java两个线程间隔递增打印数字 （synchronized wait notify的使用）

Question

如下代码

package liupeng.cn.Algorithm.thread;

import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicInteger;



public class ThreadTest {
    public static int counter  = 0;
    public static Integer lock = 0;;
    public static void main(String ... arg){
        final Semaphore sp = new Semaphore(2);
        Thread t1 = new Thread(){
            public void run(){
                for(int i=0;i<10;i++){
                    synchronized (lock) {
                        try {
                            while(lock%2==0){
                                lock.wait();
                            }
                            System.out.println("1 "+lock);
                            lock+=1;
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        lock.notify();
                    }
                }
                
            }
        };
        Thread t2 = new Thread(){
            public void run(){
                for(int i=0;i<10;i++){
                    synchronized (lock) {
                        try {
                            while(lock%2==1){
                                lock.wait();
                            }
                            System.out.println("2 "+lock);
                            lock+=1;
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        lock.notify();
                    }
                }
            }
        };
        t1.start();
        t2.start();
        
    }
}

输出报错，求教为什么会这样，感觉没有地方错啊！！！

2 0
Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
    at java.lang.Object.notify(Native Method)
    at liupeng.cn.Algorithm.thread.ThreadTest$2.run(ThreadTest.java:45)

Answer 1

public class ThreadTest { public static int counter = 0; public static Integer lock = 0;; public static void main(String ... arg){ Thread t1 = new Thread(){ public void run(){ for(int i=0;i<10;i++){ synchronized (lock) { lock.notify(); try { System.out.println("thread" + Thread.currentThread().getId() + " print " + i); lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } } } }; Thread t2 = new Thread(){ public void run(){ for(int i=0;i<10;i++){ synchronized (lock) { lock.notify(); try { System.out.println("thread" + Thread.currentThread().getId() + " print " + i); lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } } } }; t1.start(); t2.start(); } } 你在lock对象上面上锁，但是你居然修改lock的值。 你的代码为报错原因是： This method（notify/wait） should only be called by a thread that is the owner of this object's monitor. hread t1 = new Thread(){ public void run(){ for(int i=0;i<10;i++){ synchronized (lock) {//获取lock=0的对象的监视器 try { while(lock%2==0){ lock.wait(); } System.out.println("1 "+lock); lock+=1;//你居然修改了这个对象 } catch (InterruptedException e) { e.printStackTrace(); } lock.notify();//此时lock的值不为0了，但是该线程拥有的是lock=0的对象监视器，这个时候调用notify方法就报错了，因为notify必须使用线程拥有的监视器对应对象调用。 } } } };