jmm - java如何把数据同步到主内存？

Question

Consumer.run()里面执行n.conusme()时，count值为5
但是之后的n.getCount()得到的是4

public class ThreadDemo {

    public static void main(String[] args) {
        Apple n = new Apple();

        Thread a1 = new Thread(new Producer(n), "Producer");
        Thread a2 = new Thread(new Consumer(n), "Consumer");

        a1.start();
        try{
            Thread.sleep(1000) ;
        }catch(InterruptedException e){
            e.printStackTrace() ;
        }
        a2.start();
    }

}

class Producer implements Runnable {
    Apple n;

    public Producer(Apple n) {
        this.n = n;
    }

    volatile boolean keepRunning = true;

    @Override
    public void run() {
        while (true) {
            while (keepRunning) {
                if (n.getCount() < 5) {
                    n.produce();
                    System.out.println(Thread.currentThread().getName()
                            + " produced an apple，" + n.getCount() + " apple(s) left");
                }
                if (n.getCount() >= 5) {
                    keepRunning = false;
                }
                Thread.yield();
            }
            if (n.getCount() < 5) {
                keepRunning = true;

            }
        }

    }
}

class Consumer implements Runnable {
    Apple n;

    public Consumer(Apple n) {
        this.n = n;
    }

    volatile boolean keepRunning = true;

    @Override
    public void run() {
        while (true) {
            while (keepRunning) {
                if (n.getCount() <= 5 && n.getCount() > 0) {
                    n.consume();
                    System.out.println(Thread.currentThread().getName()
                            + " consumed an apple," + n.getCount() + " apple(s) left");////here is 5

                }
                if (n.getCount() <= 0) {
                    keepRunning = false;
                }

                Thread.yield();
            }
            if (n.getCount() > 0) {
                keepRunning = true;
            }
        }

    }

}

class Apple {
    private int count = 0;

    public int getCount() {
        return count;
    }

    public synchronized void produce() {
        count++;

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void consume() {
        count--;
        System.out.println("count="+count);//here is 4
    }

}

Answer 1

因為getcount方法沒有使用sychronized，所以當你呼叫consumer方法時，getcount方法也會同時執行

Answer 2

加鎖 或是 使用volatile 會保證多執行緒間資料的修改是可見的。