alternative text placed within an APPLET element. So, for example, you would replace

<APPLET CODE="MyApplet.class"         WIDTH=475 HEIGHT=350>   <B>Error: this example requires Java.</B> </APPLET> 

with

<jsp:plugin type="applet"             code="MyApplet.class"             width="475" height="350">   <jsp:fallback>     <B>Error: this example requires Java.</B>   </jsp:fallback> </jsp:plugin> 
A jsp:plugin Example

Listing 20.12 shows a JSP page that uses the jsp:plugin element to generate an entry for the Java 2 Plug-In. Listings 20.13 through 20.15 show the code for the applet itself (which uses Swing and Java 2D), and Figure 20-10 shows the result.

Figure 20-10. Result of PluginApplet.jsp in Internet Explorer when the Java 2 Plug-In is installed.

graphics/20fig10.gif
 

Listing 20.12 PluginApplet.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Using jsp:plugin</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       Using jsp:plugin</TABLE> <P> <CENTER> <jsp:plugin type="applet"             code="PluginApplet.class"             width="370" height="420"> </jsp:plugin> </CENTER> </BODY> </HTML> 
Listing 20.13 PluginApplet.java
import javax.swing.*; /** An applet that uses Swing and Java 2D and thus requires  *  the Java Plug-In.  */ public class PluginApplet extends JApplet {   public void init() {     WindowUtilities.setNativeLookAndFeel();     setContentPane(new TextPanel());   } } 
Listing 20.14 TextPanel.java
import java.awt.*; import java.awt.event.*; import javax.swing.*; /** JPanel that places a panel with text drawn at various angles  *  in the top part of the window and a JComboBox containing  *  font choices in the bottom part.  */ public class TextPanel extends JPanel                        implements ActionListener {   private JComboBox fontBox;   private DrawingPanel drawingPanel;   public TextPanel() {     GraphicsEnvironment env =       GraphicsEnvironment.getLocalGraphicsEnvironment();     String[] fontNames = env.getAvailableFontFamilyNames();     fontBox = new JComboBox(fontNames);     setLayout(new BorderLayout());     JPanel fontPanel = new JPanel();     fontPanel.add(new JLabel("Font:"));     fontPanel.add(fontBox);     JButton drawButton = new JButton("Draw");     drawButton.addActionListener(this);     fontPanel.add(drawButton);     add(fontPanel, BorderLayout.SOUTH);     drawingPanel = new DrawingPanel();     fontBox.setSelectedItem("Serif");     drawingPanel.setFontName("Serif");     add(drawingPanel, BorderLayout.CENTER);   }   public void actionPerformed(ActionEvent e) {     drawingPanel.setFontName((String)fontBox.getSelectedItem());     drawingPanel.repaint();   } } 
Listing 20.15 DrawingPanel.java
import java.awt.*; import java.awt.geom.*; import javax.swing.*; /** A window with text drawn at an angle. The font is  *  set by means of the setFontName method.  */ class DrawingPanel extends JPanel {   private Ellipse2D.Double circle =     new Ellipse2D.Double(10, 10, 350, 350);   private GradientPaint gradient =     new GradientPaint(0, 0, Color.red, 180, 180, Color.yellow,                       true); // true means to repeat pattern   private Color[] colors = { Color.white, Color.black };   public void paintComponent(Graphics g) {     super.paintComponent(g);     Graphics2D g2d = (Graphics2D)g;     g2d.setPaint(gradient);     g2d.fill(circle);     g2d.translate(185, 185);     for (int i=0; i<16; i++) {       g2d.rotate(Math.PI/8.0);       g2d.setPaint(colors[i%2]);       g2d.drawString("jsp:plugin", 0, 0);     }   }   public void setFontName(String fontName) {     setFont(new Font(fontName, Font.BOLD, 35));   } } 

20.6 Using JavaBeans with JSP

The JavaBeans API provides a standard format for Java classes. Visual manipulation tools and other programs can automatically discover information about classes that follow this format and can then create and manipulate the classes without the user having to explicitly write any code.

Full coverage of JavaBeans is beyond the scope of this book. If you want details, pick up one of the many books on the subject or see the documentation and tutorials at http://java.sun.com/beans/docs/. For the purposes of this chapter, all you need to know about beans are three simple points:

  1. A bean class must have a zero-argument (empty) constructor. You can satisfy this requirement either by explicitly defining such a constructor or by omitting all constructors, which results in an empty constructor being created automatically. The empty constructor will be called when JSP elements create beans.

  2. A bean class should have no public instance variables (fields). We hope you already follow this practice and use accessor methods instead of allowing direct access to the instance variables. Use of accessor methods lets you do three things: (1) impose constraints on variable values (e.g., have the setSpeed method of your Car class disallow negative speeds); (2) change your internal data structures without changing the class interface (e.g., change from English units to metric units internally, but still have getSpeedInMPH and getSpeedInKPH methods); (3) perform side effects automatically when values change (e.g., update the user interface when setPosition is called).

  3. Persistent values should be accessed through methods called getXxx and setXxx. For example, if your Car class stores the current number of passengers, you might have methods named getNumPassengers (which takes no arguments and returns an int) and setNumPassengers (which takes an int and has a void return type). In such a case, the Car class is said to have a property named numPassengers (notice the lowercase n in the property name, but the uppercase N in the method names). If the class has a getXxx method but no corresponding setXxx, the class is said to have a read-only property named xxx.

    The one exception to this naming convention is with boolean properties: they use a method called isXxx to look up their values. So, for example, your Car class might have methods called isLeased (which takes no arguments and returns a boolean) and setLeased (which takes a boolean and has a void return type), and would be said to have a boolean property named leased (again, notice the lowercase leading letter in the property name).

    Although you can use JSP scriptlets or expressions to access arbitrary methods of a class, standard JSP actions for accessing beans can only make use of methods that use the getXxx/setXxx or isXxx/setXxx design pattern.

Basic Bean Use

The jsp:useBean action lets you load a bean to be used in the JSP page. Beans provide a very useful capability because they let you exploit the reusability of Java classes without sacrificing the convenience that JSP adds over servlets alone.

The simplest syntax for specifying that a bean should be used is:

<jsp:useBean id="name" class="package.Class" /> 

This usually means "instantiate an object of the class specified by Class, and bind it to a variable with the name specified by id." The bean class definition should be in the server's class path (generally, in the same directories where servlets can be installed), not in the directory that contains the JSP file. So, for example, the JSP action

<jsp:useBean id="book1" class="cwp.Book" /> 

can normally be thought of as equivalent to the scriptlet

<% cwp.Book book1 = new cwp.Book(); %> 

Although it is convenient to think of jsp:useBean as being equivalent to building an object, jsp:useBean has additional options that make it more powerful. As we'll see later, you can specify a scope attribute that makes the bean associated with more than just the current page. If beans can be shared, it is useful to obtain references to existing beans, so the jsp:useBean action specifies that a new object is instantiated only if there is no existing one with the same id and scope.

Rather than using the class attribute, you are permitted to use beanName instead. The difference is that beanName can refer either to a class or to a file containing a serialized bean object. The value of the beanName attribute is passed to the instantiate method of java.beans.Bean.

In most cases, you want the local variable to have the same type as the object being created. In a few cases, however, you might want the variable to be declared to have a type that is a superclass of the actual bean type or is an interface that the bean implements. Use the type attribute to control this, as in the following example:

<jsp:useBean id="thread1" class="MyClass" type="Runnable" /> 

This use results in code similar to the following being inserted into the _jspService method:

Runnable thread1 = new MyClass(); 

Note that since jsp:useBean uses XML syntax, the format differs in three ways from HTML syntax: the attribute names are case sensitive, either single or double quotes can be used (but one or the other must be used), and the end of the tag is marked with />, not just >. The first two syntactic differences apply to all JSP elements that look like jsp:xxx. The third difference applies unless the element is a container with a separate start and end tag.

There are also a few character sequences that require special handling in order to appear inside attribute values. To get ' within an attribute value, use \'. Similarly, to get ", use \"; to get \ use \\; to get %>, use %\>; and to get <%, use <\%.

Accessing Bean Properties

Once you have a bean, you can access its properties with jsp:getProperty, which takes a name attribute that should match the id given in jsp:useBean and a property attribute that names the property of interest. Alternatively, you could use a JSP expression and explicitly call a method on the object that has the variable name specified with the id attribute. For example, assuming that the Book class has a String property called title and that you've created an instance called book1 by using the jsp:useBean example just given, you could insert the value of the title property into the JSP page in either of the following two ways:

<jsp:getProperty id="book1" property="title" /> <%= book1.getTitle() %> 

The first approach is preferable in this case, since the syntax is more accessible to Web page designers who are not familiar with the Java programming language. However, direct access to the variable is useful when you are using loops, conditional statements, and methods not represented as properties.

If you are not familiar with the concept of bean properties, the standard interpretation of the statement "this bean has a property of type T called foo" is "this class has a method called getFoo that returns something of type T and has another method called setFoo that takes a T as an argument and stores it for later access by getFoo."

Setting Bean Properties: Simple Case

To modify bean properties, you normally use jsp:setProperty. This action has several different forms, but with the simplest form you just supply three attributes: name (which should match the id given by jsp:useBean), property (the name of the property to change), and value (the new value). Later in this section we will present some alternate forms of jsp:setProperty that let you automatically associate a property with a request parameter. That section also explains how to supply values that are computed at request time (rather than fixed strings) and discusses the type conversion conventions that let you supply string values for parameters that expect numbers, characters, or boolean values.

An alternative to using the jsp:setProperty action is to use a scriptlet that explicitly calls methods on the bean object. For example, given the book1 object shown earlier in this section, you could use either of the following two forms to modify the title property:

<jsp:setProperty id="book1"                  property="title"                  value="Core Web Programming" /> <% book1.setTitle("Core Web Programming"); %> 

Using jsp:setProperty has the advantage that it is more accessible to the nonprogrammer, but direct access to the object lets you perform more complex operations such as setting the value conditionally or calling methods other than getXxx or setXxx on the object.

Example: StringBean

Listing 20.16 presents a simple class called StringBean that is in the cwp package. Because the class has no public instance variables (fields) and has a zero-argument constructor since it doesn't declare any explicit constructors, it satisfies the basic criteria for being a bean. Since StringBean has a method called getMessage that returns a String and another method called setMessage that takes a String as an argument, in beans terminology the class is said to have a String parameter called message.

Listing 20.17 shows a JSP file that uses the StringBean class. First, an instance of StringBean is created with the jsp:useBean action as follows:

<jsp:useBean id="stringBean" class="cwp.StringBean" /> 

After this, the message property can be inserted into the page in either of the following two ways:

<jsp:getProperty id="stringBean" property="message" /> <%= stringBean.getMessage() %> 

The message property can be modified in either of the following two ways:

<jsp:setProperty id="stringBean"                  property="message"                  value="some message" /> <% stringBean.setMessage("some message"); %> 

Figure 20-11 shows the result.

Figure 20-11. Result of StringBean.jsp.

graphics/20fig11.gif
 

Listing 20.16 StringBean.java
package cwp; /** A simple bean that has a single String property  *  called message.  */ public class StringBean {   private String message = "No message specified";   public String getMessage() {     return(message);   }   public void setMessage(String message) {     this.message = message;   } } 
Listing 20.17 StringBean.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Using JavaBeans with JSP</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       Using JavaBeans with JSP</TABLE> <jsp:useBean id="stringBean" class="cwp.StringBean" /> <OL> <LI>Initial value (getProperty):     <I><jsp:getProperty id="stringBean"                         property="message" /></I> <LI>Initial value (JSP expression):     <I><%= stringBean.getMessage() %></I> <LI><jsp:setProperty id="stringBean"                      property="message"                      value="Best string bean: Fortex" />     Value after setting property with setProperty:     <I><jsp:getProperty id="stringBean"                         property="message" /></I> <LI><% stringBean.setMessage("My favorite: Kentucky Wonder"); %>     Value after setting property with scriptlet:     <I><%= stringBean.getMessage() %></I> </OL> </BODY> </HTML> 

Setting Bean Properties

You normally use jsp:setProperty to set bean properties. The simplest form of this action takes three attributes: name (which should match the id given by jsp:useBean), property (the name of the property to change), and value (the new value).

For example, the SaleEntry class shown in Listing 20.18 has an itemID property (a String), a numItems property (an int), a discountCode property (a double), and two read-only properties itemCost and totalCost (each of type double). Listing 20.19 shows a JSP file that builds an instance of the SaleEntry class by means of:

<jsp:useBean id="entry" class="cwp.SaleEntry" /> 

The results are shown in Figure 20-12.

Figure 20-12. Result of SaleEntry1.jsp.

graphics/20fig12.gif
 

Once the bean is instantiated, using an input parameter to set the itemID is straightforward, as shown below:

<jsp:setProperty     id="entry"     property="itemID"     value='<%= request.getParameter("itemID") %>' /> 

Notice that we used a JSP expression for the value parameter. Most JSP attribute values have to be fixed strings, but the value and name attributes of jsp:setProperty are permitted to be request-time expressions. If the expression uses double quotes internally, recall that single quotes can be used instead of double quotes around attribute values and that \' and \" can be used to represent single or double quotes within an attribute value.

Listing 20.18 SaleEntry.java
package cwp; /** Simple bean to illustrate the various forms  *  of jsp:setProperty.  */ public class SaleEntry {   private String itemID = "unknown";   private double discountCode = 1.0;   private int numItems = 0;   public String getItemID() {     return(itemID);   }   public void setItemID(String itemID) {     if (itemID != null) {       this.itemID = itemID;     } else {       this.itemID = "unknown";     }   }   public double getDiscountCode() {     return(discountCode);   }   public void setDiscountCode(double discountCode) {     this.discountCode = discountCode;   }   public int getNumItems() {     return(numItems);   }   public void setNumItems(int numItems) {     this.numItems = numItems;   }   // In real life, replace this with database lookup.   public double getItemCost() {     double cost;     if (itemID.equals("a1234")) {       cost = 12.99*getDiscountCode();     } else {       cost = -9999;     }     return(roundToPennies(cost));   }   private double roundToPennies(double cost) {     return(Math.floor(cost*100)/100.0);   }   public double getTotalCost() {     return(getItemCost() * getNumItems());   } } 
Listing 20.19 SaleEntry1.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Using jsp:setProperty</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       Using jsp:setProperty</TABLE> <jsp:useBean id="entry" class="cwp.SaleEntry" /> <jsp:setProperty     id="entry"     property="itemID"     value='<%= request.getParameter("itemID") %>' /> <% int numItemsOrdered = 1; try {   numItemsOrdered =     Integer.parseInt(request.getParameter("numItems")); } catch(NumberFormatException nfe) {} %> <jsp:setProperty     id="entry"     property="numItems"     value="<%= numItemsOrdered %>" /> <% double discountCode = 1.0; try {   String discountString =     request.getParameter("discountCode");   // Double.parseDouble not available in JDK 1.1.   discountCode =     Double.valueOf(discountString).doubleValue(); } catch(NumberFormatException nfe) {} %> <jsp:setProperty     id="entry"     property="discountCode"     value="<%= discountCode %>" /> <BR> <TABLE ALIGN="CENTER" BORDER=1> <TR CLASS="COLORED">   <TH>Item ID<TH>Unit Price<TH>Number Ordered<TH>Total Price <TR ALIGN="RIGHT">   <TD><jsp:getProperty id="entry" property="itemID" />   <TD>$<jsp:getProperty id="entry" property="itemCost" />   <TD><jsp:getProperty id="entry" property="numItems" />   <TD>$<jsp:getProperty id="entry" property="totalCost" /> </TABLE> </BODY> </HTML> 
Associating Individual Properties with Input Parameters

Setting the itemID property is easy since its value is a String. Setting the numItems and discountCode properties is a bit more problematic since their values must be numbers and getParameter returns a String. Here is the somewhat cumbersome code required to set numItems:

<% int numItemsOrdered = 1; try {   numItemsOrdered =     Integer.parseInt(request.getParameter("numItems")); } catch(NumberFormatException nfe) {} %> <jsp:setProperty     id="entry"     property="numItems"     value="<%= numItemsOrdered %>" /> 

Fortunately, JSP has a nice solution to this problem. It lets you associate a property with a request parameter and automatically perform type conversion from strings to numbers, characters, and boolean values. Instead of using the value attribute, you use param to name an input parameter. The value of this parameter is automatically used as the value of the property, and simple type conversions are performed automatically. If the specified input parameter is missing from the request, no action is taken (the system does not pass null to the associated property). So, for example, setting the numItems property can be simplified to:

<jsp:setProperty     id="entry"     property="numItems"     param="numItems" /> 

Listing 20.20 shows the entire JSP page reworked in this manner.

Listing 20.20 SaleEntry2.jsp
... <jsp:useBean id="entry" class="cwp.SaleEntry" /> <jsp:setProperty     id="entry"     property="itemID"     param="itemID" /> <jsp:setProperty     id="entry"     property="numItems"     param="numItems" /> <jsp:setProperty     id="entry"     property="discountCode"     param="discountCode" /> ... 
Automatic Type Conversions

When bean properties are associated with input parameters, the system automatically performs simple type conversions for properties that expect primitive types (byte, int, double, etc.) or the corresponding wrapper types (Byte, Integer, Double, etc.). One warning is in order, however: both JSWDK 1.0.1 and the Java Web Server 2.0 have a bug that causes them to crash at page translation time for pages that try to perform automatic type conversions for properties that expect double values. Tomcat and most recent servers work as expected.

Associating All Properties with Input Parameters

Associating a property with an input parameter saves you the bother of performing conversions for many of the simple built-in types. JSP lets you take the process one step further by associating all properties with identically named input parameters. All you have to do is to supply "*" for the property parameter. So, for example, all three of the jsp:setProperty statements of Listing 20.20 can be replaced by the following simple line. Listing 20.21 shows the complete page.

<jsp:setProperty id="entry" property="*" /> 

Although this approach is simple, four small warnings are in order. First, as with individually associated properties, no action is taken when an input parameter is missing. In particular, the system does not supply null as the property value. Second, the JSWDK and the Java Web Server both fail for conversions to properties that expect double values. Third, automatic type conversion does not guard against illegal values as effectively as does manual type conversion. So, you might consider error pages when using automatic type conversion. Fourth, since both property names and input parameters are case sensitive, the property name and input parameter must match exactly.

Listing 20.21 SaleEntry3.jsp
... <jsp:useBean id="entry" class="cwp.SaleEntry" /> <jsp:setProperty id="entry" property="*" /> ... 

Sharing Beans

Up to this point, we have treated the objects that were created with jsp:useBean as though they were simply bound to local variables in the _jspService method (which is called by the service method of the servlet that is generated from the page). Although the beans are indeed bound to local variables, that is not the only behavior. They are also stored in one of four different locations, depending on the value of the optional scope attribute of jsp:useBean. The scope attribute has the following possible values:

  • page This is the default value. It indicates that, in addition to being bound to a local variable, the bean object should be placed in the PageContext object for the duration of the current request. Storing the object there means that servlet code can access it by calling getAttribute on the predefined pageContext variable.

  • application This very useful value means that, in addition to being bound to a local variable, the bean will be stored in the shared ServletContext available through the predefined application variable or by a call to getServletContext(). The ServletContext is shared by all servlets in the same Web application (or all servlets in the same server or servlet engine if no explicit Web applications are defined). Values in the ServletContext can be retrieved by the getAttribute method. This sharing has a couple of ramifications.

    First, it provides a simple mechanism for multiple servlets and JSP pages to access the same object. See the following subsection (Conditional Bean Creation) for details and an example.

    Second, it lets a servlet create a bean that will be used in JSP pages, not just access one that was previously created. This approach lets a servlet handle complex user requests by setting up beans, storing them in the ServletContext, then forwarding the request to one of several possible JSP pages to present results appropriate to the request data. For details on this approach, see Section 20.8 (Integrating Servlets and JSP).

  • session This value means that, in addition to being bound to a local variable, the bean will be stored in the HttpSession object associated with the current request, where it can be retrieved with getAttribute.

  • request This value signifies that, in addition to being bound to a local variable, the bean object should be placed in the ServletRequest object for the duration of the current request, where it is available by means of the getAttribute method. Storing values in the request object is common when using the MVC (Model 2) architecture. For details, see Section 20.8 (Integrating Servlets and JSP)

Conditional Bean Creation

To make bean sharing more convenient, there are two situations where bean-related elements are evaluated conditionally.

First, a jsp:useBean element results in a new bean being instantiated only if no bean with the same id and scope can be found. If a bean with the same id and scope is found, the preexisting bean is simply bound to the variable referenced by id. A typecast is performed if the preexisting bean is of a more specific type than the bean being declared, and a ClassCastException results if this typecast is illegal.

Second, instead of

<jsp:useBean ... /> 

you can use

<jsp:useBean ...>statements</jsp:useBean> 

The point of using the second form is that the statements between the jsp:useBean start and end tags are executed only if a new bean is created, not if an existing bean is used. This conditional execution is convenient for setting initial bean properties for beans that are shared by multiple pages. Since you don't know which page will be accessed first, you don't know which page should contain the initialization code. No problem: they can all contain the code, but only the page first accessed actually executes it. For example, Listing 20.22 shows a simple bean that can be used to record cumulative access counts to any of a set of related pages. It also stores the name of the first page that was accessed. Since there is no way to predict which page in a set will be accessed first, each page that uses the shared counter has statements like the following:

<jsp:useBean id="counter"              class="cwp.AccessCountBean"              scope="application">   <jsp:setProperty id="counter"                    property="firstPage"                    value="Current Page Name" /> </jsp:useBean> Collectively, the pages using the counter have been accessed <jsp:getProperty id="counter" property="accessCount" /> times. 

Listing 20.23 shows the first of three pages that use this approach. The source code archive at http://www.corewebprogramming.com/ contains the other two nearly identical pages. Figure 20-13 shows a typical result.

Figure 20-13. Result of a user visiting SharedCounts3.jsp. The first page visited by any user was SharedCounts2.jsp. SharedCounts1.jsp, SharedCounts2.jsp, and SharedCounts3.jsp were collectively visited a total of twelve times after the server was last started but prior to the visit shown in this figure.

graphics/20fig13.gif
 

Listing 20.22 AccessCountBean.java
package cwp; /** Simple bean to illustrate sharing beans through  *  use of the scope attribute of jsp:useBean.  */ public class AccessCountBean {   private String firstPage;   private int accessCount = 1;   public String getFirstPage() {     return(firstPage);   }   public void setFirstPage(String firstPage) {     this.firstPage = firstPage;   }   public int getAccessCount() {     return(accessCount++);   } } 
Listing 20.23 SharedCounts1.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Shared Access Counts: Page 1</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <TABLE BORDER=5 ALIGN="CENTER">   <TR><TH CLASS="TITLE">       Shared Access Counts: Page 1</TABLE> <P> <jsp:useBean id="counter"              class="cwp.AccessCountBean"              scope="application">   <jsp:setProperty id="counter"                    property="firstPage"                    value="SharedCounts1.jsp" /> </jsp:useBean> Of SharedCounts1.jsp (this page), <A HREF="SharedCounts2.jsp">SharedCounts2.jsp</A>, and <A HREF="SharedCounts3.jsp">SharedCounts3.jsp</A>, <jsp:getProperty id="counter" property="firstPage" /> was the first page accessed. <P> Collectively, the three pages have been accessed <jsp:getProperty id="counter" property="accessCount" /> times. </BODY> </HTML> 

20.7 Defining Custom JSP Tags

JSP 1.1 introduced an extremely valuable new capability: the ability to create your own JSP tags. You define how a tag, its attributes, and its body are interpreted, then group your tags into collections called tag libraries that can be used in any number of JSP files. The ability to define tag libraries in this way permits Java developers to boil down complex server-side behaviors into simple and easy-to-use elements that content developers can easily incorporate into their JSP pages.

Custom tags accomplish some of the same goals as beans that are accessed with jsp:useBean (see Section 20.6) encapsulating complex behaviors into simple and accessible forms. There are several differences, however. First, beans cannot manipulate JSP content; custom tags can. Second, complex operations can be reduced to a significantly simpler form with custom tags than with beans. Third, custom tags require quite a bit more work to set up than do beans. Fourth, beans are often defined in one servlet and then used in a different servlet or JSP page (see the following section on integrating servlets and JSP), whereas custom tags usually define more self-contained behavior. Finally, custom tags are available only in JSP 1.1, but beans can be used in both JSP 1.0 and 1.1.

The Components That Make Up a Tag Library

To use custom JSP tags, you need to define three separate components: the tag handler class that defines the tag's behavior, the tag library descriptor file that maps the XML element names to the tag implementations, and the JSP file that uses the tag library. The rest of this subsection gives an overview of each of these components, and the following subsections give details on how to build these components for various styles of tags.

The Tag Handler Class

When defining a new tag, your first task is to define a Java class that tells the system what to do when it sees the tag. This class must implement the javax.servlet.jsp.tagext.Tag interface. This is usually accomplished by extending the TagSupport or BodyTagSupport class. Listing 20.24 is an example of a simple tag that just inserts "Custom tag example (cwp.tags.ExampleTag)" into the JSP page wherever the corresponding tag is used. Don't worry about understanding the exact behavior of this class; that will be made clear in the next subsection. For now, just note that it is in the cwp.tags class and is called ExampleTag. Thus, with Tomcat 3, the class file would be in install_dir/webapps/ROOT/WEB-INF/ classes/cwp/tags/ExampleTag.class.

Listing 20.24 ExampleTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; /** Very simple JSP tag that just inserts a string  *  ("Custom tag example...") into the output.  *  The actual name of the tag is not defined here;  *  that is given by the Tag Library Descriptor (TLD)  *  file that is referenced by the taglib directive  *  in the JSP file.  */ public class ExampleTag extends TagSupport {   public int doStartTag() {     try {       JspWriter out = pageContext.getOut();       out.print("Custom tag example " +                 "(cwp.tags.ExampleTag)");     } catch(IOException ioe) {       System.out.println("Error in ExampleTag: " + ioe);     }     return(SKIP_BODY);   } } 
The Tag Library Descriptor File

Once you have defined a tag handler, your next task is to identify the class to the server and to associate it with a particular XML tag name. This task is accomplished by means of a tag library descriptor file (in XML format) like the one shown in Listing 20.25. This file contains some fixed information, an arbitrary short name for your library, a short description, and a series of tag descriptions. The nonbold part of the listing is the same in virtually all tag library descriptors and can be copied verbatim from the source code archive at http://www.corewebprogramming.com/ or from the Tomcat 3 standard examples (install_dir/webapps/examples/ WEB-INF/jsp).

The format of tag descriptions is described in later sections. For now, just note that the tag element defines the main name of the tag (really tag suffix, as will be seen shortly) and identifies the class that handles the tag. Since the tag handler class is in the cwp.tags package, the fully qualified class name of cwp.tags.ExampleTag is used. Note that this is a class name, not a URL or relative path name. The class can be installed anywhere on the server that beans or other supporting classes can be put. With Tomcat 3, the standard base location is install_dir/webapps/ROOT/WEB-INF/classes, so ExampleTag would be in install_dir/webapps/ROOT/WEB-INF/classes/cwp/tags. Although it is always a good idea to put your servlet classes in packages, a surprising feature of Tomcat 3.1 is that tag handlers are required to be in packages.

Listing 20.25 cwp-taglib.tld
<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE taglib  PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"  "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd"> <taglib>   <tlibversion>1.0</tlibversion>   <jspversion>1.1</jspversion>   <shortname>cwp</shortname>   <info>     A tag library from Core Web Programming Java 2 Edition,     http://www.corewebprogramming.com/.   </info>   <tag>     <name>example</name>     <tagclass>cwp.tags.ExampleTag</tagclass>     <info>Simplest example: inserts one line of output</info>   </tag> </taglib> 
The JSP File

Once you have a tag handler implementation and a tag library description, you are ready to write a JSP file that makes use of the tag. Listing 20.26 gives an example. Somewhere before the first use of your tag, you need to use the taglib directive. This directive has the following form:

<%@ taglib uri="..." prefix="..." %> 

The required uri attribute can be either an absolute or relative URL referring to a tag library descriptor file like the one shown in Listing 20.25.

The prefix attribute, also required, specifies a prefix that will be used in front of whatever tag name the tag library descriptor defined. For example, if the TLD file defines a tag named tag1 and the prefix attribute has a value of test, the actual tag name would be test:tag1. This tag could be used in either of the following two ways, depending on whether it is defined to be a container that makes use of the tag body:

<test:tag1>Arbitrary JSP</test:tag1> 

or just

<test:tag1 /> 

To illustrate, the descriptor file of Listing 20.25 is called cwp-taglib.tld, and resides in the same directory as the JSP file shown in Listing 20.26. Thus, the taglib directive in the JSP file uses a simple relative URL giving just the filename, as shown below.

<%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> 

Furthermore, since the prefix attribute is cwp (for Core Web Programming), the rest of the JSP page uses cwp:example to refer to the example tag defined in the descriptor file. Figure 20-14 shows the result.

Figure 20-14. Result of SimpleExample.jsp.

graphics/20fig14.gif
 

Listing 20.26 SimpleExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <TITLE><cwp:example /></TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1><cwp:example /></H1> <cwp:example /> </BODY> </HTML> 

Defining a Basic Tag

This subsection gives details on defining simple tags without attributes or tag bodies; the tags are thus of the form <prefix:tagname />.

A Basic Tag: Tag Handler Class

Tags that either have no body or that merely include the body verbatim should extend the TagSupport class. This is a built-in class in the javax.servlet.jsp.tagext package that implements the Tag interface and contains much of the standard functionality basic tags need. Because of other classes you will use, your tag should normally import classes in the javax.servlet.jsp and java.io packages as well. So, most tag implementations contain the following import statements after the package declaration:

import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; 

We recommend that you download an example from http://www.corewebprogramming.com/ and use it as the starting point for your own implementations.

For a tag without attributes or body, all you need to do is override the doStartTag method, which defines code that gets called at request time where the element's start tag is found. To generate output, the method should obtain the JspWriter (the specialized PrintWriter available in JSP pages through use of the predefined out variable) from the pageContext field by means of getOut. In addition to the getOut method, the pageContext field (of type PageContext) has methods for obtaining other data structures associated with the request. The most important ones are getRequest, getResponse, getServletContext, and getSession.

Since the print method of JspWriter throws IOException, the print statements should be inside a try/catch block. To report other types of errors to the client, you can declare that your doStartTag method throws a JspException and then throw one when the error occurs.

If your tag does not have a body, your doStartTag should return the SKIP_BODY constant. This instructs the system to ignore any content between the tag's start and end tags. As we will see shortly, SKIP_BODY is sometimes useful even when there is a tag body (e.g., if you sometimes include it and other times omit it), but the simple tag we're developing here will be used as a stand-alone tag (<prefix:tagname />) and thus does not have body content.

Listing 20.27 shows a tag implementation that uses this approach to generate a random 50-digit prime through use of the Primes class shown in the previous chapter.

Listing 20.27 SimplePrimeTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import java.math.*; import cwp.*; /** Generates a prime of approximately 50 digits. */ public class SimplePrimeTag extends TagSupport {   protected int len = 50;   public int doStartTag() {     try {       JspWriter out = pageContext.getOut();       BigInteger prime = Primes.nextPrime(Primes.random(len));       out.print(prime);     } catch(IOException ioe) {       System.out.println("Error generating prime: " + ioe);     }     return(SKIP_BODY);   } } 
A Basic Tag: Tag Library Descriptor File

The general format of a descriptor file is almost always the same: it should contain an XML version identifier followed by a DOCTYPE declaration followed by a taglib container element. To get started, just download a sample from http://www.corewebprogramming.com/. The important part to understand is what goes in the taglib element: the tag element. For tags without attributes, the tag element should contain four elements between <tag> and </tag>:

  1. name, whose body defines the base tag name to which the prefix of the taglib directive will be attached. In this case, we use

    <name>simplePrime</name> 

    to assign a base tag name of simplePrime.

  2. tagclass, which gives the fully qualified class name of the tag handler. In this case, we use

    <tagclass>cwp.tags.SimplePrimeTag</tagclass> 
  3. info, which gives a short description. Here, we use

    <info>Outputs a random 50-digit prime.</info> 
  4. bodycontent, which can be omitted, but if present should have the value empty for tags without bodies. Tags with normal bodies that might be interpreted as normal JSP use a value of JSP, and the rare tags whose handlers completely process the body themselves use a value of tagdependent. For the SimplePrimeTag discussed here, we would use empty as below:

    <bodycontent>empty</bodycontent> 

    Unfortunately, however, Tomcat 3.1 does not support the bodycontent element, and TLD files that contain it will not work. Tomcat 3.2, JRun, and most other servers support bodycontent properly.

Core Warning

graphics/corenote.gif
 

Do not use the bodycontent element with Tomcat 3.1.

Listing 20.28 shows the relevant part of the TLD file.

Listing 20.28 cwp-taglib.tld (Excerpt)
 <tag>     <name>simplePrime</name>     <tagclass>cwp.tags.SimplePrimeTag</tagclass>     <info>Outputs a random 50-digit prime.</info>   </tag> 
A Basic Tag: JSP File

JSP documents that make use of custom tags need to use the taglib directive, supplying a uri attribute that gives the location of the tag library descriptor file and a prefix attribute that specifies a short string that will be attached (along with a colon) to the main tag name. Listing 20.29 shows a JSP document that uses

<%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> 

to use the TLD file just shown in Listing 20.28 with a prefix of cwp. Since the base tag name is simplePrime, the full tag used is

<cwp:simplePrime /> 

Figure 20-15 shows the result.

Figure 20-15. Result of SimplePrimeExample.jsp.

graphics/20fig15.gif
 

Listing 20.29 SimplePrimeExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Some 50-Digit Primes</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>Some 50-Digit Primes</H1> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <UL>   <LI><cwp:simplePrime />   <LI><cwp:simplePrime />   <LI><cwp:simplePrime />   <LI><cwp:simplePrime /> </UL> </BODY> </HTML> 

Assigning Attributes to Tags

Allowing tags like

<prefix:name attribute1="value1" attribute2="value2" ... /> 

adds significant flexibility to your tag library. This subsection explains how to add attribute support to your tags.

Tag Attributes: Tag Handler Class

Providing support for attributes is straightforward. Use of an attribute called attribute1 simply results in a call to a method called setAttribute1 in your class that extends TagSupport (or that otherwise implements the Tag interface). The attribute value is supplied to the method as a String. Consequently, adding support for an attribute named attribute1 is merely a matter of implementing the following method:

public void setAttribute1(String value1) {   doSomethingWith(value1); } 

Note that an attribute of attributeName (lowercase a) corresponds to a method called setAttributeName (uppercase A).

One of the most common things to do in the attribute handler is to simply store the attribute in a field that will later be used by doStartTag or a similar method. For example, the following is a section of a tag implementation that adds support for the message attribute.

private String message = "Default Message"; public void setMessage(String message) {   this.message = message; } 

If the tag handler will be accessed from other classes, it is a good idea to provide a getAttributeName method in addition to the setAttributeName method. Only setAttributeName is required, however.

Listing 20.30 shows a subclass of SimplePrimeTag that adds support for the length attribute. When such an attribute is supplied, it results in a call to setLength, which converts the input String to an int and stores it in the len field already used by the doStartTag method in the parent class.

Listing 20.30 PrimeTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; /** Generates an N-digit random prime (default N = 50).  *  Extends SimplePrimeTag, adding a length attribute  *  to set the size of the prime. The doStartTag  *  method of the parent class uses the len field  *  to determine the approximate length of the prime.  */ public class PrimeTag extends SimplePrimeTag {   public void setLength(String length) {     try {       len = Integer.parseInt(length);     } catch(NumberFormatException nfe) {       len = 50;     }   } } 
Tag Attributes: Tag Library Descriptor File

Tag attributes must be declared inside the tag element by means of an attribute element. The attribute element has three nested elements that can appear between <attribute> and </attribute>.

  1. name, a required element that defines the case-sensitive attribute name. In this case, we use <name>length</name>

  2. required, a required element that stipulates whether the attribute must always be supplied (true) or is optional (false). In this case, to indicate that length is optional, we use <required>false</required> If you omit the attribute, no call is made to the setAttributeName method. So, be sure to give default values to the fields that the method sets.

  3. rtexprvalue, an optional attribute that indicates whether the attribute value can be a JSP expression like <%= expression %> (true) or whether it must be a fixed string (false). The default value is false, so this element is usually omitted except when you want to allow attributes to have values determined at request time.

Listing 20.31 shows the relevant tag element within the tag library descriptor file. In addition to supplying an attribute element to describe the length attribute, the tag element also contains the standard name (prime), tagclass (cwp.tags.PrimeTag), and info (short description) elements.

Listing 20.31 cwp-taglib.tld (Excerpt)
<tag>   <name>prime</name>   <tagclass>cwp.tags.PrimeTag</tagclass>   <info>Outputs a random N-digit prime.</info>   <attribute>     <name>length</name>     <required>false</required>   </attribute> </tag> 
Tag Attributes: JSP File

Listing 20.32 shows a JSP document that uses the taglib directive to load the tag library descriptor file and to specify a prefix of cwp. Since the prime tag is defined to permit a length attribute, Listing 20.32 uses

<cwp:prime length="xxx" /> 

Remember that custom tags follow XML syntax, which requires attribute values to be enclosed in either single or double quotes. Also, since the length attribute is not required, it is permissible to just use

<cwp:prime /> 

The tag handler is responsible for using a reasonable default value in such a case. Figure 20-16 shows the result of Listing 20.32.

Figure 20-16. Result of PrimeExample.jsp.

graphics/20fig16.gif
 

Listing 20.32 PrimeExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Some N-Digit Primes</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>Some N-Digit Primes</H1> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <UL>   <LI>20-digit: <cwp:prime length="20" />   <LI>40-digit: <cwp:prime length="40" />   <LI>80-digit: <cwp:prime length="80" />   <LI>Default (50-digit): <cwp:prime /> </UL> </BODY> </HTML> 

Including the Tag Body

Up to this point, all of the custom tags you have seen ignore the tag body and thus are used as stand-alone tags of the form

<prefix:tagname /> 

In this section, we see how to define tags that use their body content and are thus used in the following manner:

<prefix:tagname>body</prefix:tagname> 
Tag Bodies: Tag Handler Class

In the previous examples, the tag handlers defined a doStartTag method that returned SKIP_BODY. To instruct the system to make use of the body that occurs between the new element's start and end tags, your doStartTag method should return EVAL_BODY_INCLUDE instead. The body content can contain JSP scripting elements, directives, and actions, just like the rest of the page. The JSP constructs are translated into servlet code at page translation time, and that code is invoked at request time.

If you make use of a tag body, then you might want to take some action after the body as well as before it. Use the doEndTag method to specify this action. In almost all cases, you want to continue with the rest of the page after finishing with your tag, so the doEndTag method should return EVAL_PAGE. If you want to abort the processing of the rest of the page, you can return SKIP_PAGE instead.

Listing 20.33 defines a tag for a heading element that is more flexible than the standard HTML H1 through H6 elements. This new element allows a precise font size, a list of preferred font names (the first entry that is available on the client system will be used), a foreground color, a background color, a border, and an alignment (LEFT, CENTER, RIGHT). Only the alignment capability is available with the H1 through H6 elements. The heading is implemented through use of a one-cell table enclosing a SPAN element that has embedded style sheet attributes. The doStartTag method generates the TABLE and SPAN start tags, then returns EVAL_BODY_INCLUDE to instruct the system to include the tag body. The doEndTag method generates the </SPAN> and </TABLE> tags, then returns EVAL_PAGE to continue with normal page processing. Various setAttributeName methods are used to handle the attributes like bgColor and fontSize.

Listing 20.33 HeadingTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; /** Generates an HTML heading with the specified background  *  color, foreground color, alignment, font, and font size.  *  You can also turn on a border around it, which normally  *  just barely encloses the heading, but which can also  *  stretch wider. All attributes except the background  *  color are optional.  */ public class HeadingTag extends TagSupport {   private String bgColor; // The one required attribute   private String color = null;   private String align="CENTER";   private String fontSize="36";   private String fontList="Arial, Helvetica, sans-serif";   private String border="0";   private String width=null;   public void setBgColor(String bgColor) {     this.bgColor = bgColor;   }   public void setColor(String color) {     this.color = color;   }   public void setAlign(String align) {     this.align = align;   }     public void setFontSize(String fontSize) {     this.fontSize = fontSize;   }   public void setFontList(String fontList) {     this.fontList = fontList;   }   public void setBorder(String border) {     this.border = border;   }   public void setWidth(String width) {     this.width = width;   }   public int doStartTag() {     try {       JspWriter out = pageContext.getOut();       out.print("<TABLE BORDER=" + border +                 " BGCOLOR=\"" + bgColor + "\"" +                 " ALIGN=\"" + align + "\"");       if (width != null) {         out.print(" WIDTH=\"" + width + "\"");       }       out.print("><TR><TH>");       out.print("<SPAN STYLE=\"" +                 "font-size: " + fontSize + "px; " +                 "font-family: " + fontList + "; ");       if (color != null) {         out.println("color: " + color + ";");       }       out.print("\"> "); // End of <SPAN ...>     } catch(IOException ioe) {       System.out.println("Error in HeadingTag: " + ioe);     }     return(EVAL_BODY_INCLUDE); // Include tag body   }   public int doEndTag() {     try {       JspWriter out = pageContext.getOut();       out.print("</SPAN></TABLE>");     } catch(IOException ioe) {       System.out.println("Error in HeadingTag: " + ioe);     }     return(EVAL_PAGE); // Continue with rest of JSP page   } } 
Tag Bodies: Tag Library Descriptor File

There is only one new feature in the use of the tag element for tags that use body content: the bodycontent element should contain the value JSP as below.

<bodycontent>JSP</bodycontent> 

Remember, however, that Tomcat 3.1 does not support bodycontent, and since bodycontent is intended primarily as a hint to development environments, we will omit it in our examples. The name, tagclass, info, and attribute elements are used in the same manner as described previously. Listing 20.34 gives the relevant part of the code.

Listing 20.34 cwp-taglib.tld (Excerpt)
<tag>   <name>heading</name>   <tagclass>cwp.tags.HeadingTag</tagclass>   <info>Outputs a 1-cell table used as a heading.</info>   <attribute>     <name>bgColor</name>     <required>true</required>  <!-- bgColor is required -->   </attribute>   <attribute>     <name>color</name>     <required>false</required>   </attribute>   <attribute>     <name>align</name>     <required>false</required>   </attribute>   <attribute>     <name>fontSize</name>     <required>false</required>   </attribute>   <attribute>     <name>fontList</name>     <required>false</required>   </attribute>   <attribute>     <name>border</name>     <required>false</required>   </attribute>   <attribute>     <name>width</name>     <required>false</required>   </attribute> </tag> 
Tag Bodies: JSP File

Listing 20.35 shows a document that uses the heading tag just defined. Since the bgColor attribute was defined to be required, all uses of the tag include it. Figure 20-17 shows the result.

Figure 20-17. The custom cwp:heading element gives you much more control over heading format than do the standard H1 through H6 elements in HTML.

graphics/20fig17.gif
 

Listing 20.35 HeadingExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Some Tag-Generated Headings</TITLE> </HEAD> <BODY> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <cwp:heading bgColor="#C0C0C0"> Default Heading </cwp:heading> <P> <cwp:heading bgColor="BLACK" color="WHITE"> White on Black Heading </cwp:heading> <P> <cwp:heading bgColor="#EF8429" fontSize="60" border="5"> Large Bordered Heading </cwp:heading> <P> <cwp:heading bgColor="CYAN" width="100%"> Heading with Full-Width Background </cwp:heading> <P> <cwp:heading bgColor="CYAN" fontSize="60"                 fontList="Brush Script MT, Times, serif"> Heading with Non-Standard Font </cwp:heading> </BODY> </HTML> 

Optionally Including the Tag Body

Most tags either never make use of body content or always do so. This section shows you how to use request time information to decide whether or not to include the tag body. Although the body can contain JSP that is interpreted at page translation time, the result of that translation is servlet code that can be invoked or ignored at request time.

Optional Body Inclusion: Tag Handler Class

Optionally including the tag body is a trivial exercise: just return EVAL_BODY_INCLUDE or SKIP_BODY depending on the value of some request time expression. The important thing to know is how to discover that request time information, since doStartTag does not have HttpServletRequest and HttpServletResponse arguments as do service, _jspService, doGet, and doPost. The solution to this dilemma is to use getRequest to obtain the HttpServletRequest from the automatically defined pageContext field of TagSupport. Strictly speaking, the return type of getRequest is ServletRequest, so you have to do a typecast to HttpServletRequest if you want to call a method that is not inherited from ServletRequest. However, in this case we just use getParameter, so no typecast is required.

Listing 20.36 defines a tag that ignores its body unless a request time debug parameter is supplied. Such a tag provides a useful capability whereby you embed debugging information directly in the JSP page during development but activate it only when a problem occurs.

Listing 20.36 DebugTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import javax.servlet.*; /** A tag that includes the body content only if  *  the "debug" request parameter is set.  */ public class DebugTag extends TagSupport {   public int doStartTag() {     ServletRequest request = pageContext.getRequest();     String debugFlag = request.getParameter("debug");     if ((debugFlag != null) &&         (!debugFlag.equalsIgnoreCase("false"))) {       return(EVAL_BODY_INCLUDE);     } else {       return(SKIP_BODY);     }   } } 
Optional Body Inclusion: Tag Library Descriptor File

If your tag ever makes use of its body, you should provide the value JSP inside the bodycontent element (if you use bodycontent at all). Other than that, all the elements within tag are used in the same way as described previously. Listing 20.37 shows the entries needed for DebugTag.

Listing 20.37 cwp-taglib.tld (Excerpt)
<tag>   <name>debug</name>   <tagclass>cwp.tags.DebugTag</tagclass>   <info>Includes body only if debug param is set.</info> </tag> 
Optional Body Inclusion: JSP File

Listing 20.38 shows a page that encloses debugging information between <cwp:debug> and </cwp:debug>. Figures20-18 and 20-19 show the normal result and the result when a request time debug parameter is supplied, respectively.

Figure 20-18. The body of the cwp:debug element is normally ignored.

graphics/20fig18.gif
 

Figure 20-19. The body of the cwp:debug element is included when a debug request parameter is supplied.

graphics/20fig19.gif
 

Listing 20.38 DebugExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Using the Debug Tag</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>Using the Debug Tag</H1> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> Top of regular page. Blah, blah, blah. Yadda, yadda, yadda. <P> <cwp:debug> <B>Debug:</B> <UL>   <LI>Current time: <%= new java.util.Date() %>   <LI>Requesting hostname: <%= request.getRemoteHost() %>   <LI>Session ID: <%= session.getId() %> </UL> </cwp:debug> <P> Bottom of regular page. Blah, blah, blah. Yadda, yadda, yadda. </BODY> </HTML> 

Manipulating the Tag Body

The cwp:prime element ignored any body content, the cwp:heading element used body content, and the cwp:debug element ignored or used it depending on a request time parameter. The common thread among these elements is that the tag body was never modified; it was either ignored or included verbatim (after JSP translation). This section shows you how to process the tag body.

Manipulating Body: Tag Handler Class

Up to this point, all of the tag handlers have extended the TagSupport class. This is a good standard starting point, as it implements the required Tag interface and performs a number of useful setup operations like storing the PageContext reference in the pageContext field. However, TagSupport is not powerful enough for tag implementations that need to manipulate their body content, and BodyTagSupport should be used instead.

BodyTagSupport extends TagSupport, so the doStartTag and doEndTag methods are used in the same way as before. The two important new methods defined by BodyTagSupport are:

  1. doAfterBody, a method that you should override to handle the manipulation of the tag body. This method should normally return SKIP_BODY when it is done, indicating that no further body processing should be performed.

  2. getBodyContent, a method that returns an object of type BodyContent that encapsulates information about the tag body.

The BodyContent class has three important methods:

  1. getEnclosingWriter, a method that returns the JspWriter being used by doStartTag and doEndTag.

  2. getReader, a method that returns a Reader that can read the tag's body.

  3. getString, a method that returns a String containing the entire tag body.

The ServletUtilities class (see Listing 19.11) contains a static filter method that takes a string and replaces <, >, ", and & with &<, &gt;, &", and &&, respectively. This method is useful when servlets output strings that might contain characters that would interfere with the HTML structure of the page in which the strings are embedded. Listing 20.39 shows a tag implementation that gives this filtering functionality to a custom JSP tag.

Listing 20.39 FilterTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import cwp.*; /** A tag that replaces <, >, ", and & with their HTML  *  character entities (&lt;, &gt;, &quot;, and &amp;).  *  After filtering, arbitrary strings can be placed  *  in either the page body or in HTML attributes.  */ public class FilterTag extends BodyTagSupport {   public int doAfterBody() {     BodyContent body = getBodyContent();     String filteredBody =       ServletUtilities.filter(body.getString());     try {       JspWriter out = body.getEnclosingWriter();       out.print(filteredBody);     } catch(IOException ioe) {       System.out.println("Error in FilterTag: " + ioe);     }     // SKIP_BODY means we're done. If we wanted to evaluate     // and handle the body again, we'd return EVAL_BODY_TAG.     return(SKIP_BODY);   } } 
Manipulating Body: Tag Library Descriptor File

Tags that manipulate their body content should use the bodycontent element the same way as tags that simply include it verbatim; they should supply a value of JSP. Again, however, remember that Tomcat 3.1 does not properly support bodycontent, and omitting bodycontent is perfectly acceptable on any server. Other than that, nothing new is required in the descriptor file, as you can see by examining Listing 20.40, which shows the relevant portion of the TLD file.

Listing 20.40 cwp-taglib.tld (Excerpt)
<tag>   <name>filter</name>   <tagclass>cwp.tags.FilterTag</tagclass>   <info>Replaces HTML-specific characters in body.</info> </tag> 
Manipulating Body: JSP File

Listing 20.41 shows a page that uses a table to show some sample HTML and its result. Creating this table would be tedious in regular HTML since the table cell that shows the original HTML would have to change all the < and > characters to &lt; and &gt;. Doing so is particularly onerous during development when the sample HTML is frequently changing. Use of the <cwp:filter> tag greatly simplifies the process, as Listing 20.41 illustrates. Figure 20-20 shows the result.

Figure 20-20. The cwp:filter element lets you insert text without worrying about it containing special HTML characters.

graphics/20fig20.gif
 

Listing 20.41 FilterExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>HTML Logical Character Styles</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>HTML Logical Character Styles</H1> Physical character styles (B, I, etc.) are rendered consistently in different browsers. Logical character styles, however, may be rendered differently by different browsers. Here's how your browser (<%= request.getHeader("User-Agent") %>) renders the HTML 4.0 logical character styles: <P> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <TABLE BORDER=1 ALIGN="CENTER"> <TR CLASS="COLORED"><TH>Example<TH>Result <TR> <TD><PRE><cwp:filter> <EM>Some emphasized text.</EM><BR> <STRONG>Some strongly emphasized text.</STRONG><BR> <CODE>Some code.</CODE><BR> <SAMP>Some sample text.</SAMP><BR> <KBD>Some keyboard text.</KBD><BR> <DFN>A term being defined.</DFN><BR> <VAR>A variable.</VAR><BR> <CITE>A citation or reference.</CITE> </cwp:filter></PRE> <TD> <EM>Some emphasized text.</EM><BR> <STRONG>Some strongly emphasized text.</STRONG><BR> <CODE>Some code.</CODE><BR> <SAMP>Some sample text.</SAMP><BR> <KBD>Some keyboard text.</KBD><BR> <DFN>A term being defined.</DFN><BR> <VAR>A variable.</VAR><BR> <CITE>A citation or reference.</CITE> </TABLE> </BODY> </HTML> 

Including or Manipulating the Tag Body Multiple Times

Rather than just including or processing the body of the tag a single time, you sometimes want to do so more than once. The ability to support multiple body inclusion lets you define a variety of iteration tags that repeat JSP fragments a variable number of times, repeat them until a certain condition occurs, and so forth. This subsection shows you how to build such tags.

The Tag Handler Class

Tags that process the body content multiple times should start by extending BodyTagSupport and implementing doStartTag, doEndTag, and, most importantly, doAfterBody as before. The difference lies in the return value of doAfterBody. If this method returns EVAL_BODY_TAG, the tag body is evaluated again, resulting in a new call to doAfterBody. This process continues until doAfterBody returns SKIP_BODY.

Listing 20.42 defines a tag that repeats the body content the number of times specified by the reps attribute. Since the body content can contain JSP (which is converted into servlet code at page translation time but is invoked at request time), each repetition does not necessarily result in the same output to the client.

Listing 20.42 RepeatTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; /** A tag that repeats the body the specified  *  number of times.  */ public class RepeatTag extends BodyTagSupport {   private int reps;   public void setReps(String repeats) {     try {       reps = Integer.parseInt(repeats);     } catch(NumberFormatException nfe) {       reps = 1;     }   }   public int doAfterBody() {     if (reps-- >= 1) {       BodyContent body = getBodyContent();       try {         JspWriter out = body.getEnclosingWriter();         out.println(body.getString());         body.clearBody(); // Clear for next evaluation       } catch(IOException ioe) {         System.out.println("Error in RepeatTag: " + ioe);       }       return(EVAL_BODY_TAG);     } else {       return(SKIP_BODY);     }   } } 
The Tag Library Descriptor File

Listing 20.43 shows the relevant section of the TLD file that gives the name cwp:repeat to the tag just defined. To accommodate request time values in the reps attribute, the file uses an rtexprvalue element (enclosing a value of true) within the attribute element.

Listing 20.43 cwp-taglib.tld (Excerpt)
<tag>   <name>repeat</name>   <tagclass>cwp.tags.RepeatTag</tagclass>   <info>Repeats body the specified number of times.</info>   <attribute>     <name>reps</name>     <required>true</required>     <!-- rtexprvalue indicates whether attribute          can be a JSP expression. -->     <rtexprvalue>true</rtexprvalue>   </attribute> </tag> 
The JSP File

Listing 20.44 shows a JSP document that creates a numbered list of prime numbers. The number of primes in the list is taken from the request time repeats parameter. Figure 20-21 shows one possible result.

Figure 20-21. Result of RepeatExample.jsp when accessed with a repeats parameter of 20.

graphics/20fig21.gif
 

Listing 20.44 RepeatExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>Some 40-Digit Primes</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>Some 40-Digit Primes</H1> Each entry in the following list is the first prime number higher than a randomly selected 40-digit number. <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <OL> <!-- Repeats N times. A null reps value means repeat once. --> <cwp:repeat reps='<%= request.getParameter("repeats") %>'>   <LI><cwp:prime length="40" /> </cwp:repeat> </OL> </BODY> </HTML> 

Using Nested Tags

Although Listing 20.44 places the cwp:prime element within the cwp:repeat element, the two elements are independent of each other. The first generates a prime number regardless of where it is used, and the second repeats the enclosed content regardless of whether that content uses a cwp:prime element.

Some tags, however, depend on a particular nesting. For example, in standard HTML, the TD and TH elements can only appear within TR, which in turn can only appear within TABLE. The color and alignment settings of TABLE are inherited by TR, and the values of TR affect how TD and TH behave. So, the nested elements cannot act in isolation even when nested properly. Similarly, the tag library descriptor file makes use of a number of elements like taglib, tag, attribute, and required where a strict nesting hierarchy is imposed.

This subsection shows you how to define tags that depend on a particular nesting order and where the behavior of certain tags depends on values supplied by earlier ones.

The Tag Handler Classes

Class definitions for nested tags can extend either TagSupport or BodyTagSupport, depending on whether they need to manipulate their body content (these extend BodyTagSupport) or, more commonly, just ignore it or include it verbatim (these extend TagSupport).

There are two key new approaches for nested tags, however. First, nested tags can use findAncestorWithClass to find the tag in which they are nested. This method takes a reference to the current class (e.g., this) and the Class object of the enclosing class (e.g., EnclosingTag.class) as arguments. If no enclosing class is found, the method in the nested class can throw a JspTagException that reports the problem. Second, if one tag wants to store data that a later tag will use, it can place that data in the instance of the enclosing tag. The definition of the enclosing tag should provide methods for storing and accessing this data.

Suppose that we want to define a set of tags that would be used like this:

<cwp:if>   <cwp:condition><%= someExpression %></cwp:condition>   <cwp:then>JSP to include if condition is true</cwp:then>   <cwp:else>JSP to include if condition is false</cwp:else> </cwp:if> 

To accomplish this task, the first step is to define an IfTag class to handle the cwp:if tag. This handler should have methods to specify and check whether the condition is true or false (setCondition and getCondition) as well as methods to designate and check whether the condition has ever been explicitly set (setHasCondition and getHasCondition), since we want to disallow cwp:if tags that contain no cwp:condition entry. Listing 20.45 shows the code for IfTag.

The second step is to define a tag handler for cwp:condition. This class, called IfConditionTag, defines a doStartTag method that merely checks whether the tag appears within IfTag. It returns EVAL_BODY_TAG if so and throws an exception if not. The handler's doAfterBody method looks up the body content (getBodyContent), converts it to a String (getString), and compares that to "true". This approach means that an explicit value of true can be substituted for a JSP expression like <%= expression %> if, during initial page development, you want to temporarily designate that the then portion should always be used. Using a comparison to "true" also means that any other value will be considered false. Once this comparison is performed, the result is stored in the enclosing tag by means of the setCondition method of IfTag. The code for IfConditionTag is shown in Listing 20.46.

The third step is to define a class to handle the cwp:then tag. The doStartTag method of this class verifies that it is inside IfTag and also checks that an explicit condition has been set (i.e., that the IfConditionTag has already appeared within the IfTag). The doAfterBody method checks for the condition in the IfTag class, and, if it is true, looks up the body content and prints it. Listing 20.47 shows the code.

The final step in defining tag handlers is to define a class for cwp:else. This class is very similar to the one to handle the then part of the tag, except that this handler only prints the tag body from doAfterBody if the condition from the surrounding IfTag is false. The code is shown in Listing 20.48.

Listing 20.45 IfTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import javax.servlet.*; /** A tag that acts like an if/then/else. */ public class IfTag extends TagSupport {   private boolean condition;   private boolean hasCondition = false;   public void setCondition(boolean condition) {     this.condition = condition;     hasCondition = true;   }   public boolean getCondition() {     return(condition);   }   public void setHasCondition(boolean flag) {     this.hasCondition = flag;   }   /** Has the condition field been explicitly set? */   public boolean hasCondition() {     return(hasCondition);   }   public int doStartTag() {     return(EVAL_BODY_INCLUDE);   } } 
Listing 20.46 IfConditionTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import javax.servlet.*; /** The condition part of an if tag. */ public class IfConditionTag extends BodyTagSupport {   public int doStartTag() throws JspTagException {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     if (parent == null) {       throw new JspTagException("condition not inside if");     }     return(EVAL_BODY_TAG);   }   public int doAfterBody() {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     String bodyString = getBodyContent().getString();     if (bodyString.trim().equals("true")) {       parent.setCondition(true);     } else {       parent.setCondition(false);     }     return(SKIP_BODY);   } } 
Listing 20.47 IfThenTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import javax.servlet.*; /** The then part of an if tag. */ public class IfThenTag extends BodyTagSupport {   public int doStartTag() throws JspTagException {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     if (parent == null) {       throw new JspTagException("then not inside if");     } else if (!parent.hasCondition()) {       String warning =         "condition tag must come before then tag";       throw new JspTagException(warning);     }     return(EVAL_BODY_TAG);   }   public int doAfterBody() {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     if (parent.getCondition()) {       try {         BodyContent body = getBodyContent();         JspWriter out = body.getEnclosingWriter();         out.print(body.getString());       } catch(IOException ioe) {         System.out.println("Error in IfThenTag: " + ioe);       }     }     return(SKIP_BODY);   } } 
Listing 20.48 IfElseTag.java
package cwp.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; import java.io.*; import javax.servlet.*; /** The else part of an if tag. */ public class IfElseTag extends BodyTagSupport {   public int doStartTag() throws JspTagException {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     if (parent == null) {       throw new JspTagException("else not inside if");     } else if (!parent.hasCondition()) {       String warning =         "condition tag must come before else tag";       throw new JspTagException(warning);     }     return(EVAL_BODY_TAG);   }   public int doAfterBody() {     IfTag parent =       (IfTag)findAncestorWithClass(this, IfTag.class);     if (!parent.getCondition()) {       try {         BodyContent body = getBodyContent();         JspWriter out = body.getEnclosingWriter();         out.print(body.getString());       } catch(IOException ioe) {         System.out.println("Error in IfElseTag: " + ioe);       }     }     return(SKIP_BODY);   } } 
The Tag Library Descriptor File

Even though there is an explicit required nesting structure for the tags just defined, the tags must be declared separately in the TLD file. This means that nesting validation is performed only at request time, not at page translation time. In principle, you could instruct the system to do some validation at page translation time by using a TagExtraInfo class. This class has a getVariableInfo method that you can use to check that attributes exist and where they are used. Once you have defined a subclass of TagExtraInfo, you associate it with your tag in the tag library descriptor file by means of the teiclass element, which is used just like tagclass. In practice, however, TagExtraInfo is a bit cumbersome to use.

Listing 20.49 cwp-taglib.tld (Excerpt)
<tag>   <name>if</name>   <tagclass>cwp.tags.IfTag</tagclass>   <info>if/condition/then/else tag.</info> </tag> <tag>   <name>condition</name>   <tagclass>cwp.tags.IfConditionTag</tagclass>   <info>condition part of if/condition/then/else tag.</info> </tag> <tag>   <name>then</name>   <tagclass>cwp.tags.IfThenTag</tagclass>   <info>then part of if/condition/then/else tag.</info> </tag> <tag>   <name>else</name>   <tagclass>cwp.tags.IfElseTag</tagclass>   <info>else part of if/condition/then/else tag.</info> </tag> 
The JSP File

Listing 20.50 shows a page that uses the cwp:if tag three different ways. In the first instance, a value of true is hardcoded for the condition. In the second instance, a parameter from the HTTP request is used for the condition, and in the third case, a random number is generated and compared to a fixed cutoff. Figure 20-22 shows a typical result.

Figure 20-22. Result of IfExample.jsp.

graphics/20fig22.gif
 

Listing 20.50 IfExample.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD> <TITLE>If Tag Example</TITLE> <LINK REL=STYLESHEET       HREF="JSP-Styles.css"       TYPE="text/css"> </HEAD> <BODY> <H1>If Tag Example</H1> <%@ taglib uri="cwp-taglib.tld" prefix="cwp" %> <cwp:if>   <cwp:condition>true</cwp:condition>   <cwp:then>Condition is true</cwp:then>   <cwp:else>Condition is false</cwp:else> </cwp:if> <P> <cwp:if>   <cwp:condition><%= request.isSecure() %></cwp:condition>   <cwp:then>Request is using SSL (https)</cwp:then>   <cwp:else>Request is not using SSL</cwp:else> </cwp:if> <P> Some coin tosses:<BR> <cwp:repeat reps="10">   <cwp:if>     <cwp:condition><%= Math.random() < 0.5 %></cwp:condition>     <cwp:then><B>Heads</B><BR></cwp:then>     <cwp:else><B>Tails</B><BR></cwp:else>   </cwp:if> </cwp:repeat> </BODY> </HTML> 

20.8 Integrating Servlets and JSP

Servlets are great when your application requires a lot of real programming to accomplish its task. As you've seen in the previous chapter, servlets can manipulate HTTP status codes and headers, use cookies, track sessions, save information between requests, compress pages, access databases, generate GIF images on-the-fly, and perform many other tasks flexibly and efficiently. But, generating HTML with servlets can be tedious and can yield a result that is hard to modify. That's where JSP comes in; it lets you separate much of the presentation from the dynamic content. That way, you can write the HTML in the normal manner, even using HTML-specific tools and putting your Web content developers to work on your JSP documents. JSP expressions, scriptlets, and declarations let you insert simple Java code into the servlet that results from the JSP page, and directives let you control the overall layout of the page. For more complex requirements, you can wrap up Java code inside beans or define your own JSP tags.

Great. We have everything we need, right? Well, no, not quite. The assumption behind a JSP document is that it provides a single overall presentation. What if you want to give totally different results depending on the data that you receive? Beans and custom tags, although extremely powerful and flexible, don't overcome the limitation that the JSP page defines a relatively fixed top-level page appearance. The solution is to use both servlets and JavaServer Pages. If you have a complicated application that may require several substantially different presentations, a servlet can handle the initial request, partially process the data, set up beans, and then forward the results to one of a number of different JSP pages, depending on the circumstances. This approach is known as the model view controller architecture or model 2 approach to JSP. For code that supports a formalization of this approach, see the Apache Struts Framework at http://jakarta.apache.org/struts/.

Forwarding Requests

The key to letting servlets forward requests or include external content is to use a RequestDispatcher. You obtain a RequestDispatcher by calling the getRequestDispatcher method of ServletContext, supplying a URL relative to the server root. For example, to obtain a RequestDispatcher associated with http://yourhost/presentations/presentation1.jsp, you would do the following:

String url = "/presentations/presentation1.jsp"; RequestDispatcher dispatcher =   getServletContext().getRequestDispatcher(url); 

Once you have a RequestDispatcher, you use forward to completely transfer control to the associated URL and you use include to output the associated URL's content. In both cases, you supply the HttpServletRequest and HttpServlet Response as arguments. Both methods throw ServletException and IOException. For example, Listing 20.51 shows a portion of a servlet that forwards the request to one of three different JSP pages, depending on the value of the operation parameter. To avoid repeating the getRequestDispatcher call, we use a utility method called gotoPage that takes the URL, the HttpServletRequest, and the HttpServletResponse; gets a RequestDispatcher; and then calls forward on it.

Listing 20.51 Request Forwarding Example
public void doGet(HttpServletRequest request,                   HttpServletResponse response)     throws ServletException, IOException {   String operation = request.getParameter("operation");   if (operation == null) {     operation = "unknown";   }   if (operation.equals("operation1")) {     gotoPage("/operations/presentation1.jsp",              request, response);   } else if (operation.equals("operation2")) {     gotoPage("/operations/presentation2.jsp",              request, response);   } else {     gotoPage("/operations/unknownRequestHandler.jsp",              request, response);   } } private void gotoPage(String address,                       HttpServletRequest request,                       HttpServletResponse response)     throws ServletException, IOException {   RequestDispatcher dispatcher =     getServletContext().getRequestDispatcher(address);   dispatcher.forward(request, response); } 
Using Static Resources

In most cases, you forward requests to a JSP page or another servlet. In some cases, however, you might want to send the request to a static HTML page. In an e-commerce site, for example, requests that indicate that the user does not have a valid account name might be forwarded to an account application page that uses HTML forms to gather the requisite information. With GET requests, forwarding requests to a static HTML page is perfectly legal and requires no special syntax; just supply the address of the HTML page as the argument to getRequestDispatcher. However, since forwarded requests use the same request method as the original request, POST requests cannot be forwarded to normal HTML pages. The solution to this problem is to simply rename the HTML page to have a .jsp extension. Renaming somefile.html to somefile.jsp does not change its output for GET requests, but somefile.html cannot handle POST requests, whereas somefile.jsp gives an identical response for both GET and POST.

Supplying Information to the Destination Pages

There are three main places for the servlet to store the data that the JSP pages will use: in the HttpServletRequest, in the HttpSession, and in the ServletContext. These storage locations correspond to the three non-default values of the scope attribute jsp:useBean: request, session, and application.

  1. Storing data that servlet looked up and that JSP page will use only in this request. The servlet would create and store data as follows:

    SomeClass value = new SomeClass(...); request.setAttribute("key", value); 

    Then, the servlet would forward to a JSP page that uses the following to retrieve the data:

    <jsp:useBean id="key" class="SomeClass"              scope="request" /> 
  2. Storing data that servlet looked up and that JSP page will use in this request and in later requests from same client. The servlet would create and store data as follows:

    SomeClass value = new SomeClass(...); HttpSession session = request.getSession(true); session.setAttribute("key", value); 

    Then, the servlet would forward to a JSP page that uses the following to retrieve the data:

    <jsp:useBean id="key" class="SomeClass"              scope="session" /> 
  3. Storing data that servlet looked up and that JSP page will use in this request and in later requests from any client. The servlet would create and store data as follows:

    SomeClass value = new SomeClass(...); getServletContext().setAttribute("key", value); 

    Then, the servlet would forward to a JSP page that uses the following to retrieve the data:

    <jsp:useBean id="key" class="SomeClass"              scope="application" /> 
Interpreting Relative URLs in the Destination Page

Although a servlet can forward the request to arbitrary locations on the same server, the process is quite different from that of using the sendRedirect method of HttpServletResponse. First, sendRedirect requires the client to reconnect to the new resource, whereas the forward method of RequestDispatcher is handled completely on the server. Second, sendRedirect does not automatically preserve all of the request data; forward does. Third, sendRedirect results in a different final URL, whereas with forward, the URL of the original servlet is maintained.

This final point means that, if the destination page uses relative URLs for images or style sheets, it needs to make them relative to the server root, not to the destination page's actual location. For example, consider the following style sheet entry:

<LINK REL=STYLESHEET       HREF="my-styles.css"       TYPE="text/css"> 

If the JSP page containing this entry is accessed by means of a forwarded request, my-styles.css will be interpreted relative to the URL of the originating servlet, not relative to the JSP page itself, almost certainly resulting in an error. The solution is to give the full server path to the style sheet file, as follows:

<LINK REL=STYLESHEET       HREF="/path/my-styles.css"       TYPE="text/css"> 

The same approach is required for addresses used in <IMG SRC=...> and <A HREF=...>.

Alternative Means of Getting a RequestDispatcher

In servers that support version 2.2 of the servlet specification, there are two additional ways of obtaining a RequestDispatcher besides the getRequestDispatcher method of ServletContext.

First, since most servers let you register explicit names for servlets or JSP pages, it makes sense to access them by name rather than by path. Use the getNamedDispatcher method of ServletContext for this task.

Second, you might want to access a resource by a path relative to the current servlet's location, rather than relative to the server root. This approach is not common when servlets are accessed in the standard manner (http://host/servlet/ServletName), because JSP files would not be accessible by means of http://host/servlet/... since that URL is reserved especially for servlets.

However, it is common to register servlets under another path, and in such a case you can use the getRequestDispatcher method of HttpServletRequest rather than the one from ServletContext. For example, if the originating servlet is at http://host/travel/TopLevel,

getServletContext().getRequestDispatcher("/travel/cruises.jsp") 

could be replaced by

request.getRequestDispatcher("cruises.jsp"); 

Example: An On-Line Travel Agent

Consider the case of an on-line travel agent that has a quick-search page, as shown in Figure 20-23 and Listing 20.52. Users need to enter their e-mail address and password to associate the request with their previously established customer account. Each request also includes a trip origin, trip destination, start date, and end date. However, the action that will result will vary substantially based upon the action requested. For example, pressing the "Book Flights" button should show a list of available flights on the dates specified, ordered by price (see Figure 20-24). The user's real name, frequent flyer information, and credit card number should be used to generate the page. On the other hand, selecting "Edit Account" should show any previously entered customer information, letting the user modify values or add entries. Likewise, the actions resulting from choosing "Rent Cars" or "Find Hotels" will share much of the same customer data but will have a totally different presentation.

Figure 20-23. Front end to travel servlet (see Listing 20.52).

graphics/20fig23.gif
 

Figure 20-24. Result of travel servlet (Listing 20.53) dispatching request to BookFlights.jsp (Listing 20.54).

graphics/20fig24.gif
 

To accomplish the desired behavior, the front end (Listing 20.52) submits the request to the top-level travel servlet shown in Listing 20.53. This servlet looks up the customer information (see http://www.corewebprogramming.com for the actual code used, but this would be replaced by a database lookup in real life), puts it in the HttpSession object associating the value (of type cwp.TravelCustomer) with the name customer, and then forwards the request to a different JSP page corresponding to each of the possible actions. The destination page (see Listing 20.54 and the result in Figure 20-24) looks up the customer information by means of

<jsp:useBean id="customer"              class="cwp.TravelCustomer"              scope="session" /> 

then uses jsp:getProperty to insert customer information into various parts of the page.

You should pay careful attention to the TravelCustomer class (shown partially in Listing 20.55; available in full at www.corewebprogramming.com). In particular, note that the class spends a considerable amount of effort making the customer information accessible as plain strings or even HTML-formatted strings through simple properties. Every task that requires any substantial amount of programming is spun off into the bean, rather than being performed in the JSP page itself. This is typical of servlet/JSP integration the use of JSP does not entirely obviate the need to format data as strings or HTML in Java code. Significant up-front effort to make the data conveniently available to JSP more than pays for itself when multiple JSP pages access the same type of data.

Listing 20.52 /travel/quick-search.html (Excerpt)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD>   <TITLE>Online Travel Quick Search</TITLE>   <LINK REL=STYLESHEET         HREF="travel-styles.css"         TYPE="text/css"> </HEAD> <BODY> <BR> <H1>Online Travel Quick Search</H1> <FORM ACTION="/servlet/cwp.Travel" METHOD="POST"> <CENTER> Email address: <INPUT TYPE="TEXT" id="emailAddress"><BR> Password: <INPUT TYPE="PASSWORD" id="password" SIZE=10><BR> ... <TABLE CELLSPACING=1> <TR>   <TH> <IMG SRC="airplane.gif" WIDTH=100 HEIGHT=29                  ALIGN="TOP" ALT="Book Flight">    ... <TR>   <TH><SMALL>       <INPUT TYPE="SUBMIT" id="flights" VALUE="Book Flight">       </SMALL>   ... </TABLE> </CENTER> </FORM> ... </BODY> </HTML> 
Listing 20.53 Travel.java
package cwp; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; /** Top-level travel-processing servlet. This servlet sets up  *  the customer data as a bean, then forwards the request  *  to the airline booking page, the rental car reservation  *  page, the hotel page, the existing account modification  *  page, or the new account page.  */ public class Travel extends HttpServlet {   private TravelCustomer[] travelData;   public void init() {     travelData = TravelData.getTravelData();   }   /** Since password is being sent, use POST only. */   public void doPost(HttpServletRequest request,                      HttpServletResponse response)       throws ServletException, IOException {     String emailAddress = request.getParameter("emailAddress");     String password = request.getParameter("password");     TravelCustomer customer =       TravelCustomer.findCustomer(emailAddress, travelData);     if ((customer == null) || (password == null) ||         (!password.equals(customer.getPassword()))) {       gotoPage("/travel/accounts.jsp", request, response);     }     // The methods that use the following parameters will     // check for missing or malformed values.     customer.setStartDate(request.getParameter("startDate"));     customer.setEndDate(request.getParameter("endDate"));     customer.setOrigin(request.getParameter("origin"));     customer.setDestination(request.getParameter                               ("destination"));     HttpSession session = request.getSession(true);     session.setAttribute("customer", customer);     if (request.getParameter("flights") != null) {       gotoPage("/travel/BookFlights.jsp",                request, response);     } else if (request.getParameter("cars") != null) {       gotoPage("/travel/RentCars.jsp",                request, response);     } else if (request.get0.Parameter("hotels") != null) {       gotoPage("/travel/FindHotels.jsp",                request, response);     } else if (request.getParameter("cars") != null) {       gotoPage("/travel/EditAccounts.jsp",                request, response);     } else {       gotoPage("/travel/IllegalRequest.jsp",                request, response);     }   }   private void gotoPage(String address,                         HttpServletRequest request,                         HttpServletResponse response)       throws ServletException, IOException {     RequestDispatcher dispatcher =       getServletContext().getRequestDispatcher(address);     dispatcher.forward(request, response);   } } 
Listing 20.54 BookFlights.jsp
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD>   <TITLE>Best Available Flights</TITLE>   <LINK REL=STYLESHEET         HREF="/travel/travel-styles.css"         TYPE="text/css"> </HEAD> <BODY> <H1>Best Available Flights</H1> <CENTER> <jsp:useBean id="customer"              class="cwp.TravelCustomer"              scope="session" /> Finding flights for <jsp:getProperty id="customer" property="fullName" /> <P> <jsp:getProperty id="customer" property="flights" /> <P><BR><HR><BR> <FORM ACTION="/servlet/BookFlight"> <jsp:getProperty id="customer"                  property="frequentFlyerTable" /> <P> <B>Credit Card:</B> <jsp:getProperty id="customer" property="creditCard" /> <P> <INPUT TYPE="SUBMIT" id="holdButton" VALUE="Hold for 24 Hrs"> <P> <INPUT TYPE="SUBMIT" id="bookItButton" VALUE="Book It!"> </FORM> </CENTER> </BODY> </HTML> 
Listing 20.55 TravelCustomer.java (Excerpt)
package cwp; import java.util.*; import java.text.*; /** Describes a travel services customer. Implemented  *  as a bean with some methods that return data in HTML  *  format, suitable for access from JSP.  */ public class TravelCustomer {   private String emailAddress, password, firstName, lastName;   private String creditCardName, creditCardNumber;   private String phoneNumber, homeAddress;   private String startDate, endDate;   private String origin, destination;   private FrequentFlyerInfo[] frequentFlyerData;   private RentalCarInfo[] rentalCarData;   private HotelInfo[] hotelData;   public TravelCustomer(String emailAddress,                         String password,                         String firstName,                         String lastName,                         String creditCardName,                         String creditCardNumber,                         String phoneNumber,                         String homeAddress,                         FrequentFlyerInfo[] frequentFlyerData,                         RentalCarInfo[] rentalCarData,                         HotelInfo[] hotelData) {     setEmailAddress(emailAddress);     setPassword(password);     setFirstName(firstName);     setLastName(lastName);     setCreditCardName(creditCardName);     setCreditCardNumber(creditCardNumber);     setPhoneNumber(phoneNumber);     setHomeAddress(homeAddress);     setStartDate(startDate);     setEndDate(endDate);     setFrequentFlyerData(frequentFlyerData);     setRentalCarData(rentalCarData);     setHotelData(hotelData);   }   public String getEmailAddress() {     return(emailAddress);   }   public void setEmailAddress(String emailAddress) {     this.emailAddress = emailAddress;   }   ...   public String getCreditCard() {     String cardName = getCreditCardName();     String cardNum = getCreditCardNumber();     cardNum = cardNum.substring(cardNum.length() - 4);     return(cardName + " (XXXX-XXXX-XXXX-" + cardNum + ")");   }   ...   private String getFlightDescription(String airline,                                       String flightNum,                                       String price,                                       String stop1,                                       String stop2,                                       String time1,                                       String time2,                                       String flightOrigin,                                       String flightDestination,                                       String flightStartDate,                                       String flightEndDate) {     String flight =       "<P><BR>\n" +       "<TABLE WIDTH=\"100%\"><TR><TH CLASS=\"COLORED\">\n" +       "<B>" + airline + " Flight " + flightNum +       " ($" + price + ")</B></TABLE><BR>\n" +       "<B>Outgoing:</B> Leaves " + flightOrigin +       " at " + time1 + " AM on " + flightStartDate +       ", arriving in " + flightDestination +       " at " + time2 + " PM (1 stop -- " + stop1 + ").\n" +       "<BR>\n" +       "<B>Return:</B> Leaves " + flightDestination +       " at " + time1 + " AM on " + flightEndDate +       ", arriving in " + flightOrigin +       " at " + time2 + " PM (1 stop -- " + stop2 + ").\n";     return(flight);   }   ... } 

Forwarding Requests From JSP Pages

The most common request-forwarding scenario is that the request first comes to a servlet and the servlet forwards the request to a JSP page. The reason a servlet usually handles the original request is that checking request parameters and setting up beans requires a lot of programming, and it is more convenient to do this programming in a servlet than in a JSP document. The reason that the destination page is usually a JSP document is that JSP simplifies the process of creating the HTML content.

However, just because this is the usual approach doesn't mean that it is the only way of doing things. It is certainly possible for the destination page to be a servlet. Similarly, it is quite possible for a JSP page to forward requests elsewhere. For example, a request might go to a JSP page that normally presents results of a certain type and that forwards the request elsewhere only when it receives unexpected values.

Sending requests to servlets instead of JSP pages requires no changes whatsoever in the use of the RequestDispatcher. However, there is special syntactic support for forwarding requests from JSP pages. In JSP, the jsp:forward action is simpler and easier to use than wrapping up RequestDispatcher code in a scriptlet. This action takes the following form:

<jsp:forward page="Relative URL" /> 

The page attribute is allowed to contain JSP expressions so that the destination can be computed at request time. For example, the following sends about half the visitors to http://host/examples/page1.jsp and the others to http://host/examples/page2.jsp.

<% String destination;    if (Math.random() > 0.5) {      destination = "/examples/page1.jsp";    } else {      destination = "/examples/page2.jsp";    } %> <jsp:forward page="<%= destination %>" /> 

20.9 Summary

In principle, everything that can be accomplished with JSP can be accomplished with servlets alone. In practice, JSP makes the generation of HTML content easier and more reliable. In doing so, JSP encourages a separation between the Java code that generates the content and the HTML code that presents it, thus permitting the use of standard HTML development tools, a division of labor on large projects, and the ability to change either the content or the presentation without altering the other piece.

As a result, few real sites use servlets alone. That does not, however, mean that they use JSP entirely in lieu of servlets. Instead, complex sites typically combine servlets and JSP, using servlets for some pages, JSP for others, and a combination for the remainder.

In this chapter we gave you the basic building blocks of JSP and showed you how to control both the specific dynamic content that is invoked and the high-level structure of the servlet that results from each JSP page. We also showed what tools are needed to make complex pages maintainable by non-Java programmers: beans and custom tags. Finally, we showed you how to put all those pieces together in integrated servlet/JSP applications.

Whew. Give yourself a rest, then go buy an answering machine to screen all those calls that will soon be arriving from eager potential employers.

CONTENTS


Core Web Programming
Core Web Programming (2nd Edition)
ISBN: 0130897930
EAN: 2147483647
Year: 2000
Pages: 31

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