Singleton

问题：编写一个Singleton类

　简单的方式

 1 package cn.changb.singleton;
 2 
 3 /**
 4  * 简单的方式:弊端是在并发执行时，14行处可能存在多个实例
 5  */
 6 public class SingletonDemo1 {
 7     private static SingletonDemo1 INSTANCE;
 8 
 9     private SingletonDemo1() {
10     }
11 
12     public static SingletonDemo1 getInstance() {
13         if (INSTANCE == null) {
14             INSTANCE = new SingletonDemo1();
15         }
16         return INSTANCE;
17     }
18 }

　公共域方式

 1 package cn.changb.singleton;
 2 
 3 /**
 4  * 公共域方式：简单，不存在并发出现多实例的问题，但弊端是返回实例时增加一些额外的逻辑
 5  */
 6 public class SingletonDemo2 {
 7     public static final SingletonDemo2 INSTANCE = new SingletonDemo2();
 8 
 9     private SingletonDemo2() {
10     }
11 }

　工厂化方式

 1 package cn.changb.singleton;
 2 
 3 /**
 4  * 工厂化方式：可以在实例化过程中(11行处)添加逻辑
 5  */
 6 public class SingletonDemo3 {
 7     private static final SingletonDemo3 INSTANCE = new SingletonDemo3();
 8 
 9     private SingletonDemo3() {
10     }
11 
12     public static SingletonDemo3 getInstance() {
13         return INSTANCE;
14     }
15 }

　枚举类方式

 1 package cn.changb.singleton;
 2 
 3 /**
 4  * 枚举类方式：最佳实践，推荐方式
 5  * 
 6  * 不存在多个实例问题、反序列化问题！
 7  */
 8 public enum SingletonDemo4 {
 9     INSTANCE;
10 }

　SingletonTest

 1 package cn.changb.singleton;
 2 
 3 public class SingletonTest {
 4 
 5     public static void main(String[] args) {
 6         // TODO Auto-generated method stub
 7         System.out.println("SingletonDemo1" + SingletonDemo1.getInstance());
 8         System.out.println("SingletonDemo1" + SingletonDemo1.getInstance());
 9 
10         System.out.println("SingletonDemo2" + SingletonDemo2.INSTANCE);
11         System.out.println("SingletonDemo2" + SingletonDemo2.INSTANCE);
12 
13         System.out.println("SingletonDemo3" + SingletonDemo3.getInstance());
14         System.out.println("SingletonDemo3" + SingletonDemo3.getInstance());
15 
16         System.out.println("SingletonDemo4" + SingletonDemo4.INSTANCE);
17         System.out.println("SingletonDemo4" + SingletonDemo4.INSTANCE);
18     }
19 
20 }