为什么Spring要为共享的EntityManager调用创建新的EntityManager？

Question

我有一个使用spring +的示例项目。在日志中，我观察到在进行第一个rest调用之前，EntityManagers被创建了几次。请说明为什么会发生这种情况。非常感谢。

Initialized JPA EntityManagerFactory for persistence unit 'ENTITY_MGR'
SpringBoot --> 10:05:40 8766 DEBUG org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler [main] - Creating new EntityManager for shared EntityManager invocation
SpringBoot --> 10:05:41 8963 DEBUG org.hibernate.stat.internal.StatisticsInitiator [main] - Statistics initialized [enabled=true]
SpringBoot --> 10:05:41 8977 TRACE org.hibernate.internal.SessionImpl [main] - Opened Session [f35fe4ac-8ce1-46b8-90dc-e456c07015c9] at timestamp: 15339099411
SpringBoot --> 10:05:41 8980 DEBUG org.springframework.orm.jpa.EntityManagerFactoryUtils [main] - Closing JPA EntityManager
SpringBoot --> 10:05:41 8982 TRACE org.hibernate.internal.SessionImpl [main] - Closing session [f35fe4ac-8ce1-46b8-90dc-e456c07015c9]
SpringBoot --> 10:05:41 8982 INFO  org.hibernate.engine.internal.StatisticalLoggingSessionEventListener [main] - 
SpringBoot --> 10:05:41 8983 TRACE org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl [main] - Releasing JDBC resources
SpringBoot --> 10:05:41 8983 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Closing logical connection
SpringBoot --> 10:05:41 8983 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Logical connection closed
SpringBoot --> 10:05:41 8985 DEBUG org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler [main] - Creating new EntityManager for shared EntityManager invocation
SpringBoot --> 10:05:41 8986 TRACE org.hibernate.internal.SessionImpl [main] - Opened Session [26018255-8491-4014-98ad-fbce0d3b5109] at timestamp: 15339099412
SpringBoot --> 10:05:41 8986 DEBUG org.springframework.orm.jpa.EntityManagerFactoryUtils [main] - Closing JPA EntityManager
SpringBoot --> 10:05:41 8986 TRACE org.hibernate.internal.SessionImpl [main] - Closing session [26018255-8491-4014-98ad-fbce0d3b5109]
SpringBoot --> 10:05:41 8986 INFO  org.hibernate.engine.internal.StatisticalLoggingSessionEventListener [main] - 
SpringBoot --> 10:05:41 8986 TRACE org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl [main] - Releasing JDBC resources
SpringBoot --> 10:05:41 8986 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Closing logical connection
SpringBoot --> 10:05:41 8986 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Logical connection closed
SpringBoot --> 10:05:41 9069 DEBUG org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler [main] - Creating new EntityManager for shared EntityManager invocation
SpringBoot --> 10:05:41 9069 TRACE org.hibernate.internal.SessionImpl [main] - Opened Session [4d3e7b5f-46fe-436f-9526-7ba62e332846] at timestamp: 15339099412
SpringBoot --> 10:05:41 9069 DEBUG org.springframework.orm.jpa.EntityManagerFactoryUtils [main] - Closing JPA EntityManager
SpringBoot --> 10:05:41 9069 TRACE org.hibernate.internal.SessionImpl [main] - Closing session [4d3e7b5f-46fe-436f-9526-7ba62e332846]
SpringBoot --> 10:05:41 9069 INFO  org.hibernate.engine.internal.StatisticalLoggingSessionEventListener [main] - 
SpringBoot --> 10:05:41 9070 TRACE org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl [main] - Releasing JDBC resources
SpringBoot --> 10:05:41 9070 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Closing logical connection
SpringBoot --> 10:05:41 9070 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Logical connection closed
SpringBoot --> 10:05:41 9248 DEBUG org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler [main] - Creating new EntityManager for shared EntityManager invocation
SpringBoot --> 10:05:41 9248 TRACE org.hibernate.internal.SessionImpl [main] - Opened Session [1b5f5936-fa7f-42cd-831c-78570ace0f58] at timestamp: 15339099414
SpringBoot --> 10:05:41 9248 DEBUG org.springframework.orm.jpa.EntityManagerFactoryUtils [main] - Closing JPA EntityManager
SpringBoot --> 10:05:41 9248 TRACE org.hibernate.internal.SessionImpl [main] - Closing session [1b5f5936-fa7f-42cd-831c-78570ace0f58]
SpringBoot --> 10:05:41 9249 INFO  org.hibernate.engine.internal.StatisticalLoggingSessionEventListener [main] - 
SpringBoot --> 10:05:41 9249 TRACE org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl [main] - Releasing JDBC resources
SpringBoot --> 10:05:41 9249 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Closing logical connection
SpringBoot --> 10:05:41 9249 TRACE org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl [main] - Logical connection closed

Answer 1

我想知道同样的事情，下面是我所发现的。

在许多情况下，在Spring中，它被用作EntityManager字段的代理。我认为这样做的原因是，与接口的实际实现相比，代理被认为是线程安全的，而并不是线程安全的。

例如，CrudRepository接口-> SimpleJpaRepository的默认实现具有EntityManager类型的字段。在大多数情况下，我们只想扩展CrudRepository接口，并将其作为单例bean注入到某个地方。由于它是一个单例，所以EntityManager必须是线程安全的，因为存储库可以从多个线程调用。

在SharedEntityManagerCreator类的文档中，我们可以看到这个代理是如何工作的。

按照JPA规范的定义，共享EntityManager的行为就像从应用服务器的JNDI环境中获取的EntityManager一样。它将将所有调用委托给当前事务EntityManager (如果有的话)；否则，它将返回到每个操作新创建的EntityManager。

这意味着，如果创建了EntityManager (会话)，代理将使用它，如果没有，它将根据需要为您创建一个新的EntityManager (会话)。

当我们启动Spring应用程序时，Spring会调用一些EntityManager，但是这些EntityManager是代理，因为我们没有创建真正的EntityManager (会话)，所以它会为我们创建一个，并记录您所请求的这些消息。

因此，在您的示例中，这些共享EntityManager的创建可能是因为您在项目中定义了Repository接口，并且在初始化期间，在这些存储库中有对EntityManager (代理)的调用。