MyThread.java:

 1 class MyThread implements Runnable {
 2     int i = 1;
 3 
 4     public void run() {
 5         // 线程锁，一个线程执行本处代码时，其他线程等待本线程执行完毕后再执行；
 6         // 如果不加线程锁，则线程会得到同一个数；
 7         synchronized (this) {
 8             if (i <= 0) {
 9                 return;
10             }
11             System.out.println(Thread.currentThread().getName() + "--->" + i);
12             i--;
13         }
14     }
15 }

TestMyThread.java:

 1 class TestMyThread{
 2     public static void main(String[] args){
 3         MyThread myThread = new MyThread();
 4         Thread t1 = new Thread(myThread);
 5         Thread t2 = new Thread(myThread);
 6         Thread t3 = new Thread(myThread);
 7         Thread t4 = new Thread(myThread);
 8         t1.setName("线程1");
 9         t2.setName("线程2");
10         t3.setName("线程3");
11         t4.setName("线程4");
12         t1.start();
13         t2.start();
14         t3.start();
15         t4.start();
16         try{
17             Thread.sleep(1000);
18         }catch(Exception e){
19             e.printStackTrace();
20         }
21         System.out.println(myThread.i);//加锁时为0，不加锁为0到-3
22     }
23 }