spring 的OpenSessionInViewFilter简介及源码

假设在你的应用中Hibernate是通过spring 来管理它的session.如果在你的应用中没有使用OpenSessionInViewFilter或者OpenSessionInViewInterceptor。session会在transaction结束后关闭。
如果你采用了spring的声明式事务模式，它会对你的被代理对象的每一个方法进行事务包装（AOP的方式）。如下：

<bean id="txProxyTemplate" abstract="true"
class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
<property name="transactionManager" ref="transactionManager"/>
<property name="transactionAttributes">
<props>
<prop key="save*">PROPAGATION_REQUIRED</prop>
<prop key="remove*">PROPAGATION_REQUIRED</prop>
<prop key="*">PROPAGATION_REQUIRED,readOnly</prop>
</props>
</property>
</bean>

<bean id="manager" parent="txProxyTemplate">
<property name="target">
<bean class="org.appfuse.service.impl.BaseManager">
<property name="dao" ref="dao"/>
</bean>
</property>
</bean>
目标类org.appfuse.service.impl.BaseManager 的 save *方法的事务类型PROPAGATION_REQUIRED ，remove* 方法的事务类型PROPAGATION_REQUIRED
其他的方法的事务类型是PROPAGATION_REQUIRED,readOnly。
所以给你的感觉是调用这个名为“manager”的bean的方法之后session就关掉了。
如果应用中使用了OpenSessionInViewFilter或者OpenSessionInViewInterceptor，所有打开的session会被保存在一个线程变量里。在线程退出前通过
OpenSessionInViewFilter或者OpenSessionInViewInterceptor断开这些session。 为什么这么做？这主要是为了实现Hibernate的延迟加载功能。基于一个请求
一个hibernate session的原则。

spring中对OpenSessionInViewFilter的描述如下：
它是一个Servlet2.3过滤器，用来把一个Hibernate Session和一次完整的请求过程对应的线程相绑定。目的是为了实现"Open Session in View"的模式。
例如： 它允许在事务提交之后延迟加载显示所需要的对象。

这个过滤器和 HibernateInterceptor 有点类似：它是通过线程实现的。无论是没有事务的应用，还是有业务层事务的应用（通过HibernateTransactionManager 或
JtaTransactionManager的方式实现）它都适用。在后一种情况下，事务会自动采用由这个filter绑定的Session来进行相关的操作以及根据实际情况完成提交操作。

警告： 如果在你的应用中，一次请求的过程中使用了单一的一个HIbernate Session，在这种情况下，采用这个filter会产生一些以前没遇到的问题。特别需要注意的是通过
Hibernate Session重新组织持久化对象之间关系的相关操作需要在请求的最开始进行。以免与已经加载的相同对象发生冲突。

或者，我们可以通过指定"singleSession"="false"的方式把这个过滤器调到延期关闭模式。这样在一次请求的过程中不会使用一个单一的Session.每一次数据访问或事务相关
操作都使用属于它自己的session(有点像不使用Open Session in View).这些session都被注册成延迟关闭模式，即使是在这一次的请求中它相关操作已经完成。

"一次请求一个session" 对于一级缓存而言很有效，但是这样可以带来副作用。例如在saveOrUpdate的时候或事物回滚之后，虽然它和“no Open Session in View”同样安全。
但是它却允许延迟加载。

它会在spring的web应用的上下文根中查找Session工厂。它也支持通过在web.xml中定义的“SessionFactoryBeanName”的init-param元素 指定的Session工厂对应的bean的
名字来查找session工厂。默认的bean的名字是"sessionFactory".他通过每一次请求查找一次SessionFactory的方式来避免由初始化顺序引起的问题（当使用ContextLoaderServlet
来集成spring的时候 ，spring 的应用上下文是在这个filter 之后才被初始化的）。

默认的情况下，这个filter 不会同步Hibernate Session.这是因为它认为这项工作是通过业务层的事务来完成的。而且HibernateAccessors 的FlushMode为FLUSH_EAGER.如果你
想让这个filter在请求完成以后同步session.你需要覆盖它的closeSession方法，在这个方法中在调用父类的关闭session操作之前同步session.此外你需要覆盖它的getSession()
方法。返回一个session它的FlushMode 不是默认的FlushMode.NEVER。需要注意的是getSession()和closeSession()方法只有在single session的模式中才被调用。

在myfaces的wiki里提供了OpenSessionInViewFilter的一个子类如下：
public class OpenSessionInViewFilter extends org.springframework.orm.hibernate3.support.OpenSessionInViewFilter {

/**
* we do a different flushmode than in the codebase
* here
*/
protected Session getSession(SessionFactory sessionFactory) throws DataAccessResourceFailureException {
Session session = SessionFactoryUtils.getSession(sessionFactory, true);
session.setFlushMode(FlushMode.COMMIT);
return session;
}
/**
* we do an explicit flush here just in case
* we do not have an automated flush
*/
protected void closeSession(Session session, SessionFactory factory) {
session.flush();
super.closeSession(session, factory);
}
}

附加源码 OpenSessionInViewFilter 代码 ：

/*
* Copyright 2002-2007 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.orm.hibernate3.support;

import java.io.IOException;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.hibernate.FlushMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;

import org.springframework.dao.DataAccessResourceFailureException;
import org.springframework.orm.hibernate3.SessionFactoryUtils;
import org.springframework.orm.hibernate3.SessionHolder;
import org.springframework.transaction.support.TransactionSynchronizationManager;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;
import org.springframework.web.filter.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.
*
* <p>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 {@link org.springframework.orm.hibernate3.HibernateTransactionManager}
* or {@link org.springframework.transaction.jta.JtaTransactionManager} as well
* as for non-transactional execution (if configured appropriately).
*
* <p><b>NOTE</b>: This filter will by default <i>not</i> flush the Hibernate Session,
* with the flush mode set to <code>FlushMode.NEVER</code>. 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
* <code>FlushMode.AUTO</code> during a read-write transaction, with the flush
* mode reset to <code>FlushMode.NEVER</code> 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).
*
* <p><b>WARNING:</b> 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 clashes with already loaded instances of the same objects.
*
* <p>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.
*
* <p>A single session per request allows for most efficient first-level caching,
* but can cause side effects, for example on <code>saveOrUpdate</code> 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).
*
* <p>Looks up the SessionFactory in Spring's root web application context.
* Supports a "sessionFactoryBeanName" filter init-param in <code>web.xml</code>;
* 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 <i>after</i> this filter).
*
* @author Juergen Hoeller
* @since 1.2
* @see #setSingleSession
* @see #setFlushMode
* @see #lookupSessionFactory
* @see OpenSessionInViewInterceptor
* @see org.springframework.orm.hibernate3.HibernateInterceptor
* @see org.springframework.orm.hibernate3.HibernateTransactionManager
* @see org.springframework.orm.hibernate3.SessionFactoryUtils#getSession
* @see org.springframework.transaction.support.TransactionSynchronizationManager
*/
public class OpenSessionInViewFilter extends OncePerRequestFilter {

public static final String DEFAULT_SESSION_FACTORY_BEAN_NAME = "sessionFactory";

private String sessionFactoryBeanName = DEFAULT_SESSION_FACTORY_BEAN_NAME;

private boolean singleSession = true;

private FlushMode flushMode = FlushMode.NEVER;

/**
* Set the bean name of the SessionFactory to fetch from Spring's
* root application context. Default is "sessionFactory".
* @see #DEFAULT_SESSION_FACTORY_BEAN_NAME
*/
public void setSessionFactoryBeanName(String sessionFactoryBeanName) {
this.sessionFactoryBeanName = sessionFactoryBeanName;
}

/**
* Return the bean name of the SessionFactory to fetch from Spring's
* root application context.
*/
protected String getSessionFactoryBeanName() {
return this.sessionFactoryBeanName;
}

/**
* Set whether to use a single session for each request. Default is "true".
* <p>If set to "false", each data access operation or transaction will 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.
* @see SessionFactoryUtils#initDeferredClose
* @see SessionFactoryUtils#processDeferredClose
*/
public void setSingleSession(boolean singleSession) {
this.singleSession = singleSession;
}

/**
* Return whether to use a single session for each request.
*/
protected boolean isSingleSession() {
return this.singleSession;
}

/**
* Specify the Hibernate FlushMode to apply to this filter's
* {@link org.hibernate.Session}. Only applied in single session mode.
* <p>Can be populated with the corresponding constant name in XML bean
* definitions: e.g. "AUTO".
* <p>The default is "NEVER". Specify "AUTO" if you intend to use
* this filter without service layer transactions.
* @see org.hibernate.Session#setFlushMode
* @see org.hibernate.FlushMode#NEVER
* @see org.hibernate.FlushMode#AUTO
*/
public void setFlushMode(FlushMode flushMode) {
this.flushMode = flushMode;
}

/**
* Return the Hibernate FlushMode that this filter applies to its
* {@link org.hibernate.Session} (in single session mode).
*/
protected FlushMode getFlushMode() {
return this.flushMode;
}

protected void doFilterInternal(
HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
throws ServletException, IOException {

SessionFactory sessionFactory = lookupSessionFactory(request);
boolean participate = false;

if (isSingleSession()) {
// single session mode
if (TransactionSynchronizationManager.hasResource(sessionFactory)) {
// Do not modify the Session: just set the participate flag.
participate = true;
}
else {
logger.debug("Opening single Hibernate Session in OpenSessionInViewFilter");
Session session = getSession(sessionFactory);
TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));
}
}
else {
// deferred close mode
if (SessionFactoryUtils.isDeferredCloseActive(sessionFactory)) {
// Do not modify deferred close: just set the participate flag.
participate = true;
}
else {
SessionFactoryUtils.initDeferredClose(sessionFactory);
}
}

try {
filterChain.doFilter(request, response);
}

finally {
if (!participate) {
if (isSingleSession()) {
// single session mode
SessionHolder sessionHolder =
(SessionHolder) TransactionSynchronizationManager.unbindResource(sessionFactory);
logger.debug("Closing single Hibernate Session in OpenSessionInViewFilter");
closeSession(sessionHolder.getSession(), sessionFactory);
}
else {
// deferred close mode
SessionFactoryUtils.processDeferredClose(sessionFactory);
}
}
}
}

/**
* Look up the SessionFactory that this filter should use,
* taking the current HTTP request as argument.
* <p>Default implementation delegates to the <code>lookupSessionFactory</code>
* without arguments.
* @return the SessionFactory to use
* @see #lookupSessionFactory()
*/
protected SessionFactory lookupSessionFactory(HttpServletRequest request) {
return lookupSessionFactory();
}

/**
* Look up the SessionFactory that this filter should use.
* <p>Default implementation looks for a bean with the specified name
* in Spring's root application context.
* @return the SessionFactory to use
* @see #getSessionFactoryBeanName
*/
protected SessionFactory lookupSessionFactory() {
if (logger.isDebugEnabled()) {
logger.debug("Using SessionFactory '" + getSessionFactoryBeanName() + "' for OpenSessionInViewFilter");
}
WebApplicationContext wac =
WebApplicationContextUtils.getRequiredWebApplicationContext(getServletContext());
return (SessionFactory) wac.getBean(getSessionFactoryBeanName(), SessionFactory.class);
}

/**
* Get a Session for the SessionFactory that this filter uses.
* Note that this just applies in single session mode!
* <p>The default implementation delegates to the
* <code>SessionFactoryUtils.getSession</code> method and
* sets the <code>Session</code>'s flush mode to "NEVER".
* <p>Can be overridden in subclasses for creating a Session with a
* custom entity interceptor or JDBC exception translator.
* @param sessionFactory the SessionFactory that this filter uses
* @return the Session to use
* @throws DataAccessResourceFailureException if the Session could not be created
* @see org.springframework.orm.hibernate3.SessionFactoryUtils#getSession(SessionFactory, boolean)
* @see org.hibernate.FlushMode#NEVER
*/
protected Session getSession(SessionFactory sessionFactory) throws DataAccessResourceFailureException {
Session session = SessionFactoryUtils.getSession(sessionFactory, true);
FlushMode flushMode = getFlushMode();
if (flushMode != null) {
session.setFlushMode(flushMode);
}
return session;
}

/**
* Close the given Session.
* Note that this just applies in single session mode!
* <p>Can be overridden in subclasses, e.g. for flushing the Session before
* closing it. See class-level javadoc for a discussion of flush handling.
* Note that you should also override getSession accordingly, to set
* the flush mode to something else than NEVER.
* @param session the Session used for filtering
* @param sessionFactory the SessionFactory that this filter uses
*/
protected void closeSession(Session session, SessionFactory sessionFactory) {
SessionFactoryUtils.closeSession(session);
}

}