任何對象可以充當監視器。但前提得是同一個對象。

public class SaleTicket extends Thread{	PRivate static int ticket=100;//	private static Object obj = new Object();	@Override	public void run(){		while(true){				try {					Thread.currentThread().sleep(10);				} catch (InterruptedException e) {					// TODO Auto-generated catch block					e.printStackTrace();				}				show();			}		}	public void show(){		synchronized(this){			if(ticket>0){				System.out.println(ticket--);			}		}	}	public static void main(String[] args){		SaleTicket s1 = new SaleTicket();		s1.start();		SaleTicket s2 = new SaleTicket();		s2.start();		SaleTicket s3 = new SaleTicket();		s3.start();			}	}上面代碼，this充當監視器，由于三個Thread對象，所以三個this不一樣，起不到同步的效果。如果將this換成靜態Object，那么同步起作用。
public class SaleTicket2 implements Runnable {	private int ticket =100;	private Object obj = new Object();	@Override	public void run() {		while(true){			//			synchronized (this) {				try {					Thread.currentThread().sleep(10);				} catch (InterruptedException e) {					e.printStackTrace();				}				show();//			}		}	}	public void show(){		synchronized(obj){			if (ticket > 0) {				System.out.println(ticket--);			}		}			}	public static void main(String[] main){		Runnable t = new SaleTicket2();		Thread t1 = new Thread(t);		Thread t2 = new Thread(t);		Thread t3 = new Thread(t);		t1.start();		t2.start();		t3.start();	}}此代碼中，只是有一個Runnable實例，所以ticket和obj都是共享的，synchronized(this)和synchronize(obj)都能夠起到同步效果。因為是線程共享對象。2、同步方法  public synchronize 返回值 方法名（）{}同步方法默認監視器是當前對象this，
public class SaleTicket extends Thread{	private static int ticket=100;	private static Object obj = new Object();	@Override	public void run(){		while(true){				try {					Thread.currentThread().sleep(10);				} catch (InterruptedException e) {					// TODO Auto-generated catch block					e.printStackTrace();				}				show();			}		}	public synchronized void show(){//		synchronized(obj){			if(ticket>0){				System.out.println(ticket--);//			}		}	}	public static void main(String[] args){		SaleTicket s1 = new SaleTicket();		s1.start();		SaleTicket s2 = new SaleTicket();		s2.start();		SaleTicket s3 = new SaleTicket();		s3.start();			}	}上面代碼中，因為new了3個對象，this不同，同步不起作用。這就體現了單例模式的重要性。下面代碼同步起作用，因為只有一個Runnable對象，this相同。
public class SaleTicket2 implements Runnable {	private int ticket =100;	private Object obj = new Object();	@Override	public void run() {		while(true){			//			synchronized (this) {				try {					Thread.currentThread().sleep(10);				} catch (InterruptedException e) {					e.printStackTrace();				}				show();//			}		}	}	public synchronized void show(){//		synchronized(obj){			if (ticket > 0) {				System.out.println(ticket--);			}//		}			}	public static void main(String[] main){		Runnable t = new SaleTicket2();		Thread t1 = new Thread(t);		Thread t2 = new Thread(t);		Thread t3 = new Thread(t);		t1.start();		t2.start();		t3.start();	}}