Write operations are not allowed in read-only mode (FlushMode.NEVER)

使用Spring提供的Open Session In View而引起Write operations are not allowed in read-only mode (FlushMode.NEVER) 错误解决：

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>

<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

<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);

}

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);
}
}

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>

 <bean id="userService" parent="baseTransaction">

 <property name="target">

 <bean class="com.phopesoft.security.service.impl.UserServiceImpl"/>

</property>

</bean>

<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>

<bean id="userService" parent="baseTransaction"> 

<property name="target">

<bean class="com.phopesoft.security.service.impl.UserServiceImpl"/>

</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。