并发编程中3个特性需要关注（原子性，可见性，有序性）

1.原子性（不支持）

public class TestAtom {

    //并发编程：需要关注三个特性：原子性、可见性、有序性
    //volatile无法保证原子性
    private static volatile int count = 0;

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for (int j = 0; j < 1000; j++) {
                        count++;
                    }
                }
            }).start();

        }

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

        System.out.println(count);
    }

}

count 最后输出不会是10000。

2.可见性（支持）

public class TestVisible {

    //线程间不可见，使用volatile使可见
    private static volatile boolean flag = false;

    public static void main(String[] args) throws InterruptedException {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("waiting");
                while (!flag) {
                }
                System.out.println("in");
            }
        }).start();

        Thread.sleep(1000);

        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("change flag");
                flag = true; //修改flag，上面线程会打印“in”
                System.out.println("change success");
            }
        }).start();
        
    }

}

3.有序性（禁止重排序）：经典DCL

public class SingleTon {
    //volatile 禁止指令重排序
    private static volatile SingleTon singleTon;

    private SingleTon() {
    }

    public static SingleTon getInstance() {
        if (null == singleTon) {
            synchronized (SingleTon.class) {
                if (null == singleTon)
                    singleTon = new SingleTon();
            }
        }
        return singleTon;
    }

}