open session in view 研究

在没有使用Spring提供的Open Session In View情况下，因需要在service(or Dao)层里把session关闭，所以lazy loading 为true的话，要在应用层内把关系集合都初始化，如 company.getEmployees()，否则Hibernate抛session already closed Exception;    Open Session In View提供了一种简便的方法，较好地解决了lazy loading问题.     
    它有两种配置方式OpenSessionInViewInterceptor和OpenSessionInViewFilter(具体参看SpringSide)，功能相同，只是一个在web.xml配置，另一个在application.xml配置而已。     
     Open Session In View在request把session绑定到当前thread期间一直保持hibernate session在open状态，使session在request的整个期间都可以使用，如在View层里PO也可以lazy loading数据，如 ${ company.employees }。当View 层逻辑完成后，才会通过Filter的doFilter方法或Interceptor的postHandle方法自动关闭session。 
     OpenSessionInViewInterceptor配置 

<beans>
<bean name="openSessionInViewInterceptor" class="org.springframework.orm.hibernate3.support.OpenSessionInViewInterceptor">
<property name="sessionFactory">
<ref bean="sessionFactory"/>
</property>
</bean>
<bean id="urlMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
<property name="interceptors">
<list>
<ref bean="openSessionInViewInterceptor"/>
</list>
</property>
<property name="mappings">
...
</property>
</bean> ... </beans>

OpenSessionInViewFilter配置

<web-app>
...
<filter>
<filter-name>hibernateFilter</filter-name>
<filter-class> org.springframework.orm.hibernate3.support.OpenSessionInViewFilter </filter-class>
<!-- singleSession默认为true,若设为false则等于没用OpenSessionInView -->
<init-param>
<param-name>singleSession</param-name>
<param-value>true</param-value>
</init-param>
</filter> ... <filter-mapping>
<filter-name>hibernateFilter</filter-name>
<url-pattern>*.do</url-pattern>
</filter-mapping> ... </web-app>

    很多人在使用OpenSessionInView过程中提及一个错误： 
org.springframework.dao.InvalidDataAccessApiUsageException: Write operations are not allowed in read-only mode (FlushMode.NEVER) - turn your Session into FlushMode.AUTO or remove 'readOnly' marker from transaction definition 
    看看OpenSessionInViewFilter里的几个方法

protected void doFilterInternal(HttpServletRequest request,
        HttpServletResponse response,
        FilterChain filterChain) throws ServletException, IOException {
        SessionFactory sessionFactory = lookupSessionFactory();
        logger.debug("Opening Hibernate Session in OpenSessionInViewFilter");
        Session session = getSession(sessionFactory);
        TransactionSynchronizationManager.bindResource(
                sessionFactory, new SessionHolder(session));
        try {
            filterChain.doFilter(request, response);
            }
        finally {
            TransactionSynchronizationManager.unbindResource(sessionFactory);
        logger.debug("Closing Hibernate Session in OpenSessionInViewFilter");
        closeSession(session, sessionFactory);
        }
}
protected Session getSession(SessionFactory sessionFactory)
                   throws DataAccessResourceFailureException {
        Session session = SessionFactoryUtils.getSession(sessionFactory, true);
        session.setFlushMode(FlushMode.NEVER);
        return session;
}
protected void closeSession(Session session,
        SessionFactory sessionFactory)throws CleanupFailureDataAccessException {
    SessionFactoryUtils.closeSessionIfNecessary(session, sessionFactory);
}

     关于绑定session的方式，通过看spring里TransactionSynchronizationManager的实现，发现：它维护一个 java.lang.ThreadLocal类型的resources，resources负责持有线程局部变量，这里resources持有的是一个 HashMap，通过TransactionSynchronizationManager.bindResource()方法在map里绑定和线程相关的所有变量到他们的标识上，包括如上所述的绑定在sessionFactory上的线程局部session。sessionHolder只不过是存放可以 hold一个session并可以和transtaction同步的容器。可以看到 OpenSessionInViewFilter在getSession的时候,会把获取回来的session的flush mode 设为FlushMode.NEVER。然后把该sessionFactory绑定到 TransactionSynchronizationManager，使request的整个过程都使用同一个session，在请求过后再接除该 sessionFactory的绑定，最后closeSessionIfNecessary根据该session是否已和transaction绑定来决定是否关闭session。绑定以后，就可以防止每次不会新开一个Session呢？看看HibernateDaoSupport的情况：

public final void setSessionFactory(SessionFactory sessionFactory) {
this.hibernateTemplate = new HibernateTemplate(sessionFactory);
}
rotected final HibernateTemplate getHibernateTemplate() {
return hibernateTemplate;

     我们的DAO将使用这个template进行操作.

public abstract class BaseHibernateObjectDao
                extends HibernateDaoSupportimplements BaseObjectDao {
protected BaseEntityObject getByClassId(final long id) {
BaseEntityObject obj =(BaseEntityObject)getHibernateTemplate().execute(new HibernateCallback() {
public Object doInHibernate(Session session)
         throws HibernateException{
 return session.get(getPersistentClass(),new Long(id));
       }
    }
);
return obj;
}
public void save(BaseEntityObject entity) {
       getHibernateTemplate().saveOrUpdate(entity);
}
public void remove(BaseEntityObject entity) {
try {
       getHibernateTemplate().delete(entity);
} catch (Exception e) {
       throw new FlexEnterpriseDataAccessException(e);
       }
}
public void refresh(final BaseEntityObject entity) {
       getHibernateTemplate().execute(new HibernateCallback(){
            public Object doInHibernate(Session session)
           throws HibernateException   {
                 session.refresh(entity);
                 return color: #7f

  


  
    
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      2010-12-04 21:54
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