为什么wait和notify函数在同一个类锁上不能正常工作

Question

为什么wait和notify函数在同一类锁上不能正常工作？

请查看下面的代码，了解等待和通知功能及其输出。

输出：

Thread-1
Thread-2
Thread-2 after notify

预期结果：

Thread-1
Thread-2
Thread-2 after notify
Thread-1 after wait    

代码：

public class WaitAndNotify1 {    
    public static void main(String[] args) {        
        Thread t1=new Thread(new Runnable(){            
            @Override
            public void run(){                
                System.out.println("Thread-1");                
                try {
                    synchronized (this) {
                        wait();
                        System.out.println("Thread-1 after wait");
                    }                    
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        Thread t2=new Thread(new Runnable(){            
            @Override
            public void run(){
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread-2");
                synchronized (this) {
                    notify();
                    System.out.println("Thread-2 after notify");
                }
            }
        });
        t1.start();
        t2.start();
    }
}

Answer 1

您正在使用来自匿名内部类的this -因此它引用了该匿名内部类的实例。有两个不同的实例(来自不同的匿名内部类)，所以您在一个不同于调用notify()的对象上调用wait()。

您目前实际上还没有要同步的WaitAndNotify1实例。您可以将代码移到一个实例方法中，然后使用WaitAndNotify1.this来引用该实例--在这一点上您将获得预期的输出：

public class WaitAndNotify1 {
    public static void main(String[] args) {
        new WaitAndNotify1().test();
    }

    public void test() {
        Thread t1=new Thread(new Runnable(){            
            @Override
            public void run(){                
                System.out.println("Thread-1");                
                try {
                    synchronized (WaitAndNotify1.this) {
                        WaitAndNotify1.this.wait();
                        System.out.println("Thread-1 after wait");
                    }                    
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        Thread t2=new Thread(new Runnable(){            
            @Override
            public void run(){
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread-2");
                synchronized (WaitAndNotify1.this) {
                    WaitAndNotify1.this.notify();
                    System.out.println("Thread-2 after notify");
                }
            }
        });
        t1.start();
        t2.start();
    }
}