Should EntityManaer be Closed in a Desktop Application Environment

Question

Hi All,
I'm developing a desktop application with persistence api to connection to database. The application consists of two simple classes Order and Customer, where Customer is used as a field in Order. There is a ManyToOne relationship from Order to Customer and a OneToMany relationship from Customer to Order.
I have another class DBManager for managing all the database activities. All the methods in the class will create an entityManager and close the entityManager once they finish their own operation. An example method is as following:
public void addOrder(Order order) throws Exception
        try {
            entityManager = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT).createEntityManager();
            entityManager.getTransaction().begin();           
            entityManager.persist(order);
            entityManager.getTransaction().commit();
        } finally {
            if (entityManager.isOpen())
                entityManager.close();
    }As all entity instances will become detached once the entity manager is close. If I execute the above method, I'll receive an error saying that "Cannot persist detached object.", where the object refers to the Customer object stored in the Order.
I found that there are two ways to fix this problem, I can either merge the Customer object back to the context as following:
public void addOrder(Order order) throws Exception
        try {
            entityManager = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT).createEntityManager();
            entityManager.getTransaction().begin();
            order.setCustomer(entityManager.merge(order.getCustomer()));
            entityManager.persist(order);
            entityManager.getTransaction().commit();
        } finally {
            if (entityManager.isOpen())
                entityManager.close();
    }or I can leave the entityManager unclosed after any operation as following:
public void addOrder(Order order) throws Exception
            entityManager = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT).createEntityManager();
            entityManager.getTransaction().begin();           
            entityManager.persist(order);
            entityManager.getTransaction().commit();
    }Both ways would work but I'm not sure which one is the best practice to handle the situation. I know that in EJB environment the entityManager should never be closed, but according to article http://java.sun.com/developer/technicalArticles/J2SE/Desktop/persistenceapi/, entityManager should always be closed when it is no longer needed. I'm therefore very confused about which way to go. Can anyone please give me some advice on that?
Thanks in advance. You help would be greatly appreciated.
Edited by: EWL on Feb 18, 2008 4:37 PM

Answer 1

There are a couple of combining usage issues that are providing the problem. The issue is not specifically with you closing the EM but that combined with your usage of the persist() api. This test case would have the same issue in an EJB environment even if you were not closing the EM.
When using the persist() api you are depending on the behaviour that managed objects are ignored by the persist when infact you should be using multiple APIs depending on the state of the Entities that your application is interacting with.
The best approach is to interact with the EM within your application code. This allows you to share the same EM across all of a particular business call or logical application transaction. This could be wrapped within a DBManager but you would need to expose the concept of transactions to the application. This ensures you can track the lifecycle state of the Entities involved while allowing you to easily manage the EM resource.
If your architecture requires the DBManager pattern you have developed then you should only use persist() on Entities that you know to be new and merge() otherwise. In the example you have provided you could simply swap out persist() for merge() in your addOrder method (without the additional customer merge) and the code should work as you expect allowing you to close the EM.
You will need to close() or clear() the EM at some point within your application. Otherwise you are sure to run out of memory.
--Gordon