转自:
http://www.iteye.com/topic/11738前几天解释了Spring的抽象事务机制。这次讲讲Spring中的DataSource 事务。
DataSource事务相关的类比较多,我们一步步来拨开其中的密团。
1 如何获得连接
看DataSourceUtils代码
- protected static Connection doGetConnection(DataSource dataSource, boolean allowSynchronization);
- throws SQLException {
-
- ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
- if (conHolder != null); {
- conHolder.requested();;
- return conHolder.getConnection();;
- }
-
-
- Connection con = dataSource.getConnection();;
- if (allowSynchronization && TransactionSynchronizationManager.isSynchronizationActive();); {
- conHolder = new ConnectionHolder(con);;
- TransactionSynchronizationManager.bindResource(dataSource, conHolder);;
- TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(conHolder, dataSource););;
- conHolder.requested();;
- }
-
- return con;
- }
protected static Connection doGetConnection(DataSource dataSource, boolean allowSynchronization);
throws SQLException {
ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
if (conHolder != null); {
conHolder.requested();;
return conHolder.getConnection();;
}
Connection con = dataSource.getConnection();;
if (allowSynchronization && TransactionSynchronizationManager.isSynchronizationActive();); {
conHolder = new ConnectionHolder(con);;
TransactionSynchronizationManager.bindResource(dataSource, conHolder);;
TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(conHolder, dataSource););;
conHolder.requested();;
}
return con;
}
原来连接是从TransactionSynchronizationManager中获取,如果TransactionSynchronizationManager中已经有了,那么拿过来然后调用conHolder.requested()。否则从原始的DataSource这创建一个连接,放到一个ConnectionHolder,然后再调用TransactionSynchronizationManager.bindResource绑定。
好,我们又遇到两个新的类TransactionSynchronizationManager和ConnectionHolder和。继续跟踪
2 TransactionSynchronizationManager
看其中的一些代码
- private static ThreadLocal resources = new ThreadLocal();;
- public static Object getResource(Object key); {
- Map map = (Map); resources.get();;
- if (map == null); {
- return null;
- }
- Object value = map.get(key);;
- return value;
- }
- public static void bindResource(Object key, Object value); throws IllegalStateException {
- Map map = (Map); resources.get();;
- if (map == null); {
- map = new HashMap();;
- resources.set(map);;
- }
- map.put(key, value);;
- }
private static ThreadLocal resources = new ThreadLocal();;
public static Object getResource(Object key); {
Map map = (Map); resources.get();;
if (map == null); {
return null;
}
Object value = map.get(key);;
return value;
}
public static void bindResource(Object key, Object value); throws IllegalStateException {
Map map = (Map); resources.get();;
if (map == null); {
map = new HashMap();;
resources.set(map);;
}
map.put(key, value);;
}
原来TransactionSynchronizationManager内部建立了一个ThreadLocal的resources,这个resources又是和一个map联系在一起的,这个map在某个线程第一次调用bindResource时生成。
联系前面的DataSourceUtils代码,我们可以总结出来。
某个线程使用DataSourceUtils,当第一次要求创建连接将在TransactionSynchronizationManager中创建出一个ThreadLocal的map。然后以DataSource作为键,ConnectionHolder为值放到map中。等这个线程下一次再请求的这个DataSource的时候,就从这个map中获取对应的ConnectionHolder。用map是为了解决同一个线程上多个DataSource。
然后我们来看看ConnectionHolder又是什么?
3 对连接进行引用计数
看ConnectionHolder代码,这个类很简单,看不出个所以然,只好再去看父类代码ResourceHolderSupport,我们感兴趣的是这两个方法
- public void requested(); {
- this.referenceCount++;
- }
-
- public void released(); {
- this.referenceCount--;
- }
public void requested(); {
this.referenceCount++;
}
public void released(); {
this.referenceCount--;
}
看得出这是一个引用计数的技巧。原来Spring中对Connection是竟量使用已创建的对象,而不是每次都创建一个新对象。这就是DataSourceUtils中
- if (conHolder != null); {
- conHolder.requested();;
- return conHolder.getConnection();;
- }
if (conHolder != null); {
conHolder.requested();;
return conHolder.getConnection();;
}
的原因
4 释放连接
完成事物后DataSourceTransactionManager有这样的代码
- protected void doCleanupAfterCompletion(Object transaction); {
- DataSourceTransactionObject txObject = (DataSourceTransactionObject); transaction;
-
-
- TransactionSynchronizationManager.unbindResource(this.dataSource);;
- txObject.getConnectionHolder();.clear();;
-
-
- }
protected void doCleanupAfterCompletion(Object transaction); {
DataSourceTransactionObject txObject = (DataSourceTransactionObject); transaction;
// Remove the connection holder from the thread.
TransactionSynchronizationManager.unbindResource(this.dataSource);;
txObject.getConnectionHolder();.clear();;
//... DataSourceUtils.closeConnectionIfNecessary(con, this.dataSource);;
}
DataSourceUtils
- protected static void doCloseConnectionIfNecessary(Connection con, DataSource dataSource); throws SQLException {
- if (con == null); {
- return;
- }
-
- ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
- if (conHolder != null && con == conHolder.getConnection();); {
-
- conHolder.released();;
- return;
- }
-
-
-
- if (!(dataSource instanceof SmartDataSource); || ((SmartDataSource); dataSource);.shouldClose(con);); {
- logger.debug("Closing JDBC connection");;
- con.close();;
- }
- }
protected static void doCloseConnectionIfNecessary(Connection con, DataSource dataSource); throws SQLException {
if (con == null); {
return;
}
ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
if (conHolder != null && con == conHolder.getConnection();); {
// It's the transactional Connection: Don't close it.
conHolder.released();;
return;
}
// Leave the Connection open only if the DataSource is our
// special data source, and it wants the Connection left open.
if (!(dataSource instanceof SmartDataSource); || ((SmartDataSource); dataSource);.shouldClose(con);); {
logger.debug("Closing JDBC connection");;
con.close();;
}
}
恍然大悟。如果事物完成,那么就
TransactionSynchronizationManager.unbindResource(this.dataSource);将当前的ConnectionHolder
从TransactionSynchronizationManager上脱离,然后doCloseConnectionIfNecessary。最后会把连接关闭掉。
5 两个辅助类JdbcTemplate和TransactionAwareDataSourceProxy
JdbcTemplate中的execute方法的第一句和最后一句
- public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action);
- throws DataAccessException {
-
- Connection con = DataSourceUtils.getConnection(getDataSource(););;
-
- DataSourceUtils.closeConnectionIfNecessary(con, getDataSource(););;
- }
- }
public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action);
throws DataAccessException {
Connection con = DataSourceUtils.getConnection(getDataSource(););;
//其他代码
DataSourceUtils.closeConnectionIfNecessary(con, getDataSource(););;
}
}
作用不言自明了吧
从TransactionAwareDataSourceProxy中获取的连接是这个样子的
- public Connection getConnection(); throws SQLException {
- Connection con = DataSourceUtils.doGetConnection(getTargetDataSource();, true);;
- return getTransactionAwareConnectionProxy(con, getTargetDataSource(););;
- }
public Connection getConnection(); throws SQLException {
Connection con = DataSourceUtils.doGetConnection(getTargetDataSource();, true);;
return getTransactionAwareConnectionProxy(con, getTargetDataSource(););;
}
万变不离其宗,不过我们还是看看getTransactionAwareConnectionProxy
- protected Connection getTransactionAwareConnectionProxy(Connection target, DataSource dataSource); {
- return (Connection); Proxy.newProxyInstance(
- ConnectionProxy.class.getClassLoader();,
- new Class[] {ConnectionProxy.class},
- new TransactionAwareInvocationHandler(target, dataSource););;
- }
protected Connection getTransactionAwareConnectionProxy(Connection target, DataSource dataSource); {
return (Connection); Proxy.newProxyInstance(
ConnectionProxy.class.getClassLoader();,
new Class[] {ConnectionProxy.class},
new TransactionAwareInvocationHandler(target, dataSource););;
}
原来返回的是jdk的动态代理。继续看TransactionAwareInvocationHandler
- public Object invoke(Object proxy, Method method, Object[] args); throws Throwable {
-
- if (this.dataSource != null); {
- DataSourceUtils.doCloseConnectionIfNecessary(this.target, this.dataSource);;
- }
- return null;
- }
-
- }
public Object invoke(Object proxy, Method method, Object[] args); throws Throwable {
//... if (method.getName();.equals(CONNECTION_CLOSE_METHOD_NAME);); {
if (this.dataSource != null); {
DataSourceUtils.doCloseConnectionIfNecessary(this.target, this.dataSource);;
}
return null;
}
}
TransactionAwareDataSourceProxy会先从DataSourceUtils获取连接。然后将这个连接用jdk的动态代理包一下返回。外部代码如果调用的这个冒牌的Connection,就会先调用TransactionAwareInvocationHandler的invoke,在这个invoke 中,完成原来调用DataSourceUtils的功能。
总结上面的流程
Spring 对DataSource进行事务管理的关键在于ConnectionHolder和TransactionSynchronizationManager。
0.先从TransactionSynchronizationManager中尝试获取连接
1.如果前一步失败则在每个线程上,对每个DataSouce只创建一个Connection
2.这个Connection用ConnectionHolder包装起来,由TransactionSynchronizationManager管理
3.再次请求同一个连接的时候,从TransactionSynchronizationManager返回已经创建的ConnectionHolder,然后调用ConnectionHolder的request将引用计数+1
4.释放连接时要调用ConnectionHolder的released,将引用计数-1
5.当事物完成后,将ConnectionHolder从TransactionSynchronizationManager中解除。当谁都不用,这个连接被close
以上所有都是可以调用DataSourceUtils化简代码,而JdbcTemplate又是调用DataSourceUtils的。所以在Spring文档中要求尽量首先使用JdbcTemplate,其次是用DataSourceUtils来获取和释放连接。至于TransactionAwareDataSourceProxy,那是下策的下策。不过可以将Spring事务管理和遗留代码无缝集成。
所以如某位朋友说要使用Spring的事务管理,但是又不想用JdbcTemplate,那么可以考虑TransactionAwareDataSourceProxy。这个类是原来DataSource的代理。
其次,想使用Spring事物,又不想对Spring进行依赖是不可能的。与其试图自己模拟DataSourceUtils,不如直接使用现成的。