数据库连接池连接复用原理

发表于

HikariCP的ProxyConnection类有如下方法：

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// **********************************************************************
//              "Overridden" java.sql.Connection Methods
// **********************************************************************

/** {@inheritDoc} */
@Override
public final void close() throws SQLException
{
   // Closing statements can cause connection eviction, so this must run before the conditional below
   closeStatements();

   if (delegate != ClosedConnection.CLOSED_CONNECTION) {
      leakTask.cancel();

      try {
         if (isCommitStateDirty && !isAutoCommit) {
            delegate.rollback();
            lastAccess = currentTime();
            LOGGER.debug("{} - Executed rollback on connection {} due to dirty commit state on close().", poolEntry.getPoolName(), delegate);
         }

         if (dirtyBits != 0) {
           // 重置catalog、schema、sReadOnly、AutoCommit、transactionIsolation、NetworkTimeout这些属性
            poolEntry.resetConnectionState(this, dirtyBits);
            lastAccess = currentTime();
         }

         delegate.clearWarnings();
      }
      catch (SQLException e) {
         // when connections are aborted, exceptions are often thrown that should not reach the application
         if (!poolEntry.isMarkedEvicted()) {
            throw checkException(e);
         }
      }
      finally {
         delegate = ClosedConnection.CLOSED_CONNECTION;
         //PoolEntry会被设为STATE_NOT_IN_USE,PoolEntry保存了Connection
         poolEntry.recycle(lastAccess);
      }
   }
}

从上代码可知，ProxyConnection重写了Connection的Close方法，在我们结合spring调用完如数据库查询会调用connection.close(),但是ProxyConnection并不会真正去关闭连接，而是重置该连接的状态,使得该连接可以继续被使用。