Spring事务，connection获取，用DataSourceUtils的原理
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
Java代码
1.protected static Connection doGetConnection(DataSource dataSource, boolean allowSynchronization);
2. throws SQLException {
3.
4. ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
5. if (conHolder != null); {
6. conHolder.requested();;
7. return conHolder.getConnection();;
8. }
9.
10.
11. Connection con = dataSource.getConnection();;
12. if (allowSynchronization && TransactionSynchronizationManager.isSynchronizationActive();); {
13. conHolder = new ConnectionHolder(con);;
14. TransactionSynchronizationManager.bindResource(dataSource, conHolder);;
15. TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(conHolder, dataSource););;
16. conHolder.requested();;
17. }
18.
19. return con;
20. }
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
Java代码
1.private static ThreadLocal resources = new ThreadLocal();;
2.public static Object getResource(Object key); {
3. Map map = (Map); resources.get();;
4. if (map == null); {
5. return null;
6. }
7. Object value = map.get(key);;
8. return value;
9. }
10.public static void bindResource(Object key, Object value); throws IllegalStateException {
11. Map map = (Map); resources.get();;
12. if (map == null); {
13. map = new HashMap();;
14. resources.set(map);;
15. }
16. map.put(key, value);;
17. }
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
Java代码
1.public void requested(); {
2. this.referenceCount++;
3. }
4.
5. public void released(); {
6. this.referenceCount--;
7. }
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
Java代码
1.if (conHolder != null); {
2. conHolder.requested();;
3. return conHolder.getConnection();;
4. }
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
Java代码
1.protected void doCleanupAfterCompletion(Object transaction); {
2. DataSourceTransactionObject txObject = (DataSourceTransactionObject); transaction;
3.
4. // Remove the connection holder from the thread.
5. TransactionSynchronizationManager.unbindResource(this.dataSource);;
6. txObject.getConnectionHolder();.clear();;
7.
8. //... DataSourceUtils.closeConnectionIfNecessary(con, this.dataSource);;
9. }
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
Java代码
1.protected static void doCloseConnectionIfNecessary(Connection con, DataSource dataSource); throws SQLException {
2. if (con == null); {
3. return;
4. }
5.
6. ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
7. if (conHolder != null && con == conHolder.getConnection();); {
8. // It's the transactional Connection: Don't close it.
9. conHolder.released();;
10. return;
11. }
12.
13. // Leave the Connection open only if the DataSource is our
14. // special data source, and it wants the Connection left open.
15. if (!(dataSource instanceof SmartDataSource); || ((SmartDataSource); dataSource);.shouldClose(con);); {
16. logger.debug("Closing JDBC connection");;
17. con.close();;
18. }
19. }
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
Java代码
1.public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action);
2. throws DataAccessException {
3.
4. Connection con = DataSourceUtils.getConnection(getDataSource(););;
5. //其他代码
6. DataSourceUtils.closeConnectionIfNecessary(con, getDataSource(););;
7. }
8. }
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
Java代码
1.public Connection getConnection(); throws SQLException {
2. Connection con = DataSourceUtils.doGetConnection(getTargetDataSource();, true);;
3. return getTransactionAwareConnectionProxy(con, getTargetDataSource(););;
4. }
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
Java代码
1.protected Connection getTransactionAwareConnectionProxy(Connection target, DataSource dataSource); {
2. return (Connection); Proxy.newProxyInstance(
3. ConnectionProxy.class.getClassLoader();,
4. new Class[] {ConnectionProxy.class},
5. new TransactionAwareInvocationHandler(target, dataSource););;
6. }
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
Java代码
1.public Object invoke(Object proxy, Method method, Object[] args); throws Throwable {
2. //... if (method.getName();.equals(CONNECTION_CLOSE_METHOD_NAME);); {
3. if (this.dataSource != null); {
4. DataSourceUtils.doCloseConnectionIfNecessary(this.target, this.dataSource);;
5. }
6. return null;
7. }
8.
9. }
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
