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Java EE6 Decorators, advanced usage

After my first article about Java EE6 decorators, it got some attention on dzone. I got some comments telling me the example isn’t exposing the true power of the decorator feature.
The first thing I did not mention in my previous post is that we can combine a number of decorators and choose the order we want them executed.
If you have a use case for it, you can easily define 2 decorators, by just defining them in the beans.xml file like this.

<decorators>
    <class>be.styledideas.blog.decorator.HighDecorator</class>
    <class>be.styledideas.blog.decorator.LowDecorator</class>
</decorators>

So when we call our decorated class, we get the highdecorator entry, low decorator entry, actual decorated class, low decorator exit, highdecorator exit. So the decorator sequence in the file does matter.
The true power of the Decorator feature in java EE6 is the ability to combine it with CDI annotations. As example I’ll use an Social media feed processor.
So I have created an interface

public interface SocialFeedProcessor {
    Feed process(String feed);
}

and provided 2 implementations, twitter and google+

public class TwitterFeedProcessor implements SocialFeedProcessor{
 
    @Override
    public Feed process(String feed) {
        System.out.println("processing this twitter feed");
        // processing logics
        return new Feed(feed);
    }
 
}
public class GooglePlusFeedProcessor implements SocialFeedProcessor {
 
    @Override
    public Feed process(String feed) {
        System.out.println("processing this google+ feed");
        // processing logics
        return new Feed(feed);
    }
 
}

I’ll annotate these 2 beans by a custom Qualifier as described here

@javax.inject.Qualifier
@java.lang.annotation.Retention(RUNTIME)
@java.lang.annotation.Target({FIELD, PARAMETER, TYPE})
@java.lang.annotation.Documented
public @interface FeedProcessor {
}

and I annotate my 2 processors with it.

@FeedProcessor
public class TwitterFeedProcessor implements SocialFeedProcessor{
 
    @Override
    public Feed process(String feed) {
        System.out.println("processing this twitter feed");
        // processing logics
        return new Feed(feed);
    }
 
}
	
@FeedProcessor
public class GooglePlusFeedProcessor implements SocialFeedProcessor {
 
    @Override
    public Feed process(String feed) {
        System.out.println("processing this google+ feed");
        // processing logics
        return new Feed(feed);
    }
 
}

Nothing really special, but now when we write our decorator we use the power of CDI to only decorate the classes with the @FeedProcessor annotation.

@Decorator
public class SocialFeedDecorator implements SocialFeedProcessor {
    @Delegate
    private @FeedProcessor SocialFeedProcessor processor;
 
    @Override
    public Feed process(String feed) {
        System.out.println("our decorator is decorating");
        return processor.process(feed);
    }
}

the only thing that is left is registering our decorator in our beans.xml

<decorators>
    <class>be.styledideas.blog.decorator.SocialFeedDecorator</class>
</decorators>

By using the annotation, we automatically decorate all our implementations of the SocialfeedProcessor with this decorator. When we add an extra implementation of the SocialFeedProcessor without the annotation, the bean will not be decorated.

From http://styledideas.be/blog/2011/08/04/java-ee6-decorators-advanced-usage/

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