问题描述
在少数项目中，我已成功使用
In few project I have been successfully using
@PersistenceUnit(unitName = "MiddlewareJPA")
EntityManagerFactory emf;
...
EntityManager entityManager = emf.createEntityManager();
获取数据库连接的 EntityManager ，但是几天前我试图将我的项目移动到 Jboss EAP 6.2 ，它无法创建 EntityManager 。我在谷歌搜索它，我发现我应该尝试将 @PersistenceUnit 改为
to obtain EntityManager for Database connection, but some days ago I was trying to move my project to Jboss EAP 6.2 and it couldn't create EntityManager. I was googling it and I found that I should try change @PersistenceUnit to
@PersistenceContext(unitName = "MiddlewareJPA")
private EntityManager entityManager;
获取 EntityManager 。它工作但我不知道为什么。 PersistenceUnit 和 PersistenceContext 之间有什么区别？每个人的利弊是什么？我们应该在哪里使用其中一个？
to obtain EntityManager. It worked but I don't know why. What is the difference bettween PersistenceUnit and PersistenceContext? What are pros and cons of each one? Where should we be using one of them?
推荐答案
我不知道它在Java EE中是如何工作的，但在Spring中，当你指定<$ c $时c> @PersistenceContext 注释，它注入 EntityManager 。它在哪里 EntityManager ？通过调用 EntityManagerFactory.createEntityManager（）为整个应用程序生命周期创建一个 EntityManager 是错误的。因此，使用并直接实例化 EntityManager 接口的特殊实现。它有一个内部可变线程本地引用 real EntityManager 。方法的实现只是将调用重定向到此真实 EntityManager 。并且有一个servlet监听器，在每个请求之前通过调用 EMF.createEntityManager（）获取 EM 并将其分配给特殊 EM 的内部引用。此侦听器还通过调用 getTransaction（）。begin（）， .commit（）和<$ c $来管理事务。 real EM 上的c> .rollback（）。这是对已完成工作的非常简化的描述。而且我相信，JEE容器的功能和Spring一样。
I don't know how it works exactly in the Java EE, but in Spring, when you specify @PersistenceContext annotation, it injects EntityManager. Where does it get EntityManager? It is wrong to create one EntityManager for the whole application lifetime by calling EntityManagerFactory.createEntityManager(). So instead a special implementation of EntityManager interface is used and instantiated directly. It has an internal mutable thread-local reference to a real EntityManager. Implementations of methods just redirect calls to this real EntityManager. And there is a servlet listener, that before each request obtain EM by calling EMF.createEntityManager() and assign it to that inner reference of special EM. Also this listener manages transactions by calling getTransaction().begin(), .commit() and .rollback() on the real EM. It is very simplified description of performed work. And I believe, that JEE container does the same thing, as Spring does.
一般情况下，最好注入 EntityManager ，因为 EntityManagerFactory 和 @PersistenceUnit 你应该创建/销毁 EntityManager 每次都是手工管理交易。
In general case it is better to inject EntityManager, because with EntityManagerFactory and @PersistenceUnit you should create/destroy EntityManager every time by hands and manage transactions too.
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