Hibernate ThreadLocal

Hibernate有很多值得学习的地方，这里我们主要介绍Hibernate ThreadLocal，包括介绍Hibernate官方开发手册标准示例等方面。

Hibernate ThreadLocal

它会为每个线程维护一个私有的变量空间。实际上， 其实现原理是在JVM 中维护一个Map，这个Map的key 就是当前的线程对象，而value则是 线程通过Hibernate ThreadLocal.set方法保存的对象实例。当线程调用Hibernate ThreadLocal.get方法时， Hibernate ThreadLocal会根据当前线程对象的引用，取出Map中对应的对象返回。

这样，Hibernate ThreadLocal通过以各个线程对象的引用作为区分，从而将不同线程的变量隔离开来。

Hibernate官方开发手册标准示例:

    public class HibernateUtil {       private static SessionFactory sessionFactory;      static {       try {       // Create the SessionFactory sessionFactory = new Configuration().    configure().buildSessionFactory();      }       catch (HibernateException ex) {       throw new RuntimeException( "Configuration problem: " + ex.getMessage(), ex );      }       }       public static final ThreadLocal session = new ThreadLocal();      public static Session currentSession() throws HibernateException {       Session s = (Session) session.get();      // Open a new Session, if this Thread has none yet if (s == null) {       s = sessionFactory.openSession();      session.set(s);      }       return s;      }       public static void closeSession() throws HibernateException {       Session s = (Session) session.get();      session.set(null);      if (s != null) s.close();      }       } 

通过filter实现session的重用：

    public class PersistenceFilter implements Filter {       protected static ThreadLocal hibernateHolder = new ThreadLocal();      public void doFilter(ServletRequest request,     ServletResponse response, FilterChain chain)     throws IOException,ServletException {       hibernateHolder.set(getSession());      try {       ……       chain.doFilter(request, response);      ……       }       finally {       Session sess = (Session)hibernateHolder.get();      if (sess != null) { hibernateHolder.set(null);      try { sess.close(); } catch (HibernateException ex) {       throw new ServletException(ex);      }       }       }       }       ……      }