单例双重验证

public class Singleton {
    public static volatile Singleton singleton = null ;
    private Singleton(){}

    public static Singleton getInstance(){
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                   singleton = new Singleton();
                }

            }
        }
        return  singleton;
    }

    public void saySome(){
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("随便输出点什么东西吧");
    }
    public static void main(String[] args) {

        for (;;){
            new Thread(){
                @Override
                public void run() {
                    Singleton.getInstance().saySome();
                }
            }.start();

        }
    }

}

这里主要是强调 volatitle 关键字，不允许指令重排，防止出错，经过多次测试也没有出现问题