Accepting EL Expression Example

JSP Tutorials - Herong's Tutorial Examples

∟Accepting EL Expression Example

This section provides a tutorial example to test how EL expressions can be used in custom tag attributes. In JSP 2.1 EL expressions can be used directly. But in JSP 1.2, you need to use alternative approaches.

As of JSP 2.1, EL (Expression Language) expressions can be used directly in custom tag attributes. Let me use the AttValueTag.java tag class presented earlier to show you an example.

First we need to modify my TLD file, HyTaglib.tld, to turn on the "rtexprvalue" flag on my tag attributes:

<?xml version="1.0"?>
<!DOCTYPE taglib PUBLIC 
 "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN" 
 "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_2.dtd"> 
<!-- HyTaglib.tld
 - Copyright (c) 2012, HerongYang.com, All Rights Reserved.
-->
<taglib>
<tlib-version>1</tlib-version>
<jsp-version>2.1</jsp-version> 
<short-name>Herong's Tag Library</short-name> 
<tag>
 <name>attValue</name>
 <tag-class>herong.AttValueTag</tag-class>
 <body-content>empty</body-content>
 <attribute>
  <name>longValue</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>longObject</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>doubleValue</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>doubleObject</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>booleanValue</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>booleanObject</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
 <attribute>
  <name>stringObject</name>
  <required>false</required>
  <rtexprvalue>true</rtexprvalue>
 </attribute>
</tag>
</taglib>

Here is my test JSP page, AttValueTagElTest.jspx. All attribute values are using EL expressions.

<?xml version="1.0"?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page" 
   xmlns:hy="urn:jsptld:/WEB-INF/tlds/HyTaglib.tld" version="2.1"> 
<!-- AttValueTagElTest.jspx
 - Copyright (c) 2012, HerongYang.com, All Rights Reserved.
-->
<jsp:directive.page contentType="text/html"/>
<html><body>
<hy:attValue longObject="${3-2}" longValue="${1+1}"
 doubleObject="${1.1}" doubleValue="${2.2}"
 booleanObject="${2 gt 1}" booleanValue="${2 lt 1}" 
 stringObject="${'Hello world!'}"/>
</body></html>
</jsp:root>

Visit this JSP page, I am getting the same result as AttValueTagTest.jspx:

longValue = 2
longObject = 1
doubleValue = 2.2
doubleObject = 1.1
booleanValue = false
booleanObject = true
stringObject = Hello world!

Do you want to see how the JSP server converts those EL expressions into the Servlet class? Here are Java statements converted from my custom attribute values:

...
_jspx_th_hy_005fattValue_005f0.setStringObject(
   (java.lang.String) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${'Hello world!'}", java.lang.String.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false));
_jspx_th_hy_005fattValue_005f0.setBooleanValue(
   ((java.lang.Boolean) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${2 lt 1}", java.lang.Boolean.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false))
   .booleanValue());
_jspx_th_hy_005fattValue_005f0.setBooleanObject(
   (java.lang.Boolean) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${2 gt 1}", java.lang.Boolean.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false));
_jspx_th_hy_005fattValue_005f0.setDoubleValue(
   ((java.lang.Double) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${2.2}", java.lang.Double.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false))
   .doubleValue());
_jspx_th_hy_005fattValue_005f0.setDoubleObject(
   (java.lang.Double) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${1.1}", java.lang.Double.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false));
_jspx_th_hy_005fattValue_005f0.setLongValue(
   ((java.lang.Long) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${1+1}", java.lang.Long.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false))
   .longValue());
_jspx_th_hy_005fattValue_005f0.setLongObject(
   (java.lang.Long) 
   org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(
   "${3-2}", java.lang.Long.class, 
   (javax.servlet.jsp.PageContext)_jspx_page_context, null, false));
...

Note that:

Tomcat 7 uses org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate() method to evaluate my EL expressions.
All expression results are casted to objects.
If the setter method expects a primitive type, a conversion method, like doubleValue(), will be called.

Last update: 2012.

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JSP (JavaServer Pages) Overview

Tomcat 7 Installation on Windows Systems

JSP Scripting Elements

Java Servlet Introduction

JSP Implicit Objects

Syntax of JSP Pages and JSP Documents

JSP Application Session

Managing Cookies in JSP Pages

JavaBean Objects and "useBean" Action Elements