I have a java EE project using JPA (transaction-type="JTA"), hibernate as provider. I write my beans to handle the CRUD things. The program running in JBOSS 7 AS.

I have an EntityManagerDAO :

@Stateful
public class EntityManagerDao implements Serializable {

    @PersistenceContext(unitName = "dtdJpa")
    private EntityManager entityManager;

    @TransactionAttribute(TransactionAttributeType.REQUIRED)
    public Object updateObject(Object object) {
        object = entityManager.merge(object);
        return object;
    }

    @TransactionAttribute(TransactionAttributeType.REQUIRED)
    public void createObject(Object object) {
        entityManager.persist(object);
    }

    public void refresh(Object object) {
        entityManager.refresh(object);
    }

    public <T> T find(Class<T> clazz, Long id) {
        return entityManager.find(clazz, id);
    }

    @TransactionAttribute(TransactionAttributeType.REQUIRED)
    public void deleteObject(Object object) {
        entityManager.remove(object);
    }
}

but when I invoke deleteObject, this exception comes out.

java.lang.IllegalArgumentException: Removing a detached instance com.test.User#5

How is this caused and how can I solve it?

EntityManager#remove() works only on entities which are managed in the current transaction/context. In your case, you're retrieving the entity in an earlier transaction, storing it in the HTTP session and then attempting to remove it in a different transaction/context. This just won't work.

You need to check if the entity is managed by EntityManager#contains() and if not, then make it managed it EntityManager#merge().

Basically, the delete() method of your business service class should look like this:

em.remove(em.contains(entity) ? entity : em.merge(entity));