java - JPA中的事务实现

Question

我想了解什么是 JPA 中的事务。

假设我有一个持久性部署到 glassfish 的应用程序:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
  <persistence-unit name="survex" transaction-type="JTA">
    <jta-data-source>jdbc/MyDatabase</jta-data-source>
    <exclude-unlisted-classes>false</exclude-unlisted-classes>
    <properties>
    </properties>
  </persistence-unit>
</persistence>

我正在尝试调试对 MySQL 服务器的代码嗅探请求:

public String create() {

        EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("survex");
        EntityManager em = entityManagerFactory.createEntityManager();
        Log log = em.find(Log.class, 1); 

        EntityTransaction transaction = em.getTransaction();
        transaction.begin(); // NOTING HAPPENS ...
        log.setMessage("xxx");  
        em.flush(); // OH, HERE IT IS -> SET autocommit=0; UPDATE LOG SET message = 'xxx' WHERE (id = 1)
        log.setMessage("aaa");
        em.flush(); // OK -> UPDATE LOG SET message = 'aaa' WHERE (id = 1)
        transaction.commit(); // NOTHING HAPPENS! WHERE IS COMMIT?!

        log = em.find(Log.class, 1);

        transaction = em.getTransaction();
        transaction.begin(); // STILL NOTHING ...
        log.setMessage("555"); 
        em.flush(); // UPDATE LOG SET message = '555' WHERE (id = 1)
        log.setMessage("666");
        em.flush(); // UPDATE LOG SET message = '666' WHERE (id = 1)
        transaction.commit(); // NOTHING!!!

        return log.getMessage();
    }
    // SOMEWHERE OUTSIDE: COMMIT; SET autocommit=1;

为什么？!为什么 JPA 的行为如此奇怪？

我希望每次调用 transaction.commit() 时都提交，或者我误解了什么？

Answer 1

我在您的代码中看到一个问题:您将 PersistenceContext 定义为 JTA，但使用 RESOURCE_LOCAL 事务 API(即 EntityManager.getTransaction())。

当您处于 Java EE 环境中时，只需获取 UserTransaction 的注入(inject)实例:@Resource private UserTransaction transaction; 和 EntityManager 而不是创建一个。您还应该禁用 CMT(容器管理)事务，否则它们会自动打开/关闭。