Turn.java

package org.prevayler.foundation;

/** 
 * Used to sequence operations that have to be performed in order by several concurrent threads. 
 */
public class Turn {
  private Turn _next;
  private int _tickets=0;
  private boolean _isAlwaysSkipped;
  public static Turn first(){
    return new Turn(1000000);
  }
  private Turn(  int tickets){
    _tickets=tickets;
  }
  public Turn next(){
    if (_next == null)     _next=new Turn(0);
    return _next;
  }
  public synchronized void start(){
    if (_tickets == 0)     Cool.wait(this);
    _tickets--;
  }
  public void end(){
    next().haveSomeTickets(1);
  }
  private synchronized void haveSomeTickets(  int tickets){
    if (_isAlwaysSkipped) {
      next().haveSomeTickets(tickets);
      return;
    }
    _tickets+=tickets;
    notify();
  }
  public synchronized void alwaysSkip(){
    end();
    _isAlwaysSkipped=true;
    next().haveSomeTickets(_tickets);
  }
}