How to stack custom annotation in Java with @Inject annotation

问题

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.

回答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;
}

回答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

来源：https://stackoverflow.com/questions/8770465/how-to-stack-custom-annotation-in-java-with-inject-annotation

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java

java-ee

annotations

cdi