Java中如何通知特定线程

如何在线程间通信中调用特定线程？

在下面的程序中我有两个线程t1 and t2.

当我打电话时t1.notify()它提出：

Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
    at java.lang.Object.notify(Native Method)
    at Shared.methodTwo(NotifyThread.java:43)
    at Thread2.run(NotifyThread.java:77)
Error 

class Shared {

    Thread1 t1 ;
    Thread2 t2 ;

    void ThreadInit( Thread1 t1 , Thread2 t2 ) {
        this.t1 = t1 ;
        this.t2 = t2 ;
    }

    synchronized void methodOne()
    {
        Thread t = Thread.currentThread();

        System.out.println(t.getName()+" is relasing the lock and going to wait");

        try
        {
            wait();        //releases the lock of this object and waits
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        System.out.println(t.getName()+" got the object lock back and can continue with it's execution");
    }

    synchronized void methodTwo()
    {
        Thread t = Thread.currentThread();

        try
        {
            Thread.sleep(5000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        t1.notify();     

        System.out.println("A thread which is waiting for lock of this object is notified by "+t.getName());
    }
    }

    class Thread1 extends Thread 
    {
    Shared s ;
    Thread1( Shared s ) {

        this.s = s ;
    }

    public void run()
            {
                s.methodOne();   //t1 calling methodOne() of 's' object
            }

    } 

    class Thread2 extends Thread {
         Shared s ;
    Thread2( Shared s ) {

        this.s = s ;

    }

    public void run()
            {
                s.methodTwo();   //t1 calling methodOne() of 's' object
            }


    }
    public class NotifyThread 
    {
    public static void main(String[] args)
    {
        final Shared s = new Shared();

        Thread1 t1 = new Thread1(s) ;
        Thread2 t2 = new Thread2(s) ;

        s.ThreadInit(t1,t2) ;

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

You don't / can't notify a specific thread. You call notify() on a lock object. This wakes up one of the threads¹ that is waiting on the lock. In your case, the lock object is a Thread ... which rather confuses the picture. However, see below.

但是你的问题（IllegalMonitorStateException）发生是因为执行通知的线程（即current线程）不持有锁。当前线程在通知锁时必须持有锁，这是一个（硬）要求。

有关更多详细信息，请阅读 javadocsObject.wait(timeout)或（例如）这个：http://howtodoinjava.com/core-java/multi-threading/how-to-work-with-wait-notify-and-notifyall-in-java/ http://howtodoinjava.com/core-java/multi-threading/how-to-work-with-wait-notify-and-notifyall-in-java/

^{1 - If multiple threads are waiting on your lock, one thread is chosen "randomly" by the scheduler. Alternatively notifyAll will wake up all of the waiting threads.}

我不会使用Thread对象作为锁对象。它可能会起作用，但也有可能其他东西（可能是运行时系统中的某些东西）也在锁定/等待/通知Thread对象。那么事情就会变得非常混乱。

（事实上​​，阅读javadoc https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Thread.html#join(long) for Thread.join(long) !)

最好专门为此目的创建锁对象；例如

private final Object lock = new Object();

另外，编写扩展的课程Thread通常是一个坏主意。通常最好实施Runnable接口，实例化它，并将实例作为参数传递给Thread构造函数；例如

Thread t = new Thread(new Runnable() {
    public void run() {
        System.out.println("Hello world");
    }});
t.start();

实施的好处之一Runnable而不是延伸Thread是您可以更轻松地使用代码来管理线程生命周期；例如一个ExecutorService、 fork-join 线程池或经典线程池。

第二个是轻量级线程逻辑可以简洁地实现为匿名类......就像我的示例一样。