同步对JPA EntityManager的访问 [英] Synchronize access to JPA EntityManager

问题描述

我使用Spring，Hibernate和JPA Entity Manager。当同时获取UI的不同区域（通过jquery / ajax）时，我想同步对JPA实体管理器的访问，否则会导致对共享异常的共享引用。我如何实现这一目标？我有一个控制器，调用后端的服务方法，负责调用entitymanager上的get / save等。

• 实体管理器： entityManager.lock（entity，LockModeType.OPTIMISTIC） ;


• 查询： query.setLockMode（LockModeType.OPTIMISTIC）;


• NamedQuery：lockMode元素
  

锁定模式 OPTIMISTIC 与 READ & OPTIMISTIC_FORCE_INCREMENT 至写入。

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编辑：从文档

如果事务T1调用LockModeType类型的锁定。 OPTIMISTIC对
a版本化对象，实体管理器必须确保
以下现象都不会发生：

• < P1>（脏读）：事务T1修改一行。另一个事务T2然后读取该行并获取修改后的值，然后T1将
  提交或回滚。事务T2最终成功地提交
  ; T1是否提交或回滚以及
  是否在T2提交之前或之后执行都是无关紧要的。



• P2（不可重复读取） ：事务T1读取一行。在T1有
  提交之前，另一个事务T2会修改或删除该行。这两个事务最终成功提交。
  

锁定模式必须始终阻止现象P1和P2。

I am using Spring, Hibernate and JPA Entity Manager. When fetching different regions of the UI concurrently (via jquery/ajax) , I would like to synchronise access to JPA Entity Manager, as otherwise it results in shared references to collections exception. How do I achieve this ? I have a controller that calls service methods in the backend which are responsible for calling get/save etc on entitymanager.

解决方案

You can apply lock on the object with specific mode.

• Entity Manager : entityManager.lock(entity, LockModeType.OPTIMISTIC);
• Query : query.setLockMode(LockModeType.OPTIMISTIC);
• NamedQuery : lockMode element

Lock mode OPTIMISTIC is synonymous to READ & OPTIMISTIC_FORCE_INCREMENT to WRITE.

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Edit : From Documentation

If transaction T1 calls for a lock of type LockModeType.OPTIMISTIC on a versioned object, the entity manager must ensure that neither of the following phenomena can occur:

• P1 (Dirty read): Transaction T1 modifies a row. Another transaction T2 then reads that row and obtains the modified value, before T1 has committed or rolled back. Transaction T2 eventually commits successfully; it does not matter whether T1 commits or rolls back and whether it does so before or after T2 commits.

• P2 (Non-repeatable read): Transaction T1 reads a row. Another transaction T2 then modifies or deletes that row, before T1 has committed. Both transactions eventually commit successfully.

Lock modes must always prevent the phenomena P1 and P2.

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