java不执行EntityManagerFactory。createEntityManager（）是否每次都重新创建新实例？

Question

执行EntityManagerFactory。createEntityManager（）是否每次都重新创建新实例？还是每次都返回同一EntityManager的缓存副本

Answer 1

EntityManagerFactory在每次调用EntityManagerFactory时返回EntityManager的新实例。createEntityManager（）

如果你执行

emf = Persistence.createEntityManagerFactory("basicPU");
 for (int i = 0 ; i<10; i++){
     System.out.println(em = emf.createEntityManager());
 }

它打印：

org.apache.openjpa.persistence.EntityManagerImpl@18105e8
org.apache.openjpa.persistence.EntityManagerImpl@9bad5a
org.apache.openjpa.persistence.EntityManagerImpl@91f005
org.apache.openjpa.persistence.EntityManagerImpl@1250ff2
org.apache.openjpa.persistence.EntityManagerImpl@3a0ab1
org.apache.openjpa.persistence.EntityManagerImpl@940f82
org.apache.openjpa.persistence.EntityManagerImpl@864e43
org.apache.openjpa.persistence.EntityManagerImpl@17c2891
org.apache.openjpa.persistence.EntityManagerImpl@4b82d2
org.apache.openjpa.persistence.EntityManagerImpl@179d854

Answer 2

这个Javadoc是明确的：

Create a new application-managed EntityManager. This method returns a new EntityManager instance each time it is invoked.

Answer 3

对于skaffman的第二个答案，这里是JPA1.0规范的摘录：

5.9.2 Provider Responsibilities

The Provider has no knowledge of the distinction between transaction-scoped and extended persistence contexts. It provides entity managers to the container when requested and registers for synchronization notifications for the transaction.

• When EntityManagerFactory.createEntityManager is invoked, the provider must create and return a new entity manager. If a JTA transaction is active, the provider must register for synchronization notifications against the JTA transaction.