Home › JSF taglib tutorial › Using EL expression with a custom tag

Using EL expression with a custom tag 

Fang
Joined:
08/13/2009
Posts:
164

November 21, 2011 13:49:11    Last update: November 21, 2011 13:49:11
In the test for the simple taglib example, I used a literal string for the name attribute: 
<my:hello name="Jack"/>

What happens if the name attribute contains EL expresson? For example: 
<my:hello name="#{param['name']}"/>

If EL works, the tag should take the value of the "name" request parameter and print it out. But the tag as implemented in the simple taglib example prints the literal string: 
Hello #{param['name']}! I am FaceletTag.


In order to make a tag to recognize EL, we have to use TagAttribute.getValue(FaceletContext ctx) instead of TagAttribute.getValue(). The latter returns the literal value of the attribute.

The HelloTagHandler should be changed to: 
package com.example;

import java.io.IOException;

import javax.faces.component.UIComponent;
import javax.faces.component.UIComponentBase;
import javax.faces.context.FacesContext;
import javax.faces.context.ResponseWriter;
import javax.faces.view.facelets.FaceletContext;
import javax.faces.view.facelets.TagAttribute;
import javax.faces.view.facelets.TagHandler;
import javax.faces.view.facelets.TagConfig;

/**
 * Custom facelet tag that says hello.
 */
public class HelloTagHandler extends TagHandler {
    private String name = "Anonymous";

    public HelloTagHandler(TagConfig config) {
	super(config);
    }

    public void apply(FaceletContext context, UIComponent parent) 
		throws IOException {
	TagAttribute a = tag.getAttributes().get("name");
	if (a != null) {
	    name = a.getValue(context); // this call evaluates EL
	}

	UIComponentBase c = new UIComponentBase() {
	    public void encodeEnd(FacesContext ctx) throws IOException {
		ResponseWriter w = ctx.getResponseWriter();
		w.write(String.format(
		    "<p>Hello %s! I am FaceletTag.</p>",
		    name
		));
	    }

	    // abstract method in base, must override
	    public String getFamily() {
		return "com.example.facelettag.test";
	    }
	};
	parent.getChildren().add(c);
    }
}

Rebuild the taglib and test with a URL like this: 
http://localhost:8080/facelet-test/?name=Jack

The tag will print: 
Hello Jack! I am FaceletTag.
Share |
| Comment  | Tags