spring对session和事务的管理以及OpenSessionInViewFilter是如何工作的

　　为了弄清楚spring对session和事务的管理以及OpenSessionInViewFilter是如何工作的，可监控以下类的日志：
　　org.springframework.orm.hibernate3.support.OpenSessionInViewFilter，
　　org.springframework.orm.hibernate3.HibernateTransactionManager，
　　org.hibernate.transaction.JDBCTransaction，
　　org.hibernate.impl.SessionImpl
具体可参考下面的配置：

    <appender name="sessionTransactionAppender" class="org.apache.log4j.DailyRollingFileAppender">
		<param name="File" value="session_transaction.log" />
		<param name="Append" value="false" />
		<param name="Threshold" value="DEBUG" />
		<param name="DatePattern" value="'.'yyyy-MM-dd" />
		<layout class="org.apache.log4j.PatternLayout">
			<param name="ConversionPattern" value="[%t] %d %-5p [%c] %m%n" />
		</layout>
	</appender>
	<category name="org.hibernate.impl.SessionImpl">
  	  <priority value="ALL" />
      <appender-ref ref="sessionTransactionAppender"/>
	</category>
    <category name="org.springframework.orm.hibernate3.HibernateTransactionManager">
  	  <priority value="ALL" />
      <appender-ref ref="sessionTransactionAppender"/>
	</category>
	<category name="org.springframework.orm.hibernate3.support.OpenSessionInViewFilter">
  	  <priority value="debug" />
      <appender-ref ref="sessionTransactionAppender"/>
	</category>

下面是在一个不使用OpenSessionInViewFilter的项目中，一次请求(所有条目都是在一个线程里的)调用两次getAllForumGroups方法（此方法上声明了事务控制）打出的日志：
日志中可以清楚地看到：每次事务开启时会打开一个新的session，事务结束时会关闭该session,session不再和当前线程绑定。

[7143355@qtp-21544349-0] 2011-03-28 15:39:33,304 DEBUG [org.hibernate.impl.SessionImpl] opened session at timestamp: 5330116498333696
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,698 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Creating new transaction with name [oobbs.application.service.forum.ForumGroupService.getAllForumGroups]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,700 DEBUG [org.hibernate.impl.SessionImpl] opened session at timestamp: 5330116500275200
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,701 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Opened new Session [org.hibernate.impl.SessionImpl@a0e220] for Hibernate transaction
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,712 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Preparing JDBC Connection of Hibernate Session [org.hibernate.impl.SessionImpl@a0e220]
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,737 DEBUG [org.hibernate.transaction.JDBCTransaction] begin
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,739 DEBUG [org.hibernate.transaction.JDBCTransaction] current autocommit status: true
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,740 DEBUG [org.hibernate.transaction.JDBCTransaction] disabling autocommit
[7143355@qtp-21544349-0] 2011-03-28 15:39:33,746 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Exposing Hibernate transaction as JDBC transaction [org.apache.commons.dbcp.PoolableConnection@1ac9683]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,205 DEBUG [org.hibernate.impl.SessionImpl] initializing proxy: [oobbs.domainmodel.user.User#-1]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,419 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Initiating transaction commit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,423 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Committing Hibernate transaction on Session [org.hibernate.impl.SessionImpl@a0e220]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,426 DEBUG [org.hibernate.transaction.JDBCTransaction] commit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,447 DEBUG [org.hibernate.transaction.JDBCTransaction] re-enabling autocommit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,452 DEBUG [org.hibernate.transaction.JDBCTransaction] committed JDBC Connection
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,458 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Closing Hibernate Session [org.hibernate.impl.SessionImpl@a0e220] after transaction
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,464 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Creating new transaction with name [oobbs.application.service.forum.ForumGroupService.getAllForumGroups]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,465 DEBUG [org.hibernate.impl.SessionImpl] opened session at timestamp: 5330116503408640
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,466 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Opened new Session [org.hibernate.impl.SessionImpl@f7d5a6] for Hibernate transaction
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,469 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Preparing JDBC Connection of Hibernate Session [org.hibernate.impl.SessionImpl@f7d5a6]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,470 DEBUG [org.hibernate.transaction.JDBCTransaction] begin
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,474 DEBUG [org.hibernate.transaction.JDBCTransaction] current autocommit status: true
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,476 DEBUG [org.hibernate.transaction.JDBCTransaction] disabling autocommit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,478 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Exposing Hibernate transaction as JDBC transaction [org.apache.commons.dbcp.PoolableConnection@1ac9683]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,652 DEBUG [org.hibernate.impl.SessionImpl] initializing proxy: [oobbs.domainmodel.user.User#-1]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,795 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Initiating transaction commit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,796 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Committing Hibernate transaction on Session [org.hibernate.impl.SessionImpl@f7d5a6]
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,798 DEBUG [org.hibernate.transaction.JDBCTransaction] commit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,804 DEBUG [org.hibernate.transaction.JDBCTransaction] re-enabling autocommit
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,805 DEBUG [org.hibernate.transaction.JDBCTransaction] committed JDBC Connection
[7143355@qtp-21544349-0] 2011-03-28 15:39:34,810 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Closing Hibernate Session [org.hibernate.impl.SessionImpl@f7d5a6] after transaction

　　下面是在一个使用了OpenSessionInViewFilter的项目中，一次请求(所有条目都是在一个线程里的)调用两次getRole方法（此方法上声明了事务控制）打出的日志：
日志中可以清楚地看到：请求刚发起时就会创建一个session实例，并绑定到当前线程上，至请求结束时，关闭该线程。在请求期间所有事务都使用这一个session。
　　另一方面我们还可以看到：数据库连接connection保持打开的时间正是事务从开始到提交的时间，为了测试，getRole方法中故意设定了3秒的延迟，从日志上看出数据库连接也基本上维持了3秒。这是很多系统出现无法获取数据库连接问题的原因。在一个使用了连接池的系统里，连接数有一个最大上限，如果某个请求的处理耗时过长，则当前数据库连接在短时间内就得不到释放。当前并发访问量急速上升时，连接池的连接用会迅速用光，导致后续的请求因无法得到数据库连接而失败。
　　最后，至于为什么只需要保证session一直打开就能避免lazy loading时不报错，原因就在于，观察下面的log可以发现，每当在事务期间时autocommit就被禁用，事务一执行完毕就又恢复至可用。这样，在jsp页面上读取lazy对象时，并不是以事务方式进行的，而是以autocommit的方式提交select语句的。因此只需要保持session打开即可，无需事务的支持。

[http-8080-4] 2011-03-28 15:25:59,367 DEBUG [org.springframework.orm.hibernate3.support.OpenSessionInViewFilter] Using SessionFactory 'sessionFactory' for OpenSessionInViewFilter
[http-8080-4] 2011-03-28 15:25:59,367 DEBUG [org.springframework.orm.hibernate3.support.OpenSessionInViewFilter] Opening single Hibernate Session in OpenSessionInViewFilter
[http-8080-4] 2011-03-28 15:25:59,367 DEBUG [org.hibernate.impl.SessionImpl] opened session at timestamp: 5330113164767232
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Found thread-bound Session [org.hibernate.impl.SessionImpl@1d8f985] for Hibernate transaction
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Creating new transaction with name [sf.demo.RoleService.getRole]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT;
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Preparing JDBC Connection of Hibernate Session [org.hibernate.impl.SessionImpl@1d8f985]
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.hibernate.transaction.JDBCTransaction] begin
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.hibernate.transaction.JDBCTransaction] current autocommit status: true
[http-8080-4] 2011-03-28 15:25:59,439 DEBUG [org.hibernate.transaction.JDBCTransaction] disabling autocommit
[http-8080-4] 2011-03-28 15:25:59,440 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Exposing Hibernate transaction as JDBC transaction [com.mchange.v2.c3p0.impl.NewProxyConnection@16f7142]
[http-8080-4] 2011-03-28 15:26:02,440 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Initiating transaction commit
[http-8080-4] 2011-03-28 15:26:02,440 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Committing Hibernate transaction on Session [org.hibernate.impl.SessionImpl@1d8f985]
[http-8080-4] 2011-03-28 15:26:02,440 DEBUG [org.hibernate.transaction.JDBCTransaction] commit
[http-8080-4] 2011-03-28 15:26:02,442 DEBUG [org.hibernate.transaction.JDBCTransaction] re-enabling autocommit
[http-8080-4] 2011-03-28 15:26:02,443 DEBUG [org.hibernate.transaction.JDBCTransaction] committed JDBC Connection
[http-8080-4] 2011-03-28 15:26:02,443 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Not closing pre-bound Hibernate Session [org.hibernate.impl.SessionImpl@1d8f985] after transaction
[http-8080-4] 2011-03-28 15:26:02,443 DEBUG [org.hibernate.impl.SessionImpl] disconnecting session
[http-8080-4] 2011-03-28 15:26:02,445 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Found thread-bound Session [org.hibernate.impl.SessionImpl@1d8f985] for Hibernate transaction
[http-8080-4] 2011-03-28 15:26:02,445 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Creating new transaction with name [sf.demo.RoleService.getRole]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT;
[http-8080-4] 2011-03-28 15:26:02,445 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Preparing JDBC Connection of Hibernate Session [org.hibernate.impl.SessionImpl@1d8f985]
[http-8080-4] 2011-03-28 15:26:02,445 DEBUG [org.hibernate.transaction.JDBCTransaction] begin
[http-8080-4] 2011-03-28 15:26:02,445 DEBUG [org.hibernate.transaction.JDBCTransaction] current autocommit status: true
[http-8080-4] 2011-03-28 15:26:02,446 DEBUG [org.hibernate.transaction.JDBCTransaction] disabling autocommit
[http-8080-4] 2011-03-28 15:26:02,446 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Exposing Hibernate transaction as JDBC transaction [com.mchange.v2.c3p0.impl.NewProxyConnection@1fb8de1]
[http-8080-4] 2011-03-28 15:26:05,447 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Initiating transaction commit
[http-8080-4] 2011-03-28 15:26:05,447 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Committing Hibernate transaction on Session [org.hibernate.impl.SessionImpl@1d8f985]
[http-8080-4] 2011-03-28 15:26:05,447 DEBUG [org.hibernate.transaction.JDBCTransaction] commit
[http-8080-4] 2011-03-28 15:26:05,449 DEBUG [org.hibernate.transaction.JDBCTransaction] re-enabling autocommit
[http-8080-4] 2011-03-28 15:26:05,450 DEBUG [org.hibernate.transaction.JDBCTransaction] committed JDBC Connection
[http-8080-4] 2011-03-28 15:26:05,450 DEBUG [org.springframework.orm.hibernate3.HibernateTransactionManager] Not closing pre-bound Hibernate Session [org.hibernate.impl.SessionImpl@1d8f985] after transaction
[http-8080-4] 2011-03-28 15:26:05,450 DEBUG [org.hibernate.impl.SessionImpl] disconnecting session
[http-8080-4] 2011-03-28 15:26:08,680 DEBUG [org.springframework.orm.hibernate3.support.OpenSessionInViewFilter] Closing single Hibernate Session in OpenSessionInViewFilter