JSP Implicit Objects

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In the service methods of servlets we have references to many server-side objects that represent the incoming request, response, and session. JSP provides a set of implicit objects that can be used to access such objects. These objects are actually defined in the _JSP pageservice() method of the page implementation class and initialized with appropriate references.

JSP defines four scopes for the objects that can be used by the JSP authors:

Scope

Description

page

Objects can be accessed only within the JSP page in which they are referenced.

request

Objects can be accessed within all the pages that serve the current request. These include pages that are forwarded to, and included in, the original JSP page to which the request was routed.

session

Objects can only be accessed within the JSP pages accessed within the session for which the objects are defined.

application

Application scope objects can be accessed by all JSP pages in a given context.

The following table lists the implicit objects that can be used in scriptlets:

Implicit Object

Type

Scope

Description

request

Protocol dependent sub type of javax.servlet.ServletRequest

request

A reference to the current request

response

Protocol dependent sub type of javax.servlet.ServletResponse

page

The response to the request

pageContext

javax.servlet.jsp.PageContext

page

Provides a common point to access the request, response, session, and application, associated with the page being served

session

javax.servlet.http.HttpSession

session

The session associated with the current request

application

javax.servlet.ServletContext

application

The servlet context to which the page belongs

out

javax.servlet.jsp.JspWriter

page

The object that writes to the response output stream

config

javax.servlet.ServletConfig

page

The servlet configuration for the current page

Page

java.lang.Object

page

An instance of the page implementation class that is serving the request (synonymous with the this keyword if Java is used as the scripting language)

exception

java.lang.Throwable

page

Available with JSP pages that act as error pages for other JSP pages

The following JSP page illustrates the use of implicit objects in JSP pages. Save this as %CATALINA_HOME%/webapps/jsp/Implicit.jsp:

     <html>       <head>         <title>Implicit Objects</title>       </head>       <body style="font-family:verdana;font-size:10pt">         <p>           Using Request parameters...<br> 

We print the request parameter name:

             Using Request parameters...<br>             Name: <%= request.getParameter("name") %>           </p> 

We print a sentence using the out implicit variable:

           <p>             <% out.println("This is printed using the out implicit variable"); %>           </p> 

We store and retrieve an object of type String using the session implicit variable:

           <p>           Storing a string to the session...<br>           <% session.setAttribute("name", "Meeraj"); %>           Retrieving the string from session...<br>           <b>Name:</b> <%= session.getAttribute("name") %>         </p> 

Store and retrieve an object of type String in the servlet context using the application implicit variable:

         <p>           Storing a string to the application...<br>           <% application.setAttribute("name", "Meeraj"); %>           Retrieving the string from application...<br>           <b>Name:</b> <%= application.getAttribute("name") %>         </p> 

Finally, store and retrieve an object of type String in the page context using the pageContext implicit variable:

         <p>           Storing a string to the page context...<br>           <% pageContext.setAttribute("name", "Meeraj"); %>           Retrieving the string from page context...</br>           <b>Name:</b> <%= pageContext.getAttribute("name") %>         </p>      </body>    </html> 

Access this JSP page at http://localhost:8080/jsp/Implicit.jsp?name=Meeraj and you will see:

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Professional Java Servlets 2.3
Professional Java Servlets 2.3
ISBN: 186100561X
EAN: 2147483647
Year: 2006
Pages: 130

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