while(Thread.activeCount() > 1)

今天看到深入理解JVM第367页多线程volatile部分照着书本敲着代码发现了一个问题 Thread.activeCount()会一直大于2

public class VolatileTest {
public static volatile int race = 0;

public static void increase() {
    race++;
}

private static final int THREADS_COUNT = 20;

public static void main(String[] args) {
    Thread[] threads = new Thread[THREADS_COUNT];
    for (int i = 0; i < THREADS_COUNT; i++) {
        threads[i] = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10000; i++) {
                    increase();
                }
            }
        });
        threads[i].start();
    }
    while (Thread.activeCount() > 1) {
        Thread.yield();
    }
    System.out.println(race);
}
}

陷入了死循环.......why？

Thread.yield();//应该主线程先让出cpu使用权

问题在这Thread.activeCount() 还有个守护线程！！！所以就会一直陷入无限循环。

加了一句Thread.currentThread().getThreadGroup().list();

    while (Thread.activeCount() > 1) {
        Thread.currentThread().getThreadGroup().list();
        Thread.yield();
    }