Decorator Design Pattern Case Study

Decorator pattern is basically a way to extend functionality of an object by decorating with other objects which can wrap the target object and add their own behavior to it. If you never used or heard of decorators, I highly recommend reading chapter 3 of Head First Design Patterns.

Pretty much like other patterns mentioned in my posts before, JavaEE has an easy an elegant way to use the decorator pattern. Lets start with a simple stateless session bean.


package com.devchronicles.decorator;

import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;

/**
 *
 * @author murat
 */
@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class EventService {
    
    public void startService(){
        System.out.println("do something important here...");
    }
}

To start implementing the decorator pattern, we need an interface so we can bind the decorators and the object to be decorated together.


package com.devchronicles.decorator;

/**
 *
 * @author murat
 */
public interface ServiceInterface {
    public void startService();
}

The interface has the method which the decorators will add functionality on. Next we need some changes on our existing EventService bean to make it decoratable.


package com.devchronicles.decorator;

import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;

/**
 *
 * @author murat
 */
@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class EventService implements ServiceInterface{
    
    public void startService(){
        System.out.println("do something important here...");
    }
}


Now we are ready to add as much as decorator we need. All we need to do is to annotate our class, implement the ServiceInterface and to inject our service delegate.


package com.devchronicles.decorator;

import javax.decorator.Decorator;
import javax.decorator.Delegate;
import javax.inject.Inject;

/**
 *
 * @author murat
 */
@Decorator //declares this class as a decorator
public class DecoratorService implements ServiceInterface{ //must implement the service interface
    
    @Inject //inject the service
    @Delegate //and annotate as the delegate
    ServiceInterface service;

    @Override
    public void startService() { //implement the startService method to add functionality
        System.out.println("decorating the existing service!");
        service.startService(); //let the execution chain continue
    } 
}

Several decorators can be using the service interface.


package com.devchronicles.decorator;

import javax.decorator.Decorator;
import javax.decorator.Delegate;
import javax.inject.Inject;

/**
 *
 * @author murat
 */
@Decorator
public class Decorator2Service implements ServiceInterface{
     @Inject
    @Delegate
    ServiceInterface service;

    @Override
    public void startService() {
        System.out.println("decorating the service even further!!!");
        service.startService();
    }
}

Most of the configuration can be done via annotation in JavaEE6. However we still need to add some xml configuration to make decorators work. It might seem disappointing since we already annotated our decorators but still the configuration is pretty simple and needed in order to declare the order of execution. Add the following lines to the empty beans.xml.


<?xml version="1.0" encoding="UTF-8"?>
<beans 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/beans_1_0.xsd">
           <decorators>
               <class>com.devchronicles.decorator.DecoratorService</class>
           <class>com.devchronicles.decorator.Decorator2Service</class>
           </decorators>
</beans>

When the startService method of our EventService is executed, the decorators will decorate the ejb and add their own behavior to the execution.

...

INFO: WEB0671: Loading application [Decorator] at [/Decorator]
INFO: Decorator was successfully deployed in 2,534 milliseconds.
INFO: decorating the existing service!
INFO: decorating the service even further!!!
INFO: do something important here...
...

Reference: JavaEE Revisits Design Patterns: Decorator