private PooledConnection popConnection(String username, String password)
      throws SQLException { // 获取一个连接
    boolean countedWait = false;
    PooledConnection conn = null;
    long t = System.currentTimeMillis();
    int localBadConnectionCount = 0;

    while (conn == null) {
      synchronized (state) {
        if (!state.idleConnections.isEmpty()) { // pool里还有空闲连接
          // Pool has available connection
          conn = state.idleConnections.remove(0); // 从空闲list移出一个conn,用做新连接
          if (log.isDebugEnabled()) {
            log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
          }
        } else { // pool里没有空闲连接了
          // Pool does not have available connection
          if (state.activeConnections.size() < poolMaximumActiveConnections) { // 活动list没有满
            // Can create new connection
            conn = new PooledConnection(dataSource.getConnection(), this); // 新建一个PooledConnection
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          } else { // 活动list已经满了,无法创建新连接
            // Cannot create new connection
            PooledConnection oldestActiveConnection = state.activeConnections.get(0); // 获取最早开始活动的连接
            long longestCheckoutTime =
                oldestActiveConnection.getCheckoutTime(); // 获得最老的活动连接的checkout时间
            if (longestCheckoutTime > poolMaximumCheckoutTime) { // 如果checkout时间大于最大值
              // Can claim overdue connection
              state.claimedOverdueConnectionCount++; // 过期连接计数++
              state.accumulatedCheckoutTimeOfOverdueConnections +=
                  longestCheckoutTime; // 累加过期连接的checkout时间
              state.accumulatedCheckoutTime += longestCheckoutTime; // 累加checkout时间
              state.activeConnections.remove(oldestActiveConnection); // 从活动list移出最老的活动连接,留出空位来啦
              if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
                oldestActiveConnection.getRealConnection().rollback(); // 回滚
              }
              conn =
                  new PooledConnection(
                      oldestActiveConnection.getRealConnection(), this); // 新建连接,用最老连接的真连接
              oldestActiveConnection.invalidate(); // 最老连接标记无效
              if (log.isDebugEnabled()) {
                log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
              }
            } else { // 如果checkout时间并没有超过最大值,必须等待
              // Must wait
              try {
                if (!countedWait) { // countedWait原来是false
                  state.hadToWaitCount++; // 必须等待的计数++
                  countedWait = true; // countedWait变为true
                }
                if (log.isDebugEnabled()) {
                  log.debug(
                      "Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
                }
                long wt = System.currentTimeMillis();
                state.wait(
                    poolTimeToWait); // 等poolTimeToWait微秒,pushConnection移出一个连接时会notifyAll来唤醒wait的线程
                state.accumulatedWaitTime += System.currentTimeMillis() - wt;
              } catch (InterruptedException e) {
                break;
              }
            }
          }
        }
        if (conn != null) { // conn非空,确实建出来了
          if (conn.isValid()) { // conn有效
            if (!conn.getRealConnection().getAutoCommit()) {
              conn.getRealConnection().rollback(); // 这里回滚是干啥? hama
            }
            conn.setConnectionTypeCode(
                assembleConnectionTypeCode(dataSource.getUrl(), username, password));
            conn.setCheckoutTimestamp(System.currentTimeMillis());
            conn.setLastUsedTimestamp(System.currentTimeMillis());
            state.activeConnections.add(conn); // 加入活动list
            state.requestCount++;
            state.accumulatedRequestTime += System.currentTimeMillis() - t;
          } else { // conn无效
            if (log.isDebugEnabled()) {
              log.debug(
                  "A bad connection ("
                      + conn.getRealHashCode()
                      + ") was returned from the pool, getting another connection.");
            }
            state.badConnectionCount++;
            localBadConnectionCount++;
            conn = null;
            if (localBadConnectionCount > (poolMaximumIdleConnections + 3)) {
              if (log.isDebugEnabled()) {
                log.debug("PooledDataSource: Could not get a good connection to the database.");
              }
              throw new SQLException(
                  "PooledDataSource: Could not get a good connection to the database.");
            }
          }
        }
      }
    } // 到这里while结束

    if (conn == null) { // 出现了奇葩的Unknown错吧
      if (log.isDebugEnabled()) {
        log.debug(
            "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
      }
      throw new SQLException(
          "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
    }

    return conn;
  }
  private PooledConnection popConnection(String username, String password) throws SQLException {
    boolean countedWait = false;
    PooledConnection conn = null;
    long t = System.currentTimeMillis();
    int localBadConnectionCount = 0;

    while (conn == null) {
      synchronized (state) {
        if (state.idleConnections.size() > 0) {
          // Pool has available connection
          conn = (PooledConnection) state.idleConnections.remove(0);
          if (log.isDebugEnabled()) {
            log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
          }
        } else {
          // Pool does not have available connection
          if (state.activeConnections.size() < poolMaximumActiveConnections) {
            // Can create new connection
            conn = new PooledConnection(dataSource.getConnection(), this);
            @SuppressWarnings("unused")
            // used in logging, if enabled
            Connection realConn = conn.getRealConnection();
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          } else {
            // Cannot create new connection
            PooledConnection oldestActiveConnection =
                (PooledConnection) state.activeConnections.get(0);
            long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
            if (longestCheckoutTime > poolMaximumCheckoutTime) {
              // Can claim overdue connection
              state.claimedOverdueConnectionCount++;
              state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
              state.accumulatedCheckoutTime += longestCheckoutTime;
              state.activeConnections.remove(oldestActiveConnection);
              if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
                oldestActiveConnection.getRealConnection().rollback();
              }
              conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
              oldestActiveConnection.invalidate();
              if (log.isDebugEnabled()) {
                log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
              }
            } else {
              // Must wait
              try {
                if (!countedWait) {
                  state.hadToWaitCount++;
                  countedWait = true;
                }
                if (log.isDebugEnabled()) {
                  log.debug(
                      "Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
                }
                long wt = System.currentTimeMillis();
                state.wait(poolTimeToWait);
                state.accumulatedWaitTime += System.currentTimeMillis() - wt;
              } catch (InterruptedException e) {
                break;
              }
            }
          }
        }
        if (conn != null) {
          if (conn.isValid()) {
            if (!conn.getRealConnection().getAutoCommit()) {
              conn.getRealConnection().rollback();
            }
            conn.setConnectionTypeCode(
                assembleConnectionTypeCode(dataSource.getUrl(), username, password));
            conn.setCheckoutTimestamp(System.currentTimeMillis());
            conn.setLastUsedTimestamp(System.currentTimeMillis());
            state.activeConnections.add(conn);
            state.requestCount++;
            state.accumulatedRequestTime += System.currentTimeMillis() - t;
          } else {
            if (log.isDebugEnabled()) {
              log.debug(
                  "A bad connection ("
                      + conn.getRealHashCode()
                      + ") was returned from the pool, getting another connection.");
            }
            state.badConnectionCount++;
            localBadConnectionCount++;
            conn = null;
            if (localBadConnectionCount > (poolMaximumIdleConnections + 3)) {
              if (log.isDebugEnabled()) {
                log.debug("PooledDataSource: Could not get a good connection to the database.");
              }
              throw new SQLException(
                  "PooledDataSource: Could not get a good connection to the database.");
            }
          }
        }
      }
    }

    if (conn == null) {
      if (log.isDebugEnabled()) {
        log.debug(
            "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
      }
      throw new SQLException(
          "PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
    }

    return conn;
  }