spring事务管理源码分析

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。

    原文作者:Spring Cloud
    原文地址: https://blog.csdn.net/ch_space/article/details/18767875
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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