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

Answer 1

There are two types of managing EntityManager: container managed and application managed. For application managed, the preferred method of obtaining EntityManager is through EntityManagerFactory. Java EE tutorial says this:

Container-Managed Entity Managers

With a container-managed entity manager, an EntityManager instance’s persistence context is automatically propagated by the container to all application components that use the EntityManager instance within a single Java Transaction API (JTA) transaction.

JTA transactions usually involve calls across application components. To complete a JTA transaction, these components usually need access to a single persistence context. This occurs when an EntityManager is injected into the application components by means of the javax.persistence.PersistenceContext annotation. The persistence context is automatically propagated with the current JTA transaction, and EntityManager references that are mapped to the same persistence unit provide access to the persistence context within that transaction. By automatically propagating the persistence context, application components don’t need to pass references to EntityManager instances to each other in order to make changes within a single transaction. The Java EE container manages the lifecycle of container-managed entity managers.

To obtain an EntityManager instance, inject the entity manager into the application component:

@PersistenceContext 
EntityManager em; 

Application-Managed Entity Managers

With an application-managed entity manager, on the other hand, the persistence context is not propagated to application components, and the lifecycle of EntityManager instances is managed by the application.

Application-managed entity managers are used when applications need to access a persistence context that is not propagated with the JTA transaction across EntityManager instances in a particular persistence unit. In this case, each EntityManager creates a new, isolated persistence context. The EntityManager and its associated persistence context are created and destroyed explicitly by the application. They are also used when directly injecting EntityManager instances can’t be done because EntityManager instances are not thread-safe. EntityManagerFactory instances are thread-safe.

http://docs.oracle.com/javaee/6/tutorial/doc/bnbqw.html