spring jdbc 源码 分析

Collections.unmodifiableMap(map)-------------------------返回一个不可修改的map

转自：http://www.iteye.com/topic/11738

1 如何获得连接

看DataSourceUtils代码

Java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

Java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

Java代码







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

Java代码







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

Java代码







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);;

}

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







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();;
}
}

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







public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action);

throws DataAccessException {

Connection con = DataSourceUtils.getConnection(getDataSource(););;

//其他代码
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

Java代码







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

Java代码







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

Java代码







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;

}

}

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。