在JSE中使用JPA的EntityManager ThreadLocal模式

Question

我正在使用Struts 1.3 + JPA(Hibernate作为持久性提供程序)开发一个简单的"Book Store"项目.我无法切换到Spring或任何其他更复杂的开发环境(例如,Jboss),我不能使用任何特定于Hibernate的技术(例如,Session类).

鉴于我在JSE环境中,我需要明确管理整个EntityManager的生命周期.

该Book实体定义如下:

@Entity
public class Book {

@Id private String isbn;
private String title;
private Date publishDate;

    // Getters and Setters
}

我定义了三个Action类,分别负责检索所有书籍实例,通过其ISBN检索单个书籍实例并将分离的书籍合并到数据库中.

为了增加业务逻辑代码和数据访问代码之间的关注点分离,我引入了一个简单的BookDAO对象,负责执行CRUD操作.理想情况下,应将所有与数据访问相关的调用委托给持久层.例如,ListBookAction定义如下:

public class ListBookAction extends Action {

    private BookDAO dao = new BookDAO();

    @Override
    public ActionForward execute(ActionMapping mapping, ActionForm form,
            HttpServletRequest request, HttpServletResponse response)
            throws Exception {

        // Retrieve all the books
        List<Book> books = dao.findAll();

        // Save the result set
        request.setAttribute("books", books);

        // Forward to the view
        return mapping.findForward("booklist");
    }

}

BookDAO对象需要访问EntityManager实例才能执行任何操作.鉴于这EntityManger不是线程安全的,我引入了一个名为的辅助类BookUnitSession,它将EntityManager封装在一个ThreadLocal变量中:

public class BookUnitSession {

    private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("BookStoreUnit");
    private static final ThreadLocal<EntityManager> tl = new ThreadLocal<EntityManager>();

    public static EntityManager getEntityManager() {
        EntityManager em = tl.get();

        if (em == null) {
            em = emf.createEntityManager();
            tl.set(em);
        }
        return em;
    }

}

一切似乎都有效,但我仍有一些担忧.即:

1. 这个解决方案最好吗？这种情况下最好的做法是什么？
2. 我仍然需要明确地关闭EntityManager和EntityManagerFactory.我怎样才能做到这一点？

谢谢

Answer 1

在过去的几天里,我设计了一个可能的解决方案.我试图用这个BookUnitSession类构建的实际上是EntityManagerHelper类:

public class EntityManagerHelper {

    private static final EntityManagerFactory emf; 
    private static final ThreadLocal<EntityManager> threadLocal;

    static {
        emf = Persistence.createEntityManagerFactory("BookStoreUnit");      
        threadLocal = new ThreadLocal<EntityManager>();
    }

    public static EntityManager getEntityManager() {
        EntityManager em = threadLocal.get();

        if (em == null) {
            em = emf.createEntityManager();
            threadLocal.set(em);
        }
        return em;
    }

    public static void closeEntityManager() {
        EntityManager em = threadLocal.get();
        if (em != null) {
            em.close();
            threadLocal.set(null);
        }
    }

    public static void closeEntityManagerFactory() {
        emf.close();
    }

    public static void beginTransaction() {
        getEntityManager().getTransaction().begin();
    }

    public static void rollback() {
        getEntityManager().getTransaction().rollback();
    }

    public static void commit() {
        getEntityManager().getTransaction().commit();
    } 
}

这样的类确保每个线程(即每个请求)将获得自己的EntityManager实例.因此,每个DAO对象都可以EntityManager通过调用获得正确的实例EntityManagerHelper.getEntityManager()

根据每个请求的会话模式,每个请求必须打开和关闭其自己的EntityManager实例,该实例将负责在事务中封装所需的工作单元.这可以通过拦截过滤器来实现,该过滤器实现为ServletFilter:

public class EntityManagerInterceptor implements Filter {

    @Override
    public void destroy() {}

    @Override
    public void init(FilterConfig fc) throws ServletException {}

    @Override
    public void doFilter(ServletRequest req, ServletResponse res,
            FilterChain chain) throws IOException, ServletException {

            try {
                EntityManagerHelper.beginTransaction();
                chain.doFilter(req, res);
                EntityManagerHelper.commit();
            } catch (RuntimeException e) {

                if ( EntityManagerHelper.getEntityManager() != null && EntityManagerHelper.getEntityManager().isOpen()) 
                    EntityManagerHelper.rollback();
                throw e;

            } finally {
                EntityManagerHelper.closeEntityManager();
            }
    }
}

此方法还允许View(例如,JSP页面)获取实体的字段,即使它们已被延迟初始化(视图模式中的Open Session).在JSE环境中,EntityManagerFactory需要在关闭servlet容器时显式关闭.这可以通过使用ServletContextListener对象来完成:

public class EntityManagerFactoryListener implements ServletContextListener {

    @Override
    public void contextDestroyed(ServletContextEvent e) {
        EntityManagerHelper.closeEntityManagerFactory();
    }

    @Override
    public void contextInitialized(ServletContextEvent e) {}

}

在web.xml部署描述符:

<listener>
  <description>EntityManagerFactory Listener</description>
  <listener-class>package.EntityManagerFactoryListener</listener-class>
</listener>

<filter>
  <filter-name>interceptor</filter-name>
  <filter-class>package.EntityManagerInterceptor</filter-class>
</filter>

<filter-mapping>
  <filter-name>interceptor</filter-name>
  <url-pattern>*.do</url-pattern>
</filter-mapping>