使用volatile跳过方法执行

Question

我从来不经常使用易失性。如果另一个线程正在执行它，是否可以使用它跳过方法的执行？我认为在下面的代码中，仍然可能有多个线程通过检查并执行该方法。难到不是么？

private static boolean volatile test = false;
...
    public void test() {
        if (test) {
            return;
        }
        test = true;
        try {
            System.out.println("test() started in Thread with ID " + Thread.currentThread().getId());
            Thread.sleep(10000);
            System.out.println("test() finished in Thread with ID " + Thread.currentThread().getId());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        test = false;
    }

用例:该方法可以定期运行，但同时也可以由用户手动触发。没有理由使用synchronized关键字连续运行两次。请告诉我用易失性是可行的。否则，我看不出有任何理由去理解它，除了求职面试:)其他不基于易失性的解决方案是受欢迎的。

Answer 1

您可以像这样使用volatile AtomicBoolean来满足您的需求。

// default false so that first-thread that test() can enter the logic block
// AtomicBoolean's value is inherently volatile, so no need to declare volatile here
private static final AtomicBoolean test = new AtomicBoolean(false);   


public void test() {
    if (test.compareAndSet(false, true)) {  // check if the test if previously false and if so update it to true
        try {
            System.out.println("test() started in Thread with ID " + Thread.currentThread().getId());
            Thread.sleep(10000);
            System.out.println("test() finished in Thread with ID " + Thread.currentThread().getId());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            test.set(false); // executing thread now re-sets the test value
        }
    }
}