如何使用@Inject注释在Java中堆叠自定义注释

Question

I saw this several times when browsing.. people are using @Inject annotation with their own to inject EntityManager like this:

@Inject @MyEm EnityManager em;  

because you cannot just inject the EntityManager . You can do it only with @PersistenceContext . Does anybody know how to make this work (with the custom annotation), because I didn't find any information on the net? Give a example if you can, please.

Answer 1

Basically what you need to do is create a discriminator annotation and use it in conjunction with a Producer. This allows you to simple @Inject your Entity Manager in other beans within your Java EE application. Here is an example:

@Qualifier
@Retention(RUNTIME)
@Target(METHOD, FIELD, PARAMETER, TYPE)
public interface @MyEm {
}

public class EntityProducer {
    @PersistenceContext(unitName = 'MyPU', type = PersistenceContextType.EXTENDED)
    private EntityManager entityManager;

    @Produces
    @MyEm
    public EntityManager getEntityManager() {
        return entityManager;
    }
}

public class MyDAO {
    @Inject
    @MyEm
    private EntityManager entityManager;
}

Answer 2

This is called a "qualifier". Every CDI tutorial should explain about them. In short:

• create your own annotation, and annotate it with @Qualifier
• use your qualifier annotation on your concrete classes that implement some interface, or on producer methods that create an instance
• use your custom annotation at the injection point to differentiate between two or more implementations of an interface