What is a short example of inter-type declarations in AspectJ which demonstrates the usefulness of the approach?

Question

I first thought about using ITDs to define the private static final Logger logger = ... for some unrelated cases, but it doesn\'t look enough like an obvious im

Answer 1

Java does not support multiple inheritance or mixin. But with ITD's you can make Scala like Traits or Ruby like Mixins.

Here is an example taking advantage of JPA and Spring's Configurable:

Interface:

package com.evocatus.aop;

public interface DomainNotify {

    public void postPersist();

    public void postUpdate();

    public void postRemove();

}

AspectJ ITD:

package com.evocatus.aop;

import javax.persistence.PostPersist;
import javax.persistence.PostRemove;
import javax.persistence.PostUpdate;
import javax.persistence.Transient;

import org.codehaus.jackson.annotate.JsonIgnore;
import org.springframework.beans.factory.annotation.Autowired;

import com.evocatus.service.DomainNotifyService;


    privileged aspect DomainNotifyAdvice {

        @JsonIgnore
        @Transient
        @Autowired
        transient DomainNotifyService DomainNotify.domainNotifyService;

        @PostPersist
        public void DomainNotify.postPersist() {
            this.domainNotifyService.publishSave(this);
        }

        @PostUpdate
        public void DomainNotify.postUpdate() {
            this.domainNotifyService.publishUpdate(this);
        }

        @PostRemove
        public void DomainNotify.postRemove() {
            this.domainNotifyService.publishRemove(this);
        }

    }

Now any class that implements DomainNotify will get the ITD methods weaved in.

@Entity
@Configurable
public class MyJPAObject implements DomainNotify {
}