Jersey, Guice and Hibernate - EntityManager thread safety

Question

I have used this tutorial them same way in my application: http://www.benmccann.com/hibernate-with-jpa-annotations-and-guice/

My app is JAX-RS web service which will receive many concurrent requests and make updates to database.

GenericDAOImpl.java implementation:

public class GenericDAOImpl<T> implements GenericDAO<T> {

    @Inject
    protected EntityManager entityManager;

    private Class<T> type;

    public GenericDAOImpl(){}

    public GenericDAOImpl(Class<T> type) {
        this.type = type;
    }

    @Override
    public void save(T entity) {
        entityManager.getTransaction().begin();
        entityManager.persist(entity);
        entityManager.getTransaction().commit();
    }

}

If 2 concurrent threads try to save entity, I get

java.lang.IllegalStateException: Transaction already active

Saving works well if I comment transaction.

I have tried to use

@Inject
protected Provider<EntityManager> entityManagerProvider;

or

@Inject
protected EntityManagerFactory entityManagerProvider;

and for each request:

EntityManager entityManager = entityManagerProvider.get()

But then I get:

org.hibernate.PersistentObjectException: detached entity passed to persist

What is correct way to implement Guice + Hibernate EntityManager injection / thread-safe generic DAO class?

UPDATE

Andrew Rayner comment from http://www.benmccann.com/hibernate-with-jpa-annotations-and-guice/

"The logic isn’t really production ready – at least if used in a web app.

Hibernates connection pool is very basic and is not production ready – the recommendation is to use a datasource pool such as c3p0.

EntityManager shouldn’t be reused – it is intended to be created per transaction/request. There is a good chance of polluting subsequent requests.

There is also no transaction rollback if something goes wrong.

An interesting approach – but it would be much safer for webapps to use Guices own Persist extension module for managing the lifecycle of EntityMananger instances and transactions."

Answer 1

The problem was that my endpoint was annotated with @Singleton so it reused the same EntityManager during concurrent calls. After removing @Singleton, during concurrent calls, different EntityManager objects are used. If endpoint calls are subsequent, it may be that previous/old EntityManager will be used.

Highly simplified example:

@Path("/v1/items")
public class ItemsService {

    @Inject
    private EntityManager entityManager;

    @POST
    @Path("/{id}")
    @Consumes(MediaType.APPLICATION_JSON)
    @Produces(MediaType.APPLICATION_JSON)
    public void saveItem(){
         entityManager.getTransaction().begin();
         entityManager.persist(new Item());
         entityManager.getTransaction().commit();
    }
}