synchronized(任意物件)：就相當於給程式碼加鎖了，任意物件就可以看成是一把鎖。

synchronized(任意物件)
{
 多條語句操作共享資料的程式碼 
}

程式碼演示

public class SellTicket implements Runnable {
    private  int  tickets=100;
    private Object obj = new Object();
    @Override
    public void run() {
            while(true){
                synchronized
 
 (obj){
                    if(tickets>0){
                        try {
                            Thread.sleep(100);
                        }catch (InterruptedException e){
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName()
 
+"正在出售第"+tickets+"張票");
                        tickets--;
                    }
                }

            }
    }
}

public class SellTicketDemo {
    public static void main(String[] args) {
        SellTicket st= new SellTicket();

        Thread t1=new Thread(st,"視窗1");
        Thread t2=new
 
 Thread(st,"視窗2");
        Thread t3=new Thread(st,"視窗3");

        t1.start();
        t2.start();
        t3.start();
    }
}