单例 设计模式

 1 package org1;
 2 
 3 public class Singleton {
 4     private static Singleton instance = null;
 5 
 6     private Singleton() {}
 7 
 8     public static Singleton getInstance() {
 9         if (instance == null)
10             instance = new Singleton();
11         
12         return instance;
13     }
14 }

 1 package org2;
 2 
 3 public class Singleton {
 4     private static Singleton instance = new Singleton();
 5 
 6     private Singleton() {}
 7 
 8     public static Singleton getInstance() {
 9         return instance;
10     }
11 }

 1 package org3;
 2 
 3 //这个模式将同步内容下方到if内部，提高了执行的效率，不必每次获取对象时都进行同步，只有第一次才同步，创建了以后就没必要了。
 4 public class Singleton {
 5     private static Singleton instance = null;
 6 
 7     private Singleton() {}
 8 
 9     public static Singleton getInstance() {
10         if (instance == null) {
11             synchronized (Singleton.class) {
12                 if (null == instance) {
13                     instance = new Singleton();
14                 }
15             }
16         }
17         return instance;
18     }
19 }