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In JEE managed mode, such as an EntityManager injected into a SessionBean, the EntityManager reference, represents a new persistence context for each transaction. This means objects read in one transaction become detached after the end of the transaction, and should no longer be used, or need to be merged into the next transaction. In managed mode, you never create or close an EntityManager.

do not support nested transactions

Example of injecting an EntityManager and EntityManagerFactory in a SessionBean

@Stateless

@PersistenceContext(unitName="acme") private EntityManager entityManager;