第 78 条：同步访问共享的可变数据

同步访问共享的可变数据.jpeg

// Broken! - How long would you expect this program to run?
public class StopThread {
    private static boolean stopRequested;
    public static void main(String[] args) throws InterruptedException {
        Thread backgroundThread = new Thread(() -> {
            int i = 0;
            while (!stopRequested)
                i++;
        });
        backgroundThread.start();
        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}

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// Properly synchronized cooperative thread termination
public class StopThread {
    private static boolean stopRequested;
    private static synchronized void requestStop() {
        stopRequested = true;
    }
    private static synchronized boolean stopRequested() {
        return stopRequested;
    }
    public static void main(String[] args) throws InterruptedException {
        Thread backgroundThread = new Thread(() -> {
            int i = 0;
            while (!stopRequested())
                i++;
        });
        backgroundThread.start();
        TimeUnit.SECONDS.sleep(1);
        requestStop();
    }
}

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// Cooperative thread termination with a volatile field
public class StopThread {
    private static volatile boolean stopRequested;
    public static void main(String[] args) throws InterruptedException {
        Thread backgroundThread = new Thread(() -> {
            int i = 0;
            while (!stopRequested)
                i++;
        });
        backgroundThread.start();
        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}

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// Broken - requires synchronization!
private static volatile int nextSerialNumber = 0;
public static int generateSerialNumber() {
    return nextSerialNumber++;
}


// Lock-free synchronization with java.util.concurrent.atomic
private static final AtomicLong nextSerialNum = new AtomicLong();
public static long generateSerialNumber() {
    return nextSerialNum.getAndIncrement();
}

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