Shared Connection Pool via Singleton

Question

I have just started to learn Java (more jsp & servlets) and have been going through "Core Servlets and Java Server Pages" and in there is stuff on connection pooling. The author provides a good a connection pool servlet that can be shared, but as I'm new and don't fully understand how everything works (got a basic idea).
How do i go about making the following code accessiable via a singleton class?
ConnectionPool Class
package sco;
import java.sql.*;
import java.util.*;
/** A class for preallocating, recycling, and managing
*  JDBC connections.
*  <P>
*  Taken from Core Servlets and JavaServer Pages
*  from Prentice Hall and Sun Microsystems Press,
*  http://www.coreservlets.com/.
*  &copy; 2000 Marty Hall; may be freely used or adapted.
public class ConnectionPool implements Runnable {
  private String driver, url, username, password;
  private int maxConnections;
  private boolean waitIfBusy;
  private Vector availableConnections, busyConnections;
  private boolean connectionPending = false;
  public ConnectionPool(String driver, String url,
                        String username, String password,
                        int initialConnections,
                        int maxConnections,
                        boolean waitIfBusy)
      throws SQLException {
    this.driver = driver;
    this.url = url;
    this.username = username;
    this.password = password;
    this.maxConnections = maxConnections;
    this.waitIfBusy = waitIfBusy;
    if (initialConnections > maxConnections) {
      initialConnections = maxConnections;
    availableConnections = new Vector(initialConnections);
    busyConnections = new Vector();
    for(int i=0; i<initialConnections; i++) {
      availableConnections.addElement(makeNewConnection());
  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());
  // 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);
  public synchronized void free(Connection connection) {
    busyConnections.removeElement(connection);
    availableConnections.addElement(connection);
    // Wake up threads that are waiting for a connection
    notifyAll();
  public synchronized int totalConnections() {
    return(availableConnections.size() +
           busyConnections.size());
  /** Close all the connections. Use with caution:
   *  be sure no connections are in use before
   *  calling. Note that you are not <I>required</I> to
   *  call this when done with a ConnectionPool, since
   *  connections are guaranteed to be closed when
   *  garbage collected. But this method gives more control
   *  regarding when the connections are closed.
  public synchronized void closeAllConnections() {
    closeConnections(availableConnections);
    availableConnections = new Vector();
    closeConnections(busyConnections);
    busyConnections = new Vector();
  private void closeConnections(Vector connections) {
    try {
      for(int i=0; i<connections.size(); i++) {
        Connection connection =
          (Connection)connections.elementAt(i);
        if (!connection.isClosed()) {
          connection.close();
    } catch(SQLException sqle) {
      // Ignore errors; garbage collect anyhow
  public synchronized String toString() {
    String info =
      "ConnectionPool(" + url + "," + username + ")\n" +
      ", available=" + availableConnections.size() + "\n" +
      ", busy=" + busyConnections.size() + "\n" +
      ", max=" + maxConnections;
    return(info);
ScoPool Class (singleton to access the connection pool)
package sco;
public class ScoPool extends ConnectionPool {
  private ScoPool pool = null;
  private ScoPool() {
    super(); // Call parent constructor
  public static synchronized ScoPool getInstance() {
    if(pool == null) {
      pool = new ScoPool();
    return(pool);
}Please help a newbie.

Answer 1

Figured it out.
package sco;
import java.sql.SQLException;
public class ScoPool extends ConnectionPool {
  private static ScoPool pool;
  private ScoPool(String driver, String url, String username, String password,
                  int initialConnections, int maxConnections, boolean waitIfBusy) throws SQLException {
    super(driver, url, username, password, initialConnections, maxConnections, waitIfBusy); // Call parent constructor
  public static synchronized ScoPool getInstance() {
    if(pool == null) {
      String driver = "com.microsoft.jdbc.sqlserver.SQLServerDriver";
      String url = "jdbc:microsoft:sqlserver://MIM-W0432:1433;DatabaseName=sco";
      String username = "sco_user";
      String password = "123";
      int initCon = 5;
      int maxCon = 10;
      boolean waitIfBusy = true;
      try {
        pool = new ScoPool(driver, url, username, password, initCon, maxCon, waitIfBusy);
      catch(SQLException sqle) {
    return pool;
}