java 单例模式 Java多线程中的单例模式两种实现方式

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();
  }
}