为什么notifyAll（）在Integer上同步时会引发IllegalMonitorStateException？

问题描述

为什么这个测试程序导致 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

您已正确注意到必须从同步块调用 notifyAll 。

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

但是，在你的情况下，由于自动装箱，您同步的对象与您调用的实例 notifyAll 不同。实际上，新的递增的 foo 实例仍局限于堆栈，并且在等待 call。

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.

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