1 // 饿汉式
 2 public class Singleton{
 3 private Singleton(){}
 4 private static final Singleton instance = new Singleton();
 5 public static Singleton getInstance(){
 6     return instance ;
 7 }
 8 }
 9 // 静态代码块
10  public class Singleton {  
11       private Singleton instance = null;  
12       static {  
13            instance = new Singleton();  
14      }  
15       private Singleton (){}
16       public static Singleton getInstance() {  
17          return this.instance;  
18        }  
19 }  
20 
21 // 懒汉式 在getInstance方法上加同步
22 public class Singleton {  
23     private Singleton() {}  
24     private static Singleton single=null;  
25     public static synchronized Singleton getInstance() {  
26          if (single == null) {    
27              single = new Singleton();  
28          }    
29         return single;  
30     }  
31 }  
32 // 懒汉式 双重检查锁定
33 public class Singleton {  
34     private Singleton() {}  
35     private static Singleton single=null;  
36     public static Singleton getInstance() {  
37         if (singleton == null) {    
38             synchronized (Singleton.class) {    
39                if (singleton == null) {    
40                   singleton = new Singleton();   
41                }    
42             }    
43         }    
44         return singleton;   
45     }   
46 }  
47 // 静态内部类
48 public class Singleton {    
49     private static class LazyHolder {    
50        private static final Singleton INSTANCE = new Singleton();    
51     }    
52     private Singleton (){}    
53     public static final Singleton getInstance() {    
54        return LazyHolder.INSTANCE;    
55     }    
56 }