单例模式

 1 public class Singleton {
 2 
 3     private Singleton(){
 4         //do something
 5     }
 6 
 7     private static volatile Singleton singleton = null;
 8 
 9     public static Singleton getInstance(){
10         if (singleton == null){
11             synchronized (Singleton.class){
12                 if (singleton == null){
13                     singleton = new Singleton();
14                 }
15             }
16         }
17         return singleton;
18     }
19 }