Tutorial: Add Logic to Your JSP Pages with the JSP Expression Language:

Add Logic to Your JSP Pages with the JSP Expression Language

Most JSP programmers have complaints about how difficult it is to add logic into a JSP page. Working with tables, styles, and alignments while modifying them to support the presentation logic can be extremely cumbersome.

The new JSP Expression language allows you to separate the HTML and style code from the presentation code by trying to include the presentation logic in the form of expressions. The Expression Language provides a convenient way to look up data residing in the JSP container (page, request, session, and application scope) and performs operations on them before they are rendered to the screen. Many frameworks already have similar mechanisms to separate presentation logic and HTML—notable among them is the Struts-EL module provided by Struts. However, different frameworks and systems have their proprietary standards on how expressions are defined and evaluated.

The Expression Language specification defines the formal semantics of how expressions can be defined in the Java language and extends it to systems other than JSPs. This ensures some commonality and uniformity wherever expressions are used. The specification provides only the guidelines on how to define and evaluate expressions. The actual implementation depends on the actual expression engine developed by framework and tool developers.

Why Use the Expression Language?

JSP pages have the presentation logic and the HTML code intertwined in the same document, which tends to make the page less readable and less maintainable. In addition, modifying presentation logic in an existing JSP can throw off the alignment of the page or upset some HTML table—adding additional work for the GUI programmer. This problem is illustrated in the example below:


<html>
<head>
.....
<%
boolean loginFlag  = session.getAttribute("LoginFlag");
if (!loginFlag){
%>
You are Not Logged into the Application. Please Login
<%} else {%>
Welcome Mr .<%=customer.lastName%> to the Test Application. 
<%}%>

The Expression Language can alleviate these problems by providing structure to the JSP document, making it easy to understand and maintain. Here's the above code rewritten in EL:


<c:choose>
	<c:when test = �${sessionScope.LoginFlag ='False'}�>
		You are Not Logged into the Application. Please Login
 	</c:when>
	<c:otherwise>
		Welcome Mr . <c:out value = �${customer.lastName}�/> 
		to the Test Application. 
	</c:otherwise>
</c:choose>

Notice how easily the HTML code can be separated from the presentation logic. EL defines implicit objects for objects that are in default scope (Page, Session, Application, and Request) of a JSP page. These implicit objects provide easy access to variable and methods defined with any of these scopes. Apart from these scoped variables, EL provides easy access to the request parameters, header parameters, cookies, and initParams.

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