为什么这个测试程序会导致 java.lang.IllegalMonitorStateException?

Why does this test program result in a java.lang.IllegalMonitorStateException?

public class test {
    static Integer foo = new Integer(1);
    public static void main(String[] args) {
        synchronized(foo) {
            foo++;
            foo.notifyAll();
        }
        System.err.println("Success");
    }
}

结果:

Exception in thread "main" java.lang.IllegalMonitorStateException
        at java.lang.Object.notifyAll(Native Method)
        at test.main(test.java:6)

推荐答案

您已经正确地注意到 notifyAll 必须从同步块中调用.

You have noted correctly that notifyAll must be called from a synchronized block.

但是，在您的情况下，由于自动装箱，您同步的对象与您调用 notifyAll 的实例不同.事实上，新的、递增的 foo 实例仍然被限制在堆栈中，并且不可能在 wait 调用上阻塞其他线程.

However, in your case, because of auto-boxing, the object you synchronized on is not the same instance that you invoked notifyAll on. In fact, the new, incremented foo instance is still confined to the stack, and no other threads could possibly be blocked on a wait call.

您可以实现自己的可变计数器，在该计数器上执行同步.根据您的应用程序，您可能还会发现 AtomicInteger 满足您的需求.

You could implement your own, mutable counter on which synchronization is performed. Depending on your application, you might also find that AtomicInteger meets your needs.