老师你好，FutureTask源码中有一个用于返回的私有域outcome：

/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes

注释上说由于state被volatile修饰保证了outcome无需被volatile修饰，我不是很明白，难道是happens-before原则吗，但我看源码中outcome的获取都是在state赋值后发生的，不是之前呀, 如下：

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);
}
private V report(int s) throws ExecutionException {
    Object x = outcome;
    if (s == NORMAL)
        return (V)x;
    if (s >= CANCELLED)
        throw new CancellationException();
    throw new ExecutionException((Throwable)x);
}