并行运行多个 JPA 事务
我有两个（或更多）Java 线程使用 JPA 从 mysql 数据库创建、更新和删除实体。为了实现这一目标，我有一个 PersistenceLayer 类，它创建 EntityManager 并为所有实体提供保存、更新和删除方法，如下所示：
public void saveEntity(Entity entity) {
manager.getTransaction().begin();
manager.persist(entity);
manager.getTransaction().commit();
}
public void saveEntity2(Entity2 entity) {
manager.getTransaction().begin();
manager.persist(entity);
manager.getTransaction().commit();
}
如果一个线程同时进入 saveEntity 操作，另一个线程同时进入 saveEntity2 操作，则两个线程都会尝试从EntityManager 将会失败。然而，我一直认为底层数据库能够管理多个事务，特别是当两个事务都在不同的行甚至不同的表上工作时。当然，我可以同步这些块，但这意味着一次只能有一个数据库连接，这将无法扩展到创建多个线程的多个用户。
我在这里做了什么错误的假设？是否可以通过JPA向数据库提交多个事务并让DB处理并发问题？
I have two (or more) Java Threads creating, updating and deleting entities from a mysql database using JPA. To achieve this I have a PersistenceLayer class creating the EntityManager and providing save, update and delete methods for all my entities looking like:
public void saveEntity(Entity entity) {
manager.getTransaction().begin();
manager.persist(entity);
manager.getTransaction().commit();
}
public void saveEntity2(Entity2 entity) {
manager.getTransaction().begin();
manager.persist(entity);
manager.getTransaction().commit();
}
If one thread enters operation saveEntity and the other saveEntity2 at the same time, both will try to get the transaction from the EntityManager wich will fail. However I always thought that the underlying database is able to manage multiple transactions, especially if both are working on different rows or even different tables. Of course I could synchronize the blocks, but that would mean only one DB connection is possible at a time which will not scale to multiple users creating several threads.
What wrong assumption am I doing here? Is it possible to submit multiple transactions to a database via JPA and let the DB handle concurrency issues?
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EntityManager不适合由多个线程使用。您需要为每个线程获取单独的 EntityManager 实例。实际上，如果您使用 EJB 或 Spring，您可以使用事务范围的 EntityManager，它可以在多个线程中使用（它是一个代理，它将实际工作委托给单独的线程绑定实例 EntityManager ），但我认为这不是你的情况。
EntityManageris not intended to be used by multiple threads. You need to obtain separate instances ofEntityManagerfor each thread.Actually, if you use EJB or Spring you can use a transaction-scoped
EntityManager, which can be used from multiple threads (it's a proxy which delegates actual work to separate thread-bound instances ofEntityManager), but I think it's not your case.为了处理跨多个事务处理对象时的并发问题，可以在对象上获取锁。
乐观或悲观锁定都可以与适当的锁定模式一起使用。通过
entityManager.lock(entity, LockModeType.READ) 锁定版本化对象将确保防止任何脏读。 不可重复读取。LockModeType.WRITE将强制增加/更新实体的版本列。To handle concurrency issues when the object is being processed accross multiple transactions, lock can be acquired on the object. Either
OptimisticorPessimisticlocking can be used with appropriate lock-modes.Locking the versioned object by
entityManager.lock(entity, LockModeType.READ)will ensure that it will prevent any Dirty Read & Non-Repeatable Read.LockModeType.WRITEwill force incrementing/updating the version column of the entity.