java - 使用内在锁进入 block

Question

我看不出以下代码如何产生似乎违反对象锁定义的输出。当然应该只允许一个线程打印“获得的锁”消息，但他们都这样做了？

class InterruptThreadGroup {
    public static void main(String[] args) {
        Object lock = new Object();
        MyThread mt1 = new MyThread(lock);
        MyThread mt2 = new MyThread(lock);
        mt1.setName("A");
        mt1.start();
        mt2.setName("B");
        mt2.start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        // Thread.currentThread().getThreadGroup().interrupt();
    }
}

class MyThread extends Thread {
    private Object lock;

    public MyThread(Object l) {
        this.lock = l;

    }

    public void run() {
        synchronized (lock) {
            System.out.println(getName() + " acquired lock");
            try {
                lock.wait();
            } catch (InterruptedException e) {
                System.out.println(getName() + " interrupted.");
            }
            System.out.println(getName() + " terminating.");
        }
    }
}

Answer 1

是因为调用lock.wait()释放了锁，让第二个线程进入synchronized block 。摘自 javadoc

The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.

请注意，您的代码中存在一些问题，例如:

• 你不应该在 while 循环之外等待
• 任何地方都没有通知，所以你可以永远等待
• 让您的任务实现 Runnable 并将其作为参数传递给 Thread 的构造函数比直接扩展 Thread 更好。