spring-boot - 为什么 Spring Data JPA 为共享 EntityManager 调用创建新的 EntityManager？

Question

我有一个使用 spring boot + spring data JPA 的示例项目。在日志中，我观察到 EntityManagers 在我进行第一次 rest 调用之前被创建了几次。请澄清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，这就是我发现的。
在 Spring 的许多情况下，它被用作 EntityManager 的代理。领域。我想这样做的原因是据说代理是线程安全 与 EntityManager 的实际实现相比接口(interface)其中不是线程安全的 .
例如 CrudRepository 的默认实现界面 -> SimpleJpaRepository具有类型 EntityManager 的字段.在大多数情况下，我们只想扩展 CrudRepository接口(interface)并将其作为单例 bean 注入(inject)某处。由于它是单例 EntityManager必须是 线程安全 因为可以从多个线程调用存储库。
在 SharedEntityManagerCreator 的文档中类，我们可以看到这个代理是如何真正工作的。

A shared EntityManager will behave just like an EntityManager fetched froman application server's JNDI environment, as defined by the JPA specification.It will delegate all calls to the current transactional EntityManager, if any;otherwise it will fall back to a newly created EntityManager per operation.

这意味着如果创建了 EntityManager (Session)，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 (Session)。
当我们启动 Spring Boot 应用程序时，Spring 会调用一些 EntityManager s 但这些 EntityManager s 是代理，因为我们没有创建真正的 EntityManager (Session)，它会为我们创建一个并记录您要求的这些消息。
所以在你的情况下，这些共享 EntityManager s 的创建可能是因为您定义了 Repository项目中的接口(interface)和初始化期间会调用 EntityManager (代理)在这些存储库中。