Hibernate -每次操作ThreadLocal<EntityManager>与EntityManager的比较

Question

我使用Hibernate 5.1.0，Guice，Guice，泽西。我有HibernateModule，它创建EntityManagerFactory并管理EntityManager实例：

public class HibernateModule extends AbstractModule {

    private static final ThreadLocal<EntityManager> ENTITY_MANAGER_CACHE = new ThreadLocal<EntityManager>();

    @Provides @Singleton
    public EntityManagerFactory provideEntityManagerFactory(@Named("hibernate.connection.url") String url, 
        @Named("hibernate.connection.username") String userName, 
        @Named("hibernate.connection.password") String password,
        @Named("hibernate.hbm2ddl.auto") String hbm2ddlAuto,
        @Named("hibernate.show_sql") String showSql) {

        Map<String, String> properties = new HashMap<String, String>();
        properties.put("hibernate.connection.driver_class", "org.postgresql.Driver");
        properties.put("hibernate.connection.url", url);
        properties.put("hibernate.connection.username", userName);
        properties.put("hibernate.connection.password", password);
        properties.put("hibernate.connection.pool_size", "1");
        properties.put("hibernate.dialect", "org.hibernate.dialect.PostgreSQLDialect");
        properties.put("hibernate.hbm2ddl.auto", hbm2ddlAuto);
        properties.put("hibernate.show_sql", showSql);
        properties.put("hibernate.cache.use.query_cache", "false");
        properties.put("hibernate.cache.use_second_level_cache", "false");

        return Persistence.createEntityManagerFactory("db-manager", properties);
    }

    @Provides
    public EntityManager provideEntityManager(EntityManagerFactory entityManagerFactory) {

        EntityManager entityManager = ENTITY_MANAGER_CACHE.get();

        if (entityManager == null || !entityManager.isOpen()) 
            ENTITY_MANAGER_CACHE.set(entityManager = entityManagerFactory.createEntityManager());

        entityManager.clear();

        return entityManager;
    }
}

entityManager.clear()用于清除持久化上下文，并强制从dabatase中查询最新数据。GenericDAO从HibernateModule接收注入的EntityManager。主要方法：

public class GenericDAOImpl<T> implements GenericDAO<T> {

    @Inject
    protected EntityManager entityManager;

    private Class<T> type;

    public GenericDAOImpl(){}

    public GenericDAOImpl(Class<T> type) {
        this.type = type;
    }

    @Override
    public void save(T entity) {
        entityManager.getTransaction().begin();
        entityManager.persist(entity);
        entityManager.getTransaction().commit();
    }

    @Override
    public T find(Long entityId) {
        return (T) entityManager.find(type, entityId);
    }
}

在此之前，我曾尝试实现一种解决方案，为每个DB操作提供新的EntityManager，但它会产生一些副作用，比如“通过独立实体传递给持久化”。

从EntityManager重用ThreadLocal<EntityManager>是一个很好的实践吗？这个实现有什么潜在的缺点吗？

Answer 1

它应该能正常工作。Spring正在为ThreadLocal使用EntityManager类。参考资料：

Spring使创建会话和将会话透明地绑定到当前线程变得更加容易。

如果您想要重用PersistenceContextType实例，您应该记住JPA提示、FlushModeType和EntityManager。