Loopback exception example

[Lucas Smith]

Hi,

I have read in Ivan Krizsan's notes (great job!) - page 65 (Reentrant locking) about loopback exception.

Why the following code does not throw mentioned exception?:

@Singleton
@LocalBean
@Startup
public class Single {
    @PostConstruct
    @Lock(LockType.READ)
    private void post() {
        a();
    }

    @Lock(LockType.READ)
    private void a() {
        b();
    }

    @Lock(LockType.WRITE)
    private void b() {
    }

}

Could you give me an example when such exception can occur?

[E Armitage]

a calling b here is very bad because most likely the @Lock(LockType.WRITE) is ignored since transaction and (just by inference) also concurrency attributes are only applied in a client view mode.
i.e the attributes are only applied when the business method is being invoked from some injected or looked up reference of the bean.
Here post and a will not throw exceptions because the container will allow multiple threads to access the methods
and b called through a is really run in READ mode as well. (i.e b is not considered a business method when called through method a).

To get an exception, set the @AccessTimeout value to zero for method b, make method b do some longish task and call method b twice (one call after another) from a client view of the bean or two different clients of the bean. The second call should get the javax.ejb.ConcurrentAccessException.