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Chapter 18 JDBC and Database Connection Pooling

Listing 18.19 ConnectionPool.java (continued)

  // You can't just make a new connection in the foreground

  // when none are available, since this can take several

  // seconds with a slow network connection. Instead,

  // start a thread that establishes a new connection,

  // then wait. You get woken up either when the new connection

  // is established or if someone finishes with an existing

  // connection.

  private void makeBackgroundConnection() {

    connectionPending = true;

    try {

      Thread connectThread = new Thread(this);

      connectThread.start();

    } catch(OutOfMemoryError oome) {

      // Give up on new connection

    }

  }

  public void run() {

    try {

      Connection connection = makeNewConnection();

      synchronized(this) {

        availableConnections.addElement(connection);

        connectionPending = false;

        notifyAll();

      }

    } catch(Exception e) { // SQLException or OutOfMemory

      // Give up on new connection and wait for existing one

      // to free up.

    }

  }

  // This explicitly makes a new connection. Called in

  // the foreground when initializing the ConnectionPool,

  // and called in the background when running.

  private Connection makeNewConnection()

      throws SQLException {

    try {

      // Load database driver if not already loaded

      Class.forName(driver);

      // Establish network connection to database

      Connection connection =

        DriverManager.getConnection(url, username, password);

      return(connection);

    } catch(ClassNotFoundException cnfe) {

      // Simplify try/catch blocks of people using this by

      // throwing only one exception type.

      throw new SQLException("Can't find class for driver: " +

                             driver);

    }

  }

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