java - 如何使线程 - notify()/notifyAll() 私有(private)

Question

有办法阻止线程调用 notify()/notifyAll() 吗？ 然后，就可以在类内部调用notify。

一种notify的私有(private)方法。 (一种，我们知道，我们不能)

这里有一些代码，必须注意synchronized中的OtherThread，这是我想要阻止的:

public class MainThread {

    public static void main (String[] args) throws InterruptedException{
        Thread.currentThread().setName("Main Thread");
        X x = new X();
        x.start();

        OtherThread y = new OtherThread(x);
        y.start();


        synchronized (x){
            System.out.println(Thread.currentThread().getName()+" Waiting");
            x.wait();
        }
        System.out.println(Thread.currentThread().getName()+": Now I'm running");

    }

}

class X extends Thread{


    public void run(){
        // setting thread name
        Thread.currentThread().setName("X");


        try {
            Thread.sleep(1000*5); // 5 seconds here
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        synchronized (this){
            this.notify();// I want only notify/notifyAll inside X to be possible
            System.out.println("Notified from "+Thread.currentThread().getName()+" Thread");
        }

    }
}

class OtherThread extends Thread{
    Thread t;

    public OtherThread(Thread x){ // pass the X thread to this thread
        this.t = x;
    }

    public void run(){
        // setting thread name
        Thread.currentThread().setName("OtherThread");

        try {
            Thread.sleep(1000*1); // 1 second here
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        synchronized (t){
            t.notify(); // prevent this. To can't call notify from outside class X.
            System.out.println("Notified from "+Thread.currentThread().getName()+" Thread");
        }
    }

}

Answer 1

嗯，答案是你不能。我的意思是，您根本无法更改 notify/wait 可见性(它们对所有对象都设置为公共(public))

但是，我认为可见性并不是适合您的工具。您正在使用 X 的实例作为您正在等待/通知的监视器。问题是，由于其他未指定的原因，您也将该 X 线程共享给其他线程。因此每个人都可以潜在地调用 wait/notify。

我的建议是使用另一个对象作为监视器(而不是 X 的实例)并仅在需要与其交互的对象之间共享该对象。像这样的东西:

public class MainThread {

    public static void main (String[] args) throws InterruptedException{
        Thread.currentThread().setName("Main Thread");
        Object monitor = new Object();

        X x = new X(monitor);
        x.start();

        OtherThread y = new OtherThread(x);
        y.start();


        synchronized (monitor){
            System.out.println(Thread.currentThread().getName()+" Waiting");
            monitor.wait();
        }
        System.out.println(Thread.currentThread().getName()+": Now I'm running");

    }

}

class X extends Thread{

    private Object monitor;

    public X(Object monitor) {
      this.monitor = monitor;
    }

    public void run(){
        // setting thread name
        Thread.currentThread().setName("X");


        try {
            Thread.sleep(1000*5); // 5 seconds here
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        synchronized (this.monitor){
            this.monitor.notify();// I want only notify/notifyAll inside X to be possible
            System.out.println("Notified from "+Thread.currentThread().getName()+" Thread");
        }

    }
}

class OtherThread extends Thread{
    Thread t;

    public OtherThread(Thread x){ // pass the X thread to this thread
        this.t = x;
    }

    public void run(){
        // setting thread name
        Thread.currentThread().setName("OtherThread");

        try {
            Thread.sleep(1000*1); // 1 second here
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        synchronized (t){
            t.notify(); // this will work, but it wont do anything.
            System.out.println("Notified from "+Thread.currentThread().getName()+" Thread");
        }
    }

}

此外，只有一个优点，我建议您查看更高级的方法来同步它。您可以创建一个 Semaphore (或 CountDownLatch)，而不是仅仅共享一个对象来使用它的监视器，它更强大，语义更清晰(这就是我喜欢的)他们)。

**** 编辑 ****

另外，还有一个小建议。 Thread.currentThread().setName("X") 实际上可以通过 this.setName("X") 进行更改，因为您的对象是线程并且知道自己(但你不能在主线程上这样做)。不过，我建议完全删除它，因为 Thread 类公开了一个构造函数，允许您预先设置该名称，因此您可以:

class X extends Thread {

    public X() {
            super("X")
    }

    public void run(){
            try {
            ...
        }
    }

    class OtherThread extends Thread{
        Thread t;

        public OtherThread(Thread x){ // pass the X thread to this thread
            super("OtherThread");
            this.t = x;
        }

        public void run(){
            try {
            ...
        }

    }