jpa on a Desktop SWING Application

问题:

I'm developping a mono user desktop application using SWING. I had a little experience with this kind of application on which i used the java.sql api and figured out that it wasn't confortable at all ...

In my new application i'm trying to use JPA for the first time, i've read a lot of tutorials which made me understand almost all what i need, but didn't find a good example for real java Desktop applications.

I'm thinking of using the following architecture, but don't know if i'm right ...

i think of creating a MyPersistenceUnit class :

    public class MyPersistenceUnit {
        private static EntityManagerFactory factory;
        private static EntityManager entityManager;

        public static void initiate(){
            factory=Persistence.createEntityManagerFactory("PU_Name");
            entityManager=factory.createEntityManager();
        }

        public static EntityManager getEntityManager() {
            return entityManager;
        }

        public static void close(){
            entityManager.close();
            factory.close();
        }

    }

the initiate() method will be the first to be called, and the close() method will be called when the application gets closed.

While the application is running all transactions will be done through the getEntityManager() instance, which is accessible every where in the application. If my understanding is right on JSE applications the obtained entity manager has an extended persistence context which will keep all the entities on the managed state while the entity manager doesn't get closed, and that's what made me think this way ...

I don't know if i'm missing something, so any tip will be appreciated

Note that i'm using eclipselink provider with the derby embedded database.
Thanks

回答1:

As I understand, the question boils down to whether you should open EntityManager and store its reference globally and access the same instance everywhere in the application.

I think that should be okay if your application is small to medium size. Just be cautious that database connection (hence session/entityManager) may drop due to various factors. And don't do this with transactions (ie dont open them in beginning and commit in end). Keep transactions as fine grained as possible.

There have been various discussion where more experienced people discussed about it, you can follow that here : for and counter argument on this SO question - Session management using Hibernate in a Swing application

Also see this on the same topic.

Here is a sample desktop application created by a committer of hibernate. Its bit old, you can get the idea.

And finally this is great article for understanding of general JPA concepts for desktop application.

回答2:

After re-thinking my design, i decided to change it as follows:

• create a 'permanent' EM when the application starts, and keep it open until the application shutdowns. permanentEM will be used to find/refresh entities when needed (for fetching lazy relationships for example ...).
  In order to ensure an efficient management of memory by the WEAK refrence-mode, i'll avoid to permanently reference permanentEM's managed entities.
• create a 'temporary' EM to load the 'permanent' data, that are necessary for the start of the application. Once the data loaded close that temporary EM, to detach all the loaded-in-memory data.
• create a new 'temporary' EM, for each persist/merge/remove transaction, and close it once the transaction commits.

回答3:

Since its a SE application, your may use the embedded database. I know H2 normally allow one thread visit, so if your are in the same developing environment and want to avoid the headache concurrence control of entity.

my suggestion is that encapsulate the your service call in one thread and assign this thread a entitymanager.

don't allow other threads to use entities , then define data access interface which can transfer data from entity to other objects in a proper data structure .

in one world, i think it is better to put all the entities in one entity manager domain and isolate them from other thread.

回答4:

have you looked at http://docs.jboss.org/hibernate/core/4.0/manual/en-US/html/transactions.html#transactions-basics-issues. I think it would be wise to read the documentation there before proceeding using sessions object for too much time. I guess the better approach is to use the session to do a simple unit of work (for example : the CRUD operations).

Hope it helps!!.

Greetings.