volatile不是線程安全的

public class VolatileTest {
    private volatile int i = 0;

    public void add() {
        i++;
    }

    public int getI() {
        return this.i;
    }

    public static void main(String[] args) {
        final VolatileTest volatileTest = new VolatileTest();
        for (int i = 0; i < 10000; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep((int)Math.random() * 20);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    volatileTest.add();
                }
            }).start();
        }
        System.out.println(volatileTest.getI());
    }
}

　　read-update-write並非線程安全的