How can I prevent from a close method being invoked?

Question

I'm planning to write a business method for those non-container-managed clients such as JRX-RS resources.

@Stateless
public class PersistenceService {

    public <R> R apply(final Function<EntityManager, R> function) {
        return function.apply(entityManager);
    }

    @PersistenceContext
    private transient EntityManager entityManager;
}

So that a JAX-RS root resource class uses it like this.

@RequestScoped
@Path("/entities")
public class MyEntites {

    @GET
    @Path("/{id: \\d+}")
    public Response read(final long id) {

        return Response.ok(service.apply(m -> {
            m.find(MyEntity.class, id);
        })).build();
    }

    @Inject
    private transient PersistenceService service;
}

The problem is that how can I prevent clients invoking close() on specified entity manager?

service.apply(m -> {
    m.close();
    return null;
});

Answer 1

You can create a wrapper around the original entity manager, which ensures that close() is never called:

public class NoCloseEntityManager implements EntityManager {
  private EntityManager wrapped;
  public NoCloseEntityManager(EntityManager wrapped) {
    this.wrapped = wrapped;
  }

  public void close() { 
    throw new UnsupportedOperationException("Don't call close()");
  }

  // all other methods from the interface call the wrapped manager:
  public void persist(Object entity) {
    wrapped.persist(entity);
  }
  ...
}

That one can you use in your service:

@Stateless
public class PersistenceService {

    @PersistenceContext
    private transient EntityManager entityManager;

    public <R> R apply(final Function<EntityManager, R> function) {
        return function.apply(new NoCloseEntityManager(entityManager));
    }

}

But I don't know if the effort is worse the result, as entityManager.close() isn't called very often - if someone calls it, he will not use it by mistake...