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EL Implicit Objects
Posted on: March 11, 2008 at 12:00 AM

EL is the JSP 2.0 Expression Language Interpreter from Apache Corporation. EL means the expression language , it makes it possible to easily access application data stored in JavaBeans components.

EL Implicit Objects

EL is the JSP 2.0 Expression Language Interpreter from Apache Corporation. EL means the expression language , it makes it possible to easily access application data stored in JavaBeans components. The jsp expression language allows a page author to access a bean using simple syntax such as $(name). Before JSP 2.0, we could use only a scriptlet, JSP expression, or a custom tag to include server state in the jsp page output. Expression Language (EL) was first introduced in JSTL 1.0. EL makes it easier to integrate server side state with the presentation output.

EL expression is always written between the delimiters ${ and }. In the JSP page response, the result of expression evaluation replaces the expression and its delimiters in the template text. Within tags, expression can be used only in attribute values.

The JSP expression language defines various implicit objects:

pageContext: The context for the JSP page. It contains the reference of the following objects.

servletContext
session
request
response

2. param: It maps a request parameter name to a single value.

3. paramValues: It maps a request parameter name to an array of values.

4. header: It maps a request header.

5. cookie: It maps a cookie name to a single cookie.

6. initParam: It maps the context initialization parameter name.

The code of the program is given below:

<html>
  <head>
    <title>JSP 2.0 Expression Language
         - Implicit Objects</title>
  </head>
  <body>
   <h6>JSP 2.0 Expression Language - Implicit Objects</h6>

    The list of the implicit objects available 
       in the Expression Lanaguage. 
    Not available are illustrated here:
    <ul>
      <li>pageContext - the PageContext object</li>

 <li>requestScope
 - a Map that maps request-scoped attribute names 
          to their values</li>
 <li>sessionScope
 - a Map that maps session-scoped attribute names 
          to their values</li>
 <li>applicationScope 
 - a Map that maps application-scoped attribute 
          names to their values</li>
 <li>param
 - a Map that maps parameter names to a single String 
          parameter value</li>
 <li>paramValues
 - a Map that maps parameter names to a String[] of 
          all values for that parameter</li>
 </ul>
<blockquote>
<br>
      <code>
        <table border="1">
          <thead>
	    <td><b>EL Expression</b></td>
	    <td><b>Result</b></td>
	  </thead>
	  <tr>
	    <td>\${header["host"]}</td>
	    <td>${header["host"]}</td>
	  </tr>
	  <tr>
	    <td>\${header["accept"]}</td>
	    <td>${header["accept"]}</td>
	  </tr>
	  <tr>
	    <td>\${header["user-agent"]}</td>
	    <td>${header["user-agent"]}</td>
	  </tr>
	</table>
      </code>
    </blockquote>

  </body>
</html>

The output of the program is given below:

Download this example.

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