闲来无事，重新看了下spring事务管理源码，写个笔记。

1、TransactionTemplate

当需要在事务中执行一个DB操作时，执行：

transactionTemplate.execute(new TransactionCallback<MyDO>{
	public MyDO doInTransaction(TransactionStatus status){
		myDao.update();
		...;
		myDao.insert();
	}
});

事务中任何操作抛出异常，整个事务就会回滚掉，不用应用代码显式处理。

public <T> T execute(TransactionCallback<T> action) throws TransactionException {
		if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) {
			return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action);
		}
		else {
			TransactionStatus status = this.transactionManager.getTransaction(this);//这里会根据当前事务状态确定是否开启新事务
			T result;
			try {
				result = action.doInTransaction(status);
			}
			catch (RuntimeException ex) {
				// Transactional code threw application exception -> rollback
				rollbackOnException(status, ex);
				throw ex;
			}
			catch (Error err) {
				// Transactional code threw error -> rollback
				rollbackOnException(status, err);
				throw err;
			}
			catch (Exception ex) {
				// Transactional code threw unexpected exception -> rollback
				rollbackOnException(status, ex);
				throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
			}
			this.transactionManager.commit(status);
			return result;
		}
}

接下来是如何开启事务的？看transactionManager.getTransaction(this)

2、DataSourceTransactionManager

TransactionTemplate中包含一个dataSourceTransactionManager变量，用来管理事务，一切都是由他关联起来的。

public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
		Object transaction = doGetTransaction();
		if (isExistingTransaction(transaction)) {//已开启事务，就返回当前事务状态
			// Existing transaction found -> check propagation behavior to find out how to behave.
			return handleExistingTransaction(definition, transaction, debugEnabled);
		}
		...
		try {
			boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
			DefaultTransactionStatus status = newTransactionStatus(
					definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
			doBegin(transaction, definition);//这里就是开启事务啦！
			prepareSynchronization(status, definition);
			return status;
		}
}

3、来看下如何doBegin的：

protected void doBegin(Object transaction, TransactionDefinition definition) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
	Connection con = null;

	try {
		if (txObject.getConnectionHolder() == null ||
				txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
			Connection newCon = this.dataSource.getConnection();//看到没？这个就是db连接了~
			txObject.setConnectionHolder(new ConnectionHolder(newCon), true);//放到连接holder里面
		}
		
		// 这里绑定db连接到当前线程，哦，原来是依靠threadlocal实现的！注意，这里的key是Datasource
		if (txObject.isNewConnectionHolder()) {
			TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
		}
	}
	...
}

现在事务开启了，链接也放在线程变量里面了，那如何跟dao操作关联起来呢？这里以ibatis为例分析下。

4、从SqlMapClient初始化看起：

<bean id="mySqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
	<property name="dataSource" ref="myDataSource"/>
	<property name="configLocations">
	   <list>
		 <value>classpath:sqlmap/sqlmap-my.xml</value>
	   </list>  
	</property>
</bean>

这个dataSource跟事务管理器的dataSource是同一个。看org.springframework.orm.ibatis.SqlMapClientFactoryBean初始化后：

public void afterPropertiesSet() throws Exception {
		try {
			...
			// Tell the SqlMapClient to use the given DataSource, if any.
			if (this.dataSource != null) {
				TransactionConfig transactionConfig = (TransactionConfig) this.transactionConfigClass.newInstance();
				DataSource dataSourceToUse = this.dataSource;
				if (this.useTransactionAwareDataSource && !(this.dataSource instanceof TransactionAwareDataSourceProxy)) {
					dataSourceToUse = new TransactionAwareDataSourceProxy(this.dataSource);//搞了一个数据源代理
				}
				transactionConfig.setDataSource(dataSourceToUse);
				...
			}
		}
}

如果没猜错，spring在ibatis从这个代理数据源获取连接时候做了拦截处理，如果当前线程已经开启了事务，就直接用这个事务关联的连接了，看源码：

public TransactionAwareDataSourceProxy(DataSource targetDataSource) {
	super(targetDataSource);
}

5、继续看怎么取连接的：

public Connection getConnection() throws SQLException {
	DataSource ds = getTargetDataSource();
	Assert.state(ds != null, "'targetDataSource' is required");
	return getTransactionAwareConnectionProxy(ds);//连接也包装成了代理对象，看下这个代理的连接是怎么从datasource搞出来的
}

protected Connection getTransactionAwareConnectionProxy(DataSource targetDataSource) {
	return (Connection) Proxy.newProxyInstance(
			ConnectionProxy.class.getClassLoader(),
			new Class[] {ConnectionProxy.class},
			new TransactionAwareInvocationHandler(targetDataSource));
}

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	...
	Connection actualTarget = this.target;
	if (actualTarget == null) {
		actualTarget = DataSourceUtils.doGetConnection(this.targetDataSource);//怎么取的呢?
	}
	//真正用这个连接的时候返回原始对象
	if (method.getName().equals("getTargetConnection")) {
		// Handle getTargetConnection method: return underlying Connection.
		return actualTarget;
	}
	...
}

6、看下DataSourceUtils的doGetConnection这个方法：

public static Connection doGetConnection(DataSource dataSource) throws SQLException {
	//从当前线程上下文取出连接holder，这个就是开启事务时候放进去的，还记得不？
	ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
	if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
		conHolder.requested();
		if (!conHolder.hasConnection()) {
			logger.debug("Fetching resumed JDBC Connection from DataSource");
			conHolder.setConnection(dataSource.getConnection());
		}
		return conHolder.getConnection();
	}
	...
}

好了，总结一下：

1）TransactionTemplate.execute执行事务块时，DataSourceTransactionManager开启事务，并把db连接用事务同步器TransactionSynchronizationManager注册到线程变量中中。

2）事务块中实行dao方法时，sqlMapClient从数据源获取连接是通过代理数据源取到了代理连接，在操作db的时候，这个代理连接返回了当前线程上下文中的连接。这样，同一个事务块中的多个dao操作就公用了同一个db连接，实现了事务悄无声息的封装。