单例模式----懒汉式改成安全

 1 package com.yhqtv.java1;
 2 
 3 /*
 4  * @author  XMKJ  yhqtv.com Email:yhqtv@qq.com
 5  * @create 2020-04-29-12:46
 6  *
 7  */
 8 public class Test {
 9     public static void main(String[] args) {
10 
11     }
12 }
13 
14 class Dog {
15     private Dog() {
16 
17     }
18 
19     private static Dog dog = null;
20 
21     private static Dog getDog() {
22 //     方式一：效率稍差
23 //                synchronized (Dog.class) {
24 //            if (dog == null) {
25 //                dog = new Dog();
26 //
27 //            }
28 //            return dog;
29 //        }
30 //        方式二：效率高
31         if (dog==null) {
32             synchronized (Dog.class) {
33                 if (dog == null) {
34                     dog = new Dog();
35 
36                 }
37 
38             }
39         }
40         return dog;
41 
42     }
43 }