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14.5 Optionally Including the Tag Body

Most tags either never make use of body content or always do so. This section shows you how to use request time information to decide whether or not to include the tag body. Although the body can contain JSP that is interpreted at page translation time, the result of that translation is servlet code that can be invoked or ignored at request time.

The Tag Handler Class

Optionally including the tag body is a trivial exercise: just return EVAL_BODY_INCLUDE or SKIP_BODY depending on the value of some request time expression. The important thing to know is how to discover that request time information, since doStartTag does not have HttpServletRequest and HttpServletResponse arguments as do service, _jspService, doGet, and doPost. The solution to this dilemma is to use getRequest to obtain the HttpServletRequest from the automatically defined pageContext field of TagSupport. Strictly speaking, the return type of getRequest is ServletRequest, so you have to do a typecast to HttpServletRequest if you want to call a method that is not inherited from ServletRequest. However, in this case I just use getParameter, so no typecast is required.

Listing 14.13 defines a tag that ignores its body unless a request time debug parameter is supplied. Such a tag provides a useful capability whereby you embed debugging information directly in the JSP page during development, but activate it only when a problem occurs.

Listing 14.13 DebugTag.java

package coreservlets.tags;

import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import java.io.*;
import javax.servlet.*;

/** A tag that includes the body content only if
 * the "debug" request parameter is set.
 */

public class DebugTag extends TagSupport {
 public int doStartTag() {
  ServletRequest request = pageContext.getRequest();
  String debugFlag = request.getParameter("debug");
  if ((debugFlag != null) &&
    (!debugFlag.equalsIgnoreCase("false"))) {
   return(EVAL_BODY_INCLUDE);
  } else {
   return(SKIP_BODY);
  }
 }
}

The Tag Library Descriptor File

If your tag ever makes use of its body, you must provide the value JSP inside the bodycontent element. Other than that, all the elements within tag are used in the same way as described previously. Listing 14.14 shows the entries needed for DebugTag.

Listing 14.14 csajsp-taglib.tld

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE taglib
 PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"
 "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd">

<!-- a tag library descriptor -->

<taglib>
 <!-- after this the default space is
    "http://java.sun.com/j2ee/dtds/jsptaglibrary_1_2.dtd"
  -->

 <tlibversion>1.0</tlibversion>
 <jspversion>1.1</jspversion>
 <shortname>csajsp</shortname>
 <urn></urn>
 <info>
  A tag library from Core Servlets and JavaServer Pages,
  http://www.coreservlets.com/.
 </info>

 <!-- Other tags defined earlier... -->

 <tag>
  <name>debug</name>
  <tagclass>coreservlets.tags.DebugTag</tagclass>
  <info>Includes body only if debug param is set.</info>
  <bodycontent>JSP</bodycontent>
 </tag>

</taglib>

The JSP File

Listing 14.15 shows a page that encloses debugging information between <csajsp:debug> and </csajsp:debug>. Figure 14–5 and 14–6 show the normal result and the result when a request time debug parameter is supplied, respectively.

Listing 14.15 DebugExample.jsp

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Using the Debug Tag</TITLE>
<LINK REL=STYLESHEET
   HREF="JSP-Styles.css"
   TYPE="text/css">
</HEAD>

<BODY>
<H1>Using the Debug Tag</H1>

<%@ taglib uri="csajsp-taglib.tld" prefix="csajsp" %>

Top of regular page. Blah, blah, blah. Yadda, yadda, yadda.
<P>

<csajsp:debug>
<B>Debug:</B>
<UL>
 <LI>Current time: <%= new java.util.Date() %>
 <LI>Requesting hostname: <%= request.getRemoteHost() %>
 <LI>Session ID: <%= session.getId() %>
</UL>
</csajsp:debug>

<P>
Bottom of regular page. Blah, blah, blah. Yadda, yadda, yadda.

</BODY>
</HTML>

Figure 14–5 The body of the csajsp:debug element is normally ignored.

Figure 14–6 The body of the csajsp:debug element is included when a debug request parameter is supplied.

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