Java EE6 Decorators, advanced usage

After I published 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 second feature is more compelling than the first, it exposes the true power of the Decorator feature in java EE6. That 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.

4 thoughts on “Java EE6 Decorators, advanced usage”

  1. you annotate both classes with @FeedProcessor now in decorator class which class gets injected? i think you have to qualify only one class with @FeedProcessor

    1. Both classes get processed. That is the whole beauty of it, so all classes that are marked as a FeedProcessor (with the @FeedProcessor annotation) are decorated. So when I remove one of the 2 annotation, only the ones with the @FeedProcessor annotation will be decorated.

  2. Great!! i was confusing it with injection method in which if you annotate two classes with same qualifier there will be ambiguity which class to inject. thanks for help

  3. In relation to the question asked by ‘Farrukh Obaid ‘ and your answer that as both @FeedProcessor will be called. In what sequence ‘GooglePlusFeedProcessor’ and ‘TwitterFeedProcessor’ will be called ? Or can we have any control over it.

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