Question

First, I have a stateless bean which do a simple retreive, looking like this.

@Stateless
@LocalBean
public A {
    @PersistenceContext
    private EntityManager em;

    public MyEntity retrieveMethod(){
        em.createQuery(...).getSingleResult();
    }
}

I have a statefull bean used to manage long conversation with a remote client, it look like this :

@Statefull
@LocalBean
@TransactionAttribute(NOT_SUPPORTED)
public class B implements BRemote {
    @PersistenceContext(type = EXTENDED)
    private EntityManager em;

    @EJB
    A a;

    public void start(){
        OtherEntity oe = new OtherEntity();
        oe.setRelationMyEntitie(this.a.retrieveMethod());

        em.persist(oe);
    }

    @TransactionAttribute(REQUIRED)
    public void end(){
        em.flush();
    }
}

The problem come on executing em.persist(oe). oe has a reference to an instance of MyEntity which was loaded by another EntityManager. So em don’t know it complaining about persisting detached Entity.

I would like to know what is there is a way to avoid this problem. If there is no direct solution, what is the best pattern to adopt ?

---

EDIT: I don’t want to use a transaction on start() because in real application, the statefull bean is used to realize a complex entity model which need to be persist at once. I try to setup the pattern called session-per-conversation in described here http://docs.jboss.org/hibernate/core/4.0/manual/en-US/html/transactions.html#transactions-basics-apptx. So if I understand you right, the solution is to “use a transaction in start() method of bean B”, but if I do this, at the end of the method, the content is flushed to database and it’s not what I want.

Other solution I can see is to get MyEntity in the B’s EntityManager, so do a merge, or a em.find() or to delegate retrieveMethod to some DAO style class, using the em in parameter, and in bean A, do a simple delegation to the DAO, in bean B, call directly the DAO.

Any idea on what is the best approach ?

Answer 1

Here is the solution I used :

import java.lang.reflect.Field;

import javax.annotation.Resource;
import javax.interceptor.AroundInvoke;
import javax.interceptor.InvocationContext;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

public class SessionPerConversationInterceptor {
    private final static ThreadLocal<EntityManager> s_thEntityManager = new ThreadLocal<>();

    @AroundInvoke
public Object manageEntityManager(InvocationContext ctx) throws java.lang.Exception {
    EntityManager em = s_thEntityManager.get();
    if (em == null) {
        MasterPersistenceContext traversableEntityManager = ctx.getTarget().getClass().getAnnotation(MasterPersistenceContext.class);
        if (traversableEntityManager != null) {
            for (Field field : ctx.getTarget().getClass().getDeclaredFields()) {
                if (field.getAnnotation(PersistenceContext.class) != null) {
                    field.setAccessible(true);
                    em = (EntityManager) field.get(ctx.getTarget());
                    s_thEntityManager.set(em);

                    try {
                        Object oRet = ctx.proceed();
                        return oRet;
                    } finally {
                        s_thEntityManager.set(null);
                    }
                }
            }
        }
    } else if (ctx.getTarget().getClass().getAnnotation(MasterPersistenceContext.class) == null) {
        for (Field field : ctx.getTarget().getClass().getDeclaredFields()) {
            if (field.getAnnotation(PersistenceContext.class) != null) {
                field.setAccessible(true);

                EntityManager oldEntityManager = (EntityManager) field.get(ctx.getTarget());

                field.set(ctx.getTarget(), em);

                try {
                    Object oRet = ctx.proceed();
                    return oRet;
                } finally {
                    field.set(ctx.getTarget(), oldEntityManager);
                }
            }
        }
    }

    return ctx.proceed();
}
}

I hope it can help (and I hope this a not so ugly/dumb solution).