Home › Introduce JSF EL implicit variables by custom EL resolver

Introduce JSF EL implicit variables by custom EL resolver 

Fang
Joined:
08/13/2009
Posts:
164

November 10, 2011 13:19:13    Last update: December 01, 2011 19:10:43
You can add custom implicit variables to JSF pages by using a custom EL resolver, in two simple steps:
  1. Write an ELResolver class to resolve the variable
  2. Add the ELResolver to faces-config.xml


Starting from the Maven Hello World example:
  1. Add faces API and EL dependencies to pom.xml: 
    <dependencies>
	<dependency>
	    <groupId>org.apache.myfaces.core</groupId>
	    <artifactId>myfaces-api</artifactId>
	    <version>2.1.3</version>
	    <scope>provided</scope>
	</dependency>

	<dependency>
	    <groupId>javax.el</groupId>
	    <artifactId>javax.el-api</artifactId>
	    <version>2.2.3</version>
	    <scope>provided</scope>
	</dependency>

	<dependency>
	    <groupId>junit</groupId>
	    <artifactId>junit</artifactId>
	    <version>3.8.1</version>
	    <scope>test</scope>
	</dependency>
</dependencies>

  2. Add a simple greeter class (src/main/java/com/example/Greeter.java): 
    package com.example;

public class Greeter {
    public String sayHi(String name) {
	return "Hello " + name + "!";
    }
}

  3. Add our custom EL resolver (src/main/java/com/example/ELResolver.java): 
    package com.example;

import java.util.Iterator;
import java.beans.FeatureDescriptor;

import javax.el.ELContext;
import javax.el.PropertyNotWritableException;

/**
 * This class introduces an implicit variable 'Greeter' into the EL space. 
 * It does variable resolution only, property resolution is left to the built-in 
 * resolvers. For variable resolution, base is always null, property is String.
 */
public class ELResolver extends javax.el.ELResolver {
    private static final String GREETER = "Greeter";

    @Override
    public Class<?> getCommonPropertyType(ELContext ctx, Object base) {
	return null;
    }

    @Override
    public Iterator<FeatureDescriptor> getFeatureDescriptors(ELContext ctx, Object base) {
	return null;
    }

    @Override
    public Class<?> getType(ELContext ctx, Object base, Object property) {
	if ((base == null) && property.equals(GREETER)) {
	    ctx.setPropertyResolved(true);
	    return Greeter.class;
	}
	return null;
    }

    @Override
    public Object getValue(ELContext ctx, Object base, Object property) {
	if ((base == null) && property.equals(GREETER)) {
	    ctx.setPropertyResolved(true);
	    return new Greeter();
	}
	return null;
    }

    @Override
    public boolean isReadOnly(ELContext ctx, Object base, Object property) {
	return true;
    }

    @Override
    public void setValue(ELContext ctx, Object base, Object property, Object value) {
	ctx.setPropertyResolved(false);
    }
}

  4. Add the custom EL resolver to src/main/resources/META-INF/faces-config.xml 
    <?xml version="1.0" encoding="UTF-8"?>
<faces-config xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-facesconfig_2_1.xsd"
    version="2.1">
    <application>
	<el-resolver>com.example.ELResolver</el-resolver>
    </application>
</faces-config>

  5. Build JAR with 
    mvn package

  6. Drop the JAR into WEB-INF/lib of a webapp and test the new EL with: 
    <h:outputText value="#{Greeter.sayHi('Mike')}"/>



Fixed: the setValue method used to throw an exception, which is wrong. 
@Override
public void setValue(ELContext ctx, Object base, Object property, Object value) {
    // This is WRONG!
    throw new PropertyNotWritableException("I don't set value");
}

When values need to be set on a managed bean, throwing exception here stops further processing and causes the whole page to fail, even when the introduced variable is not used.
Share |
| Comment  | Tags
 

Related Notes