使用synchronized控制线程的执行顺序

//线程按顺序执行：A->B->C->D
public class ThreadTest {

 private int order = 0;
 private Object lock = new Object(); //使用synchronized控制线程顺序执行
 public static void main(String[] args) {
  ThreadTest o = new ThreadTest();
  
  ThreadA t1 = o.new ThreadA();
  ThreadB t2 = o.new ThreadB();
  ThreadC t3 = o.new ThreadC();
  ThreadD t4 = o.new ThreadD();
  t1.start();
  t2.start();
  t3.start();
  t4.start();
 }

 class ThreadA extends Thread{
  public void run() {
   synchronized(lock) {
    while(order !=0) {
     try {
      lock.wait();
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
    }
     
    if(order == 0) {
     System.out.println(order);
     order++;
     lock.notifyAll();
    }
   }
  }
 }
 
 class ThreadB extends Thread{
  public void run() {
   synchronized(lock) {
   while(order !=1) {
    try {
     lock.wait();
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
   if(order == 1) {
    System.out.println(order);
    order++;
    lock.notifyAll();
   }else {
    
   }
  }
  }
 }
 
 class ThreadC extends Thread{
  public void run() {
   synchronized(lock) {
   while(order != 2) {
    try {
     lock.wait();
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
   if(order == 2) {
    System.out.println(order);
    order++;
    lock.notifyAll();
   }
   }
  }
 }
 
 class ThreadD extends Thread{
  public void run() {
   synchronized(lock) {
   while(order != 3) {
    try {
     lock.wait();
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
   if(order == 3) {
    System.out.println(order);
    order++;
    lock.notifyAll();
   }
  }
  }
 }
}