The Buttso Blathers: Dependency Injection in Taglibs

20 November 2008

Dependency Injection in Taglibs

Helping with some migration material recently for OC4J --> WLS, a question was asked about whether OC4J 10.1.3.x supported dependency injection with its Tag library implementation.

It's known that OC4J 10.1.3.1+ supports dependency injection in the Web container as described here, but the specifics of whether that extended to tag librarieswas not mentioned.

A subsequent quick test verified that dependency injection does indeed work for tags using the SimpleTag (SimpleTagSupport) and BodyTag (BodyTagSupport).

package sab.testdi.tag;

import java.io.*;
import java.io.PrintWriter;
import javax.ejb.EJB;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import sab.testdi.ejb.CalcLocal;

public class CalcAdd extends SimpleTagSupport {
   
 @EJB(name="Calc")
 CalcLocal calc;
  
 int v1 = 0;
 int v2 = 0;
  
 public void setV1(int v1) {
   this.v1 = v1;
 }

 public void setV2(int v2) {
   this.v2 = v2;
 }

 public void doTag() throws JspException, IOException {
   PrintWriter out = new PrintWriter(this.getJspContext().getOut());
   out.printf("<div style='font-family: courier'>");
   if(calc != null) {
     out.printf("%s+%s=%s", v1, v2, calc.add(v1, v2));
   } else {
     out.printf("bugger, calc is null!");
   }
   out.println("</div>");
 }
}

The taglib.tld file defines the tag:

<?xml version = '1.0' encoding = 'windows-1252'?>
<taglib xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
   http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
   version="2.0"
   xmlns="http://java.sun.com/xml/ns/j2ee">
 <display-name>calc</display-name>
 <tlib-version>1.2</tlib-version>
 <short-name>calc</short-name>
 <uri>/webapp/calc</uri>
 <tag>
   <description>A short description...</description>
   <display-name>add</display-name>
   <name>add</name>
   <tag-class>sab.testdi.tag.CalcAdd</tag-class>
   <body-content>empty</body-content>
   <attribute>
     <name>v1</name>
     <required>true</required>
     <rtexprvalue>true</rtexprvalue>
   </attribute>
   <attribute>
     <name>v2</name>
     <required>true</required>
     <rtexprvalue>true</rtexprvalue>
   </attribute>
   <example><calc:add v1="100" v2="25"/></example>
 </tag>
</taglib>

And finally, a JSP can use it by importing the JAR file containing the tag libraru into the WEB-INF/lib directory (where it is nicely auto-discovered) and making a call to the <calc:add .../> tag.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<%@ page contentType="text/html;charset=windows-1252"%>
<%@ taglib uri="/webapp/calc" prefix="calc"%>
<html>
 <head>
   <meta http-equiv="Content-Type" content="text/html; charset=windows-1252"/>
   <title>Index</title>
 </head>
 <body style="font-family: arial">
 <h2>Testing Taglib Dependency Injection</h2>
 <p>
 <calc:add v1="12" v2="12"/></p>
 </body>
</html>

When the JSP is accessed it successfully displays 12+12=24 which is rendered by the tag library after being calculated via the injected EJB reference.

20 November 2008

Dependency Injection in Taglibs

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