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);
 }

}