JAVA: an EntityManager object in a multithread environment

Question

if I have multiple threads, each use injector to get the EntityManager object, each use the em object to select a list of other class objects. Ready to be used in a for loop.

If a thread finishes first and calls clear(), will that affect the other threads? Like the for loop will have exception?

How about close()?

If the answer is "It depends", what (class definition? method call?) and where (java code? annotation? xml?) should I look at to find out how is it depended?

I did not write the source, I am just using someone else's library without documentation.

Thank you.

Answer 1

Here is full working thread-safe Entity Manager Helper.

import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.Persistence;  public class EntityManagerHelper {      private static final EntityManagerFactory emf;     private static final ThreadLocal<EntityManager> threadLocal;      static {         emf = Persistence.createEntityManagerFactory("Persistent_Name");         threadLocal = new ThreadLocal<EntityManager>();     }      public static EntityManager getEntityManager() {         EntityManager em = threadLocal.get();          if (em == null) {             em = emf.createEntityManager();             // set your flush mode here             threadLocal.set(em);         }         return em;     }      public static void closeEntityManager() {         EntityManager em = threadLocal.get();         if (em != null) {             em.close();             threadLocal.set(null);         }     }      public static void closeEntityManagerFactory() {         emf.close();     }      public static void beginTransaction() {         getEntityManager().getTransaction().begin();     }      public static void rollback() {         getEntityManager().getTransaction().rollback();     }      public static void commit() {         getEntityManager().getTransaction().commit();     } } 

Answer 2

Entity managers are not thread-safe (source Java EE 6 tutorial) and cannot be shared among threads. Each thread needs to use its own entity manager or bad things will happen, regardless of clear() or close() calls.

But, if the injector is injecting each thread with its own entity manager then things should be OK.

Spring and possibly other DI frameworks will inject a ThreadLocal-based proxy for a real entity manager into your beans. Calls that each thread makes will proxy to the real thread-local instance of an entity manager - this is how things can work even though it might appear an entity manager is shared among multiple threads.

More details about how your entity manager is injected would help (Spring, etc.)