Guice Persist是否提供事务范围的或应用程序管理的EntityManager? [英] Does Guice Persist provide transaction scoped or application managed EntityManager?

问题描述

我们使用 Guice Persist 将EntityManager注入我们的项目. /p>

例如

public class MyDao{
   @Inject
   EntityManager em;

   public void someMethod(){
       //uses em instance
   }
}

但是我们不清楚如何使用注入的EntityManager实例.

1. 这是什么类型的EntityManager?(请参见例如:实体管理器的类型)，Guice Persist在后台通过EntityManagerFactory.createEntityManager()实例化它，所以我想说它是应用程序管理的实体管理器.但是在官方Wiki 中，他们写了关于每次交易观看次数策略，这表明EntityManager是(伪)事务作用域的.
2. 我们应该手动在其上调用close()吗?还是Guice会照顾好它?
3. 一级缓存的范围是什么?仅单个事务(例如在事务范围的实体管理器中)还是只要我使用相同的EntityManager注入实例(例如在应用程序管理的实体管理器中)?

解决方案

我对Guice-persist的源代码进行了一些研究，并阅读了Guice-persist Wiki页面下的注释，这些是我需要的答案:

1.如果通过@Inject EntityManager进行注入，则EntityManager的生命周期管理将被破坏.如Wiki上的评论之一所述:

我确认直接注入EntityManager而不是提供者 可能很危险.如果您不在UnitOfWork或方法中 用@Transaction注释，这是EntityManager的第一次注入 在一个线程中将创建一个新的EntityManager，从不销毁它，并且 始终为此线程使用此特定EntityManager(存储了EM 线程本地).这可能会导致可怕的问题，例如注入 死了的entityManager(连接已关闭等)，所以我的建议是 总是注入提供者，或者至少注入 仅在打开的UnitOfWork内部直接使用EntityManager.

所以我所问的例子不是最正确的用法.它创建EntityManager的单例实例(每个线程)，并将该实例注入到各处:-(.

但是，如果我注入了Provider并在@Transactional方法中使用了Provider，则EntityManager的实例将是按交易的.因此，此问题的答案是:如果正确注入和使用，则实体管理器将具有事务作用域.

2. 如果注射和使用正确，则无需手动关闭实体管理器(guice-persist会为我处理).如果使用不当，手动关闭将是一个非常糟糕的主意(当我@Inject EntityManager时，将在每个位置注入EntityManager的关闭实例)

3. 如果正确注入和使用，则L1缓存的范围为单笔交易.如果使用不正确，则L1缓存的范围是应用程序的生命周期(EntityManager为单例)

We use Guice Persist to inject EntityManager in our project.

E.g.

public class MyDao{
   @Inject
   EntityManager em;

   public void someMethod(){
       //uses em instance
   }
}

But it is unclear for us how injected instance of EntityManager is about to be used.

1. What type of EntityManager is this? (see e.g.: types of entity managers) Under the hood Guice Persist instantiates it via EntityManagerFactory.createEntityManager() so I'd say it's application-managed entity manager. But in official Wiki they write about seesion-per-transaction strategy, which suggests that EntityManager is (pseudo) transaction-scoped.
2. Should we invoke close() on it manually? Or Guice will take care of it?
3. What is the scope of first level cache? Only single transaction (like in transaction-scoped entity managers) or as long as I use the same injected instance of EntityManager (like in application managed entity managers)?

解决方案

I did some research of the source code of Guice-persist and read through comments under Guice-persist wiki pages and these are the answers that I needed:

1 . Lifecycle management of EntityManager is kind of broken if it's injected via @Inject EntityManager. As stated in one of the comments on the Wiki:

I confirm that inject directly an EntityManager instead of a provider can be dangerous. If you're not inside a UnitOfWork or a method annotated with @Transaction, the first injection of an EntityManager in a thread will create a new EntityManager, never destroy it, and always use this specific EntityManager for this thread (EM are stored thread-local). This can lead to terrible issues, like injection of dead entityManager (connection closed, etc) So my recommendation if to always inject a Provider, or at least to inject directly an EntityManager only inside an opened UnitOfWork.

So example in my question isn't the most correct usage. It creates singleton instance of EntityManager (per-thread) and will inject this instance everywhere :-(.

However if I've injected Provider and used it inside @Transactional method then the instance of EntityManager would be per-transaction. So the answer to this question is: if injected and used correctly, the entity manager is transaction-scoped.

2 . If injected and used correctly then I don't need to manualy close entity manager (guice-persist will handle that for me). If used incorrectly, closing manually would be very bad idea (closed instance of EntityManager would be injected in every place when I @Inject EntityManager )

3 . If injected and used correctly then the scope of L1 cache is single transaction. If used incorrectly, the scope of the L1 cache is the lifetime of application (EntityManager is singleton)

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