java - 交换器 : JVM never stops

Question

public static void main(String[] args) throws Exception {
    final Exchanger<String> exchanger = new Exchanger<String>();
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + exchanger.exchange("this came from subthread"));
            } catch (InterruptedException ex) {
                System.out.println("interrupted while waiting for message");
            }
        }
    }).start();

    System.out.println(Thread.currentThread().getName() + exchanger.exchange("this came from main thread"));
    String s = exchanger.exchange("this came from main thread");
}

输出

mainthis came from subthread
Thread-0this came from main thread

为什么 JVM 这里永远不会退出？

Answer 1

您的线程中有 1 个交换点，但主线程中有 2 个交换点。因此，第二次交换:String s = Exchanger.exchange("this come from main thread"); 永远等待并阻止 JVM 退出。如果您在该行之后添加打印语句，您将看到它不会被执行。

如果您在线程中添加第二个交换器，程序将退出:

public void run() {
    try {
        System.out.println(Thread.currentThread().getName() + exchanger.exchange("this came from subthread"));
        exchanger.exchange("allow JVM to exit here");
    } catch (InterruptedException ex) {
        System.out.println("interrupted while waiting for message");
    }
}