java - 这个并发的遗留代码是否需要可变

Question

我正在努力处理一些并发的遗留代码，并且想知道stopLatch和/或mode是否应为volatile:

public class MyClass {

    private final ExecutorService executor = Executors.newSingleThreadExecutor();
    private MyModeEnum mode = MyModeEnum.NONE;
    private CountDownLatch stopLatch;

    public synchronized void start(final MyModeEnum mode) {
        assert mode != null && mode != MyModeEnum.NONE;
        if (isRunning()) {
            // Throw an exception.
        }
        this.mode = mode;
        stopLatch = new CountDownLatch(1);
        executor.execute(() -> {
            try {
                // Pre and post operations not under control around stopLatch.await().
            } catch (final Exception e) {
                stop();
                // Further exception handling.
            } finally {
                MyClass.this.mode = MyModeEnum.NONE;
            }
        });
    }

    public synchronized void stop() {
        if (!isRunning()) {
            return;
        }
        stopLatch.countDown();
    }

    public boolean isRunning() {
        return mode != MyModeEnum.NONE;
    }

    public MyModeEnum getMode() {
        return mode;
    }

}

详细的解释非常感谢。

编辑:我无法将一般问题/答案(例如 When exactly do you use the volatile keyword in Java?)归结为这个特定问题。

Answer 1

根据最受好评的评论:

Mick Mnemonic:

isRunning() is not synchronized, so mode should be volatile.

Brian Goetz:

You have data races on mode. It is accessed from outside the sync block. First choice should be to synchronize isRunning and getMode; making mode volatile is a more advanced play. (Walk first, then fly.)