c3p0获取连接Connection后的Close()---释疑

• 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 ?
• 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.

• 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;
         }
         //....
    
     }

• 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.