单例模式

1.懒汉-线程不安全

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

2.懒汉-线程安全（效率很低，99%情况下不需要同步。）

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

3.饿汉

1 public class Singleton {  
2     private static Singleton instance = new Singleton();  
3     private Singleton (){}  
4     public static Singleton getInstance() {  
5     return instance;  
6     }  
7 }

4.饿汉2

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

5.静态内部类

1 public class Singleton {  
2     private static class SingletonHolder {  
3     private static final Singleton INSTANCE = new Singleton();  
4     }  
5     private Singleton (){}  
6     public static final Singleton getInstance() {  
7     return SingletonHolder.INSTANCE;  
8     }  
9 }

6.枚举

1 public enum Singleton {  
2     INSTANCE;  
3     public void whateverMethod() {  
4     }  
5 }

7.双重校验锁

 1 public class Singleton {  
 2     private volatile static Singleton singleton;  
 3     private Singleton (){}  
 4     public static Singleton getSingleton() {  
 5     if (singleton == null) {  
 6         synchronized (Singleton.class) {  
 7         if (singleton == null) {  
 8             singleton = new Singleton();  
 9         }  
10         }  
11     }  
12     return singleton;  
13     }  
14 }