线程安全 synchronize关键字

任何对象可以充当监视器。但前提得是同一个对象。

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();			}	}上面代码，this充当监视器，由于三个Thread对象，所以三个this不一样，起不到同步的效果。如果将this换成静态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();	}}此代码中，只是有一个Runnable实例，所以ticket和obj都是共享的，synchronized(this)和synchronize(obj)都能够起到同步效果。因为是线程共享对象。2、同步方法  public synchronize 返回值 方法名（）{}同步方法默认监视器是当前对象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();			}	}上面代码中，因为new了3个对象，this不同，同步不起作用。这就体现了单例模式的重要性。下面代码同步起作用，因为只有一个Runnable对象，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();	}}