Open Session In View模式的基本常识-zhuan
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);
}
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。在这个过程中，若HibernateTemplate 发现自当前session有不是readOnly的transaction，就会获取到FlushMode.AUTO Session，使方法拥有写权限。
public static void closeSessionIfNecessary(Session session, SessionFactory sessionFactory)throws CleanupFailureDataAccessException {
if (session == null || TransactionSynchronizationManager.hasResource(sessionFactory)){
return;
}
logger.debug("Closing Hibernate session");
try {
session.close();
}catch (JDBCException ex) {
// SQLException underneath
throw new CleanupFailureDataAccessException("Could not close Hibernate session", ex.getSQLException());
}catch (HibernateException ex) {
throw new CleanupFailureDataAccessException("Could not close Hibernate session", ex);
}
}
也即是，如果有不是readOnly的transaction就可以由Flush.NEVER转为Flush.AUTO,拥有insert,update,delete操作权限，如果没有transaction，并且没有另外人为地设flush model的话，则doFilter的整个过程都是Flush.NEVER。所以受transaction保护的方法有写权限，没受保护的则没有。
采用spring的事务声明,使方法受transaction控制
<bean id="baseTransaction" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean" abstract="true">
<property name="transactionManager" ref="transactionManager"/>
<property name="proxyTargetClass" value="true"/>
<property name="transactionAttributes">
<props>
<prop key="get*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="find*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="load*">PROPAGATION_REQUIRED,readOnly</prop>
<prop key="save*">PROPAGATION_REQUIRED</prop>
<prop key="add*">PROPAGATION_REQUIRED</prop>
<prop key="update*">PROPAGATION_REQUIRED</prop>
<prop key="remove*">PROPAGATION_REQUIRED</prop>
</props>
</property>
</bean>
对于上例，则以save,add,update,remove开头的方法拥有可写的事务，如果当前有某个方法，如命名为importExcel()，则因没有transaction而没有写权限，这时若方法内有insert,update,delete操作的话，则需要手动设置flush model为Flush.AUTO,如:
session.setFlushMode(FlushMode.AUTO); session.save(user); session.flush();
尽管Open Session In View看起来还不错，其实副作用不少。看回上面OpenSessionInViewFilter的doFilterInternal方法代码，这个方法实际上是被父类的doFilter调用的，因此，我们可以大约了解的OpenSessionInViewFilter调用流程: request(请求)->open session并开始transaction->controller->View(Jsp)->结束transaction并close session。
一切看起来很正确，尤其是在本地开发测试的时候没出现问题，但试想下如果流程中的某一步被阻塞的话，那在这期间connection就一直被占用而不释放。最有可能被阻塞的就是在写Jsp这步，一方面可能是页面内容大，response.write的时间长，另一方面可能是网速慢，服务器与用户间传输时间久。当大量这样的情况出现时，就有连接池连接不足，造成页面假死现象。
[Open Session In View是个双刃剑，放在公网上内容多流量大的网站请慎用]
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官方文档对其过滤器的解释如下：
public class OpenSessionInViewFilterextends OncePerRequestFilter
Servlet 2.3 Filter that binds a Hibernate Session to the thread for the entire processing of the request. Intended for the "Open Session in View" pattern, i.e. to allow for lazy loading in web views despite the original transactions already being completed.
This filter makes Hibernate Sessions available via the current thread, which will be autodetected by transaction managers. It is suitable for service layer transactions via HibernateTransactionManager orJtaTransactionManager as well as for non-transactional execution (if configured appropriately).
NOTE: This filter will by default not flush the Hibernate Session, with the flush mode set to FlushMode.NEVER. It assumes to be used in combination with service layer transactions that care for the flushing: The active transaction manager will temporarily change the flush mode to FlushMode.AUTO during a read-write transaction, with the flush mode reset to FlushMode.NEVER at the end of each transaction. If you intend to use this filter without transactions, consider changing the default flush mode (through the "flushMode" property).
WARNING: Applying this filter to existing logic can cause issues that have not appeared before, through the use of a single Hibernate Session for the processing of an entire request. In particular, the reassociation of persistent objects with a Hibernate Session has to occur at the very beginning of request processing, to avoid *es with already loaded instances of the same objects.
Alternatively, turn this filter into deferred close mode, by specifying "singleSession"="false": It will not use a single session per request then, but rather let each data access operation or transaction use its own session (like without Open Session in View). Each of those sessions will be registered for deferred close, though, actually processed at request completion.
A single session per request allows for most efficient first-level caching, but can cause side effects, for example on saveOrUpdate or when continuing after a rolled-back transaction. The deferred close strategy is as safe as no Open Session in View in that respect, while still allowing for lazy loading in views (but not providing a first-level cache for the entire request).
Looks up the SessionFactory in Spring's root web application context. Supports a "sessionFactoryBeanName" filter init-param in web.xml; the default bean name is "sessionFactory". Looks up the SessionFactory on each request, to avoid initialization order issues (when using ContextLoaderServlet, the root application context will get initialized after this filter).