11.12 Using Predefined Variables

11.12 Using Predefined Variables

When you wrote a doGet method for a servlet, you probably wrote something like this:

 public void doGet(HttpServletRequest  request  ,                   HttpServletResponse  response  )     throws ServletException, IOException {   response.setContentType("text/html");   HttpSession  session  = request.getSession();   PrintWriter  out  = response.getWriter();   out.println(...);   ... } 

The servlet API told you the types of the arguments to doGet , the methods to call to get the session and writer objects, and their types. JSP changes the method name from doGet to _jspService and uses a JspWriter instead of a PrintWriter . But the idea is the same. The question is, who told you what variable names to use? The answer is, nobody! You chose whatever names you wanted.

For JSP expressions and scriptlets to be useful, you need to know what variable names the autogenerated servlet uses. So, the specification tells you. You are supplied with eight automatically defined local variables in _jspService , sometimes called "implicit objects." Nothing is special about these; they are merely the names of the local variables. Local variables. Not constants. Not JSP reserved words. Nothing magic. So, if you are writing code that is not part of the _jspService method, these variables are not available. In particular, since JSP declarations result in code that appears outside the _jspService method, these variables are not accessible in declarations. Similarly, they are not available in utility classes that are invoked by JSP pages. If you need a separate method to have access to one of these variables, do what you always do in Java code: pass the variable along.

The available variables are request , response , out , session , application , config , pageContext , and page . Details for each are given below. An additional variable called exception is available, but only in error pages. This variable is discussed in Chapter 12 (Controlling the Structure of Generated Servlets: The JSP page Directive) in the sections on the errorPage and isErrorPage attributes.

• 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. Since the output stream (see out ) is normally buffered, it is usually legal to set HTTP status codes and response headers in the body of 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. If you turn buffering off, however (see the buffer attribute in Chapter 12), you must set status codes and headers before supplying any output.

• out

  This variable is the Writer used to send output to the client. However, to make it easy to set response headers at various places in the JSP page, out is not the standard PrintWriter but rather a buffered version of Writer called JspWriter . You can adjust the buffer size through use of the buffer attribute of the page directive (see Chapter 12). 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 in JSP, so this variable is bound even if there is no incoming session reference. The one exception is the use of the session attribute of the page directive (Chapter 12) to disable automatic session tracking. In that case, attempts to reference the session variable cause errors at the time the JSP page is translated into a servlet. See Chapter 9 for general information on session tracking and the HttpSession class.

• 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 and JSP pages in the Web application, whereas instance variables are available only to the same servlet that stored the data.

• config

  This variable is the ServletConfig object for this page. In principle, you can use it to read initialization parameters, but, in practice, initialization parameters are read from jspInit , not from _jspService .

• pageContext

  JSP introduced a class called PageContext to give a single point of access to many of the page attributes. The PageContext class has methods getRequest , getResponse , getOut , getSession , and so forth. The pageContext variable stores the value of the PageContext object associated with the current page. If a method or constructor needs access to multiple page- related objects, passing pageContext is easier than passing many separate references to request , response , out , and so forth.

• 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.