Java多线程中的单例模式

package com.ietree.multithread.sync;

public class Singletion {
    
    private static class InnerSingletion {
        private static Singletion single = new Singletion();
    }

    public static Singletion getInstance() {
        return InnerSingletion.single;
    }
}

package com.ietree.multithread.sync;

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 (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (DubbleSingleton.class) {
                if (ds == null) { // 这个判断很重要，如果没有那将不是单例，而是多例
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }

    public static void main(String[] args) {
        
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        }, "t1");
        
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        }, "t2");
        
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        }, "t3");

        t1.start();
        t2.start();
        t3.start();
    }
}