Java concurrent programming-singleton & multithreading

package cn.bufanli.test.singleton;

/**
 * 单例&多线程
 * 单例模式,最常见的就是饥饿模式和懒汉模式,一个直接实例化对象
 * 一个在调用方法时进行实例化对象,在多线程中,考虑到性能和线程安全问你题,
 * 我们一般选择下面两种单例模式,在提高性能的同时,又保证了线程安全
 *  dubble check instance 两次确认
 *  static inner class 静态内部类
 */
public class singletonThreadDemo {
}

Dubble check instance 

package cn.bufanli.test.singleton;

public class DubbleSingleton {
    private static DubbleSingleton ds;
    public static DubbleSingleton getDS(){
        if(ds == null){
            try {
                //模拟初始化对象的准备时间..
                Thread.sleep(3000);
            }catch(Exception e){
                e.printStackTrace();
            }
            synchronized (DubbleSingleton.class){
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            System.out.println(DubbleSingleton.getDS().hashCode());
        },"t1");
        Thread t2 = new Thread(() -> {
            System.out.println(DubbleSingleton.getDS().hashCode());
        },"t2");
        Thread t3 = new Thread(() -> {
            System.out.println(DubbleSingleton.getDS().hashCode());
        },"t3");
        t1.start();
        t2.start();
        t3.start();
    }
}

static inner class

package cn.bufanli.test.singleton;

public class Singleton {
    private static class InnerSingleton{
        private static Singleton single = new Singleton();
    }
    public static Singleton getInstance(){
        return InnerSingleton.single;
    }
}