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java单例的四个小案例

1

package com.example.youruan.day01.entity;public class Singleton1 {
       private Singleton1() {
       }    private static final Singleton1 singletonInstance = new Singleton1();    public static Singleton1 getInstance() {
           return singletonInstance;    }}

2

package com.example.youruan.day01.entity;public class Singleton2 {
       private static Singleton2 instance = null;    public static synchronized Singleton2 getInstance() {
           if (instance == null)            instance = new Singleton2();        return instance;    }}

3

package com.example.youruan.day01.entity;public class Singleton3 {
       private static volatile Singleton3 instance = null;    public static Singleton3 getInstance() {
           if (instance == null) {
               synchronized (Singleton3.class) {
                   if (instance == null) {
                       instance = new Singleton3();                }            }        }        return instance;    }}

4

package com.example.youruan.day01.entity;public class Singleton4 {
       private static class Holder {
           static final Singleton4 instance = new Singleton4();    }    public static Singleton4 getInstance() {
           return Holder.instance;    }}

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