public class Test2 {
<span class="hljs-keyword">private</span> <span class="hljs-keyword">static</span> volatile Boolean aBoolean = <span class="hljs-keyword">true</span>;

<span class="hljs-keyword">static</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span> <span class="hljs-inheritance"><span class="hljs-keyword">implements</span></span> <span class="hljs-title">Runnable</span>{</span>

    @Override

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {

        synchronized (aBoolean){

            <span class="hljs-keyword">while</span> (aBoolean){

                <span class="hljs-keyword">try</span> {

                    System.out.println(<span class="hljs-string">"进入A, 等待中"</span>);

                    aBoolean.wait();

                } <span class="hljs-keyword">catch</span> (InterruptedException e) {

                    e.printStackTrace();

                }

            }

            System.out.println(<span class="hljs-string">"A over"</span>);

        }

    }

}

<span class="hljs-keyword">static</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">B</span> <span class="hljs-inheritance"><span class="hljs-keyword">implements</span></span> <span class="hljs-title">Runnable</span>{</span>

    @Override

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {

        <span class="hljs-keyword">try</span> {

            TimeUnit.SECONDS.sleep(<span class="hljs-number">3</span>);

        } <span class="hljs-keyword">catch</span> (InterruptedException e) {

            e.printStackTrace();

        }

        synchronized (aBoolean){

            System.out.println(<span class="hljs-string">"通知A可以运行了"</span>);

            aBoolean = <span class="hljs-keyword">false</span>;

            aBoolean.notifyAll();

            <span class="hljs-comment">//Boolean.TRUE.notifyAll();</span>

        }

    }

}

<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> main(String[] args) {

    <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> A()).start();

    <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> B()).start();

}


}

运行结果:

进入A, 等待中

通知A可以运行了

Exception in thread "Thread-1" java.lang.IllegalMonitorStateException

    at java.lang.Object.notifyAll(Native Method)

    at Test2$B.run(Test2.java:39)

    at java.lang.Thread.run(Thread.java:748)

由于我们会给aBoolean重新赋值, 导致对象监视器改变, 因为false和true这两个常量对应着两个不同的对象。当aBoolean产生变化时，很可能导致不同的线程同步了不同的对象

解决方法:

将这一行 aBoolean.notifyAll();  改为: Boolean.TRUE.notifyAll();

或者是用显示锁ReentrantLock的方式