Java多线程之单例模式（线程安全）

 package org.study2.javabase.ThreadsDemo.sync;

 /**
  * @Auther:GongXingRui
  * @Date:2018/9/20
  * @Description: 单例模式 - 二次确认，提高效率
  **/
 public class DanDemo {
     public static void main(String args[]) {
         Dan dan1 = new Dan();
         Dan dan2 = new Dan();
         dan1.start();
         dan2.start();
     }
 }

 class Dan extends Thread {
     @Override
     public void run() {
         System.out.println(JVM.getInstance());
     }

 }

 /**
  * 单例模式 - 懒汉式
  */
 class JVM {
     private static JVM instance = null;

     private JVM() {

     }

     // 二次确认提高效率
     public static JVM getInstance() {
         if (null == instance) { // 提高效率
             synchronized (JVM.class) {
                 if (null == instance) { // 安全
                     instance = new JVM();
                 }
             }
         }
         return instance;
     }

     public static JVM getInstance2() {
         synchronized (JVM.class) {
             if (null == instance) {
                 instance = new JVM();
             }
         }
         return instance;
     }

     public static synchronized JVM getInstance1() {
         if (null == instance) {
             instance = new JVM();
         }
         return instance;
     }
 }

 /**
  * 单例模式 - 饿汉式
  * 线程安全
  */
 class JVM2 {
     private static JVM2 instance = new JVM2();

     private JVM2() {

     }

     public static JVM2 getInstance() {
         return instance;
     }
 }