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18.7 Connection Pooling

505

Listing 18.19 ConnectionPool.java (continued)

  public synchronized Connection getConnection()

      throws SQLException {

    if (!availableConnections.isEmpty()) {

      Connection existingConnection =

        (Connection)availableConnections.lastElement();

      int lastIndex = availableConnections.size()   1;

      availableConnections.removeElementAt(lastIndex);

      // If connection on available list is closed (e.g.,

      // it timed out), then remove it from available list

      // and repeat the process of obtaining a connection.

      // Also wake up threads that were waiting for a

      // connection because maxConnection limit was reached.

      if (existingConnection.isClosed()) {

        notifyAll(); // Freed up a spot for anybody waiting

        return(getConnection());

      } else {

        busyConnections.addElement(existingConnection);

        return(existingConnection);

      }

    } else {

      // Three possible cases:

      // 1) You haven't reached maxConnections limit. So

      //    establish one in the background if there isn't

      //    already one pending, then wait for

      //    the next available connection (whether or not

      //    it was the newly established one).

      // 2) You reached maxConnections limit and waitIfBusy

      //    flag is false. Throw SQLException in such a case.

      // 3) You reached maxConnections limit and waitIfBusy

      //    flag is true. Then do the same thing as in second

      //    part of step 1: wait for next available connection.

      if ((totalConnections() < maxConnections) &&

          !connectionPending) {

        makeBackgroundConnection();

      } else if (!waitIfBusy) {

        throw new SQLException("Connection limit reached");

      }

      // Wait for either a new connection to be established

      // (if you called makeBackgroundConnection) or for

      // an existing connection to be freed up.

      try {

        wait();

      } catch(InterruptedException ie) {}

      // Someone freed up a connection, so try again.

      return(getConnection());

    }

  }

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