How does synchronized and erratic work in the Java memory model?

Question

In the book Effective Java:

// 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(new Runnable() {
            public void run() {
                int i = 0;
                while (!stopRequested)
                    i++;
            }
        });
        backgroundThread.start();

        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}

backgroundThread does not stop after one second. Since the rise, optimization in the JVM, the HotSpot server VM does.

You can view this in the next section:
Why does HotSpot optimize the following using a lift? .

Optimization is performed as follows:

if (!done)
    while (true)
        i++;

There are two ways to fix the problem.

1. Use volatile

private static volatile boolean stopRequested;

The function of volatiles is to prohibit lifting - it ensures that any stream that reads the field will see the last recorded value

2. Use synchronized

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(new Runnable() {
            public void run() {
                int i = 0;
                while (!stopRequested())
                    i++;
            }
        });
        backgroundThread.start();

        TimeUnit.SECONDS.sleep(1);
        requestStop();
    }
}

The above code is right in the book "Effective Java", its equivalent, which is used volatilefor decoration stopRequested.

private static boolean stopRequested() {
    return stopRequested;
}

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, hoisting, synchronized.
?

Answer 1

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private static boolean stopRequested() {
   return stopRequested;
}

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Answer 2

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