public class TestClient {

   @Inject TestSFSB mySFSB;
   
   public void testIt() {
      
      TestEntity myEntity = new TestEntity();
      myEntity.setKey("key-that-exists-in-db"); // simulates user input that will violate a unique-constraint
      try {         
         mySFSB.store(myEntity);
      } catch (ApplicationException e) {
         try {
            System.out.println("Hey user we have a constraint violation, please fix your data!");
            myEntity.setKey("new key that does not exist in db"); //simulates fixed user input that would not violate
            mySFSB.store(myEntity);
         } catch (Exception e1) {
            // with the fixed data it ends here with
            // javax.ejb.EJBException: javax.persistence.PersistenceException:
            // org.hibernate.HibernateException: proxy handle is no longer valid
            e1.printStackTrace();
         }
      }
   }
}

@Stateful
public class TestSFSB {
   
   @PersistenceContext(type = PersistenceContextType.EXTENDED)
   EntityManager em;

    public TestEntity findMasterByPk(Long id) {
       TestEntity myEntity = em.find(TestEntity.class, id);
       return myEntity;
    }

    public void store(TestEntity myEntity) throws ApplicationException {
       
      if (em.contains(myEntity)) {
         em.persist(myEntity);
      } else if (myEntity.getId() != null && myEntity.getId() > 0L) {
         em.merge(myEntity);
      } else {
         em.persist(myEntity);
      }

       try {
         em.flush();
       } catch (PersistenceException e) {          
          Throwable cause1 = e.getCause();
          if (cause1 instanceof ConstraintViolationException) {
             // first time calling the method it comes here
             ConstraintViolationException cve = (ConstraintViolationException) cause1;
             throw new ApplicationException(cve.getErrorCode(), e);
          } else {
             // second time with fixed data it ends up here with
             // javax.persistence.PersistenceException: org.hibernate.HibernateException: proxy handle is no longer valid?
             throw e;
          }
       }
    }   
}