Java设计模式学习笔记——单例模式 

懒汉模式

 1 package com;
 2 
 3 public class LazySingletonTest { //懒加载
 4     public static void main(String[] args) {
 5         LazySingleton inst1 = LazySingleton.getInstance();
 6         LazySingleton inst2 = LazySingleton.getInstance();
 7         System.out.println(inst1 == inst2);
 8     }
 9 }
10 
11 class LazySingleton{
12     private static volatile LazySingleton instance; //防止多线程时指令重排带来的一致性问题
13     private LazySingleton(){
14 
15     }
16 
17     public static LazySingleton getInstance() {
18         if(instance==null){
19             synchronized (LazySingleton.class) { // 防止多线程同时访问
20                 if(instance==null) // 第二次检查
21                     instance = new LazySingleton();
22             }
23         }
24         return instance;
25     }
26 }

 

饿汉模式

 1 public class HungrySingletonTest {
 2     public static void main(String[] args) {
 3         HungrySingleton inst1 = HungrySingleton.getInstance();
 4         HungrySingleton inst2 = HungrySingleton.getInstance();
 5         System.out.println(inst1 == inst2);
 6     }
 7 }
 8 
 9 class HungrySingleton{
10     private static HungrySingleton instance = new HungrySingleton();
11     private HungrySingleton(){
12     }
13 
14     public static HungrySingleton getInstance(){
15         return instance;
16     }
17 }

 

静态内部类方式

 1 public class InnerClassSingletonTest {
 2     public static void main(String[] args) {
 3         InnerClassSingleton inst1 = InnerClassSingleton.getInstance();
 4         InnerClassSingleton inst2 = InnerClassSingleton.getInstance();
 5         System.out.println(inst1 == inst2);
 6     }
 7 }
 8 
 9 class InnerClassSingleton {
10     private static class InnerClassHolder{
11         private static InnerClassSingleton instance = new InnerClassSingleton();
12     }
13     private InnerClassSingleton(){
14 
15     }
16     public static InnerClassSingleton getInstance(){
17         return InnerClassHolder.instance;
18         //只有在实际使用的时候才会触发类的初始化，所以也是懒加载的一种形式
19         //但是利用了类的加载机制保证了线程安全
20     }
21 }

 

枚举方式 （线程安全且不支持反射创建实例）

 1 public enum EnumSingleton {
 2     INSTANCE;
 3     public void print(){
 4         System.out.println(this.hashCode());
 5     }
 6 }
 7 
 8 class EnumTest{
 9     public static void main(String[] args) {
10         EnumSingleton inst1 = EnumSingleton.INSTANCE;
11         EnumSingleton inst2 = EnumSingleton.INSTANCE;
12         System.out.println(inst1 == inst2);
13     }
14 }

 

posted @ 2021-08-29 16:56  Saturn5  阅读(22)  评论(0)    收藏  举报