协作对象之间发生死锁

class Taxi{
    
    
  private final Dispatcher dispatcher;

  public Taxi(Dispatcher dispatcher){
    
    
    this.dispatcher = dispatcher;
  }

  public synchronized point getLocation(){
    
    
    return location;
  }

  public synchronized void setLocation(Point location){
    
    
    this.location = location;
    if(location.equals(destination)){
    
    
      dispatcher.notifyAvailable(this);
    }
  }
}

class Dispatcher{
    
    
  private final Set<Taxi> taxis;
  private final Set<Taxi> availableTaxis;
  
  public Dispatcher(){
    
    
    taxis = new HashSet<Taxi>();
    availableTaxis = new HashSet<Taxi>();
  }

  public synchronized void notifyAvailable(Taxi taxi){
    
    
    availableTaxis.add(taxi);
  }

  public synchronized Image getImage(){
    
    
    Image image = new Image();
    for(Taxi t:taxis){
    
    
      image.drawMarker(t.getLocation());
    }
    return image;
  }
}

如果在持有锁时调用某个外部方法，那么将出现活跃性问题，在外部方法中可能会获得其他锁，这可能会产生死锁，活着阻塞时间过长，导致其他线程无法及时获得当前被持有的锁