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论题:
- java c3p0获取连接Connnection 之后, 调用 con.close( ) 是否真的关闭了物理连接 ?
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简答:
- c3p0采用连接池, 目的就是提前预置一定数量的连接, 在使用时候重复利用这些连接, 所以, con.close() 之后, 物理连接当然是被放回了连接池, 而没有真正的关闭 .
- c3p0中的池连接(指:Connection) 是实现了PooledConnection接口的, PooledConnection为连接池管理提供钩子 (hook) 的对象, 它会在connnection被操作时接收到消息, 从而对该操作加以干涉,将Connection连接放回连接池.
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疑问:
- 一些童鞋经过试验,发现如下问题,于是发出路上疑问, con.close()真的关闭了连接吗...
public class MyTest {
/**
* 开启了20个线程,返回的hashcode,如下:没有重复的
*1730967171 1122742512 156948859 1210106945 1217158489
*1868341939 1595154849 1873764403 1937202425 1629470861
*/
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
new Thread(new Runnable() {
@Override
public void run() { Connection con = JDBCUtil.getConnection();
System.out.println(con.hashCode());
// ....
con.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}).start();
}
}
}
public class JDBCUtil {
/**
* <!-- 最大空闲时间设置为0 , 即永不过期 -->
* <property name="maxIdleTime">0</property>
* <!-- 最多有多少个连接 -->
* <property name="maxPoolSize">5</property>
* <!-- 最少几个连接 -->
* <property name="minPoolSize">2</property>
*/
private static Connection conn;
private static ComboPooledDataSource ds = new ComboPooledDataSource(); public static Connection getConnection() {
try {
conn = ds.getConnection();
} catch (SQLException e) {
e.printStackTrace();
}
return conn;
}
}c3p0源码跟踪 [ps : 水平有限,以下未解释java事件注册相关原理 ]
abstract class AbstractPoolBackedDataSource
extends PoolBackedDataSourceBase
implements PooledDataSource{
//..... // dataSource.getConnection()所调用的就是该方法
//implementation of javax.sql.DataSource
public Connection getConnection() throws SQLException{ PooledConnection pc =
getPoolManager(). // 返回 C3P0PooledConnectionPoolManager
getPool(). // 返回 C3P0PooledConnectionPool
checkoutPooledConnection(); // 返回PooledConnection
return pc.getConnection();
} //.....
} class C3P0PooledConnectionPool{
// ....
public PooledConnection checkoutPooledConnection() throws SQLException{
//System.err.println(this + " -- CHECKOUT");
try
{
PooledConnection pc = (PooledConnection) this.checkoutAndMarkConnectionInUse();
pc.addConnectionEventListener( cl );
return pc;
}
catch (TimeoutException e)
{ throw SqlUtils.toSQLException("An attempt by a client to checkout a Connection has timed out.", e); }
catch (CannotAcquireResourceException e)
{ throw SqlUtils.toSQLException("Connections could not be acquired from the underlying database!", "08001", e); }
catch (Exception e)
{ throw SqlUtils.toSQLException(e); }
} private Object checkoutAndMarkConnectionInUse() throws TimeoutException, CannotAcquireResourceException, ResourcePoolException, InterruptedException
{
Object out = null;
boolean success = false;
while (! success)
{
try
{
out = rp.checkoutResource( checkoutTimeout );
if (out instanceof AbstractC3P0PooledConnection)
{
// cast should succeed, because effectiveStatementCache implies c3p0 pooled Connections
AbstractC3P0PooledConnection acpc = (AbstractC3P0PooledConnection) out;
/*
*以下在获取物理连接的时候,PooledcConnection中注册的事件监听器会收到消息
*无论每次对connection的何种操作,PooledConnection都会收到来自驱动的消息,
*其中的钩子(hook)对象就会完成对Connection的回收
*/
Connection physicalConnection = acpc.getPhysicalConnection();
success = tryMarkPhysicalConnectionInUse(physicalConnection);
}
else
success = true; //we don't pool statements from non-c3p0 PooledConnections
}
finally
{
try { if (!success && out != null) rp.checkinResource( out );}
catch (Exception e) { logger.log(MLevel.WARNING, "Failed to check in a Connection that was unusable due to pending Statement closes.", e); }
}
}
return out;
}
//.... }
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综述:
- 当应用程序关闭连接时，它调用
Connection方法close。完成连接池时，连接池管理器将得到通知；因为它曾使用ConnectionPool方法addConnectionEventListener作为ConnectionEventListener对象注册它自身。连接池管理器释放到PooledConnection对象的句柄，并将PooledConnection对象返回到连接池，以便再次使用。因此，当应用程序关闭其连接时，基础物理连接会被回收而不是被关闭。 - 知识水平有限,难免错误,模糊, 请多批评指正,谢谢.
- 当应用程序关闭连接时，它调用