Recipe 16.12 Accessing or Removing Request Attributes in JSPs

Problem

You want to use a JSP to access or remove a request attribute.

Solution

Use the JSTL core tags c:out and c:remove to access and optionally remove the attribute.

Discussion

Example 16-14 accesses an object attribute that is bound to the HttpServletRequest. The JSP accesses this attribute by using EL syntax inside the c:out JSTL tag.

Example 16-12 in Recipe 16.10 forwards a request attribute to a servlet using the jsp:forward standard action. The JSP in that example can forward its request attribute to the JSP in Example 16-14 by using the code:

<jsp:forward page="/requestDisplay.jsp" />

The code:

"${requestScope[\"com.jspservletcookbook.ContextObject\"].
    values}"

uses the requestScope JSTL implicit object. This variable, which the JSTL automatically makes available to EL-related code, is a java.util.Map type containing any attributes bound to the request scope. The code then displays the values the attribute contains by accessing the object attribute's values property (see Recipe 16.1 for a discussion of the object used for storing an attribute in various scopes throughout this chapter).

Example 16-14. Accessing and removing a request attribute with the JSTL

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>

<html>
<head><title>Request reading JSP</title></head>
<body>
<h2>Here are the values from the bound RequestObject</h2>

<c:out value=
    "${requestScope[\"com.jspservletcookbook.ContextObject\"].
        values}" escapeXml="false" />

<%-- c:remove may not be necessary as request attributes persist only as long as the 
request is being handled --%>
<br>Removing request attribute with c:remove ... <c:remove var=
    "com.jspservletcookbook.ContextObject" scope="request" />

</body>
</html>

The c:remove tag removes the attribute named in its var attribute from the specified scope. Use scope="request" because you are removing this attribute from the JSP's request scope. Figure 16-2 shows the output of the displayRequest.jsp page in a web browser.

Figure 16-2. The browser display after accessing and removing a request attribute in a JSP