• 論題:
  • java c3p0獲取連接Connnection 之后, 調用 con.close( ) 是否真的關閉了物理連接 ?
• 簡答:
  • c3p0采用連接池, 目的就是提前預置一定數量的連接, 在使用時候重復利用這些連接, 所以, con.close() 之后, 物理連接當然是被放回了連接池, 而沒有真正的關閉 .
  • c3p0中的池連接(指:Connection) 是實現了PooledConnection接口的, PooledConnection為連接池管理提供鉤子 (hook) 的對象, 它會在connnection被操作時接收到消息, 從而對該操作加以干涉,將Connection連接放回連接池.

• 疑問:
  • 一些童鞋經過試驗,發現如下問題,于是發出路上疑問, con.close()真的關閉了連接嗎...
  • 1 public class MyTest { 2 /** 3 * 開啟了20個線程,返回的hashcode,如下:沒有重復的 4 *1730967171 1122742512 156948859 1210106945 1217158489 5 *1868341939 1595154849 1873764403 1937202425 1629470861 6 */ 7 public static void main(String[] args) { 8 for (int i = 0; i < 10; i++) { 9 new Thread(new Runnable() { 10 @Override 11 public void run() { 12 13 Connection con = JDBCUtil.getConnection(); 14 System.out.println(con.hashCode()); 15 // .... 16 con.close(); 17 } catch (SQLException e) { 18 e.printStackTrace(); 19 } 20 } 21 }).start(); 22 } 23 } 24 } 25 public class JDBCUtil { 26 /** 27 * <!-- 最大空閑時間設置為0 , 即永不過期 --> 28 * <property name="maxIdleTime">0</property> 29 * <!-- 最多有多少個連接 --> 30 * <property name="maxPoolSize">5</property> 31 * <!-- 最少幾個連接 --> 32 * <property name="minPoolSize">2</property> 33 */ 34 private static Connection conn; 35 private static ComboPooledDataSource ds = new ComboPooledDataSource(); 36 37 public static Connection getConnection() { 38 try { 39 conn = ds.getConnection(); 40 } catch (SQLException e) { 41 e.printStackTrace(); 42 } 43 return conn; 44 } 45 } View Code

    ?

  • c3p0源碼跟蹤 [ps : 水平有限,以下未解釋java事件注冊相關原理 ]

  • 1 abstract class AbstractPoolBackedDataSource 2 extends PoolBackedDataSourceBase 3 implements PooledDataSource{ 4 //..... 5 6 // dataSource.getConnection()所調用的就是該方法 7 //implementation of javax.sql.DataSource 8 public Connection getConnection() throws SQLException{ 9 10 PooledConnection pc = 11 getPoolManager(). // 返回 C3P0PooledConnectionPoolManager 12 getPool(). // 返回 C3P0PooledConnectionPool 13 checkoutPooledConnection(); // 返回PooledConnection 14 return pc.getConnection(); 15 } 16 17 //..... 18 } 19 20 21 22 class C3P0PooledConnectionPool{ 23 // .... 24 public PooledConnection checkoutPooledConnection() throws SQLException{ 25 //System.err.println(this + " -- CHECKOUT"); 26 try 27 { 28 PooledConnection pc = (PooledConnection) this.checkoutAndMarkConnectionInUse(); 29 pc.addConnectionEventListener( cl ); 30 return pc; 31 } 32 catch (TimeoutException e) 33 { throw SqlUtils.toSQLException("An attempt by a client to checkout a Connection has timed out.", e); } 34 catch (CannotAcquireResourceException e) 35 { throw SqlUtils.toSQLException("Connections could not be acquired from the underlying database!", "08001", e); } 36 catch (Exception e) 37 { throw SqlUtils.toSQLException(e); } 38 } 39 40 private Object checkoutAndMarkConnectionInUse() throws TimeoutException, CannotAcquireResourceException, ResourcePoolException, InterruptedException 41 { 42 Object out = null; 43 boolean success = false; 44 while (! success) 45 { 46 try 47 { 48 out = rp.checkoutResource( checkoutTimeout ); 49 if (out instanceof AbstractC3P0PooledConnection) 50 { 51 // cast should succeed, because effectiveStatementCache implies c3p0 pooled Connections 52 AbstractC3P0PooledConnection acpc = (AbstractC3P0PooledConnection) out; 53 /* 54 *以下在獲取物理連接的時候,PooledcConnection中注冊的事件監聽器會收到消息 55 *無論每次對connection的何種操作,PooledConnection都會收到來自驅動的消息, 56 *其中的鉤子(hook)對象就會完成對Connection的回收 57 */ 58 Connection physicalConnection = acpc.getPhysicalConnection(); 59 success = tryMarkPhysicalConnectionInUse(physicalConnection); 60 } 61 else 62 success = true; //we don't pool statements from non-c3p0 PooledConnections 63 } 64 finally 65 { 66 try { if (!success && out != null) rp.checkinResource( out );} 67 catch (Exception e) { logger.log(MLevel.WARNING, "Failed to check in a Connection that was unusable due to pending Statement closes.", e); } 68 } 69 } 70 return out; 71 } 72 //.... 73 74 } View Code
• 綜述:
  • 當應用程序關閉連接時，它調用 Connection 方法 close。完成連接池時，連接池管理器將得到通知；因為它曾使用 ConnectionPool 方法 addConnectionEventListener 作為 ConnectionEventListener 對象注冊它自身。連接池管理器釋放到 PooledConnection 對象的句柄，并將 PooledConnection 對象返回到連接池，以便再次使用。因此，當應用程序關閉其連接時，基礎物理連接會被回收而不是被關閉。
  • 知識水平有限,難免錯誤,模糊, 請多批評指正,謝謝.

轉載于:https://www.cnblogs.com/murthy/p/6495196.html

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