单例模式

JAVA单例模式6种写法

1. 饿汉式单例

   //饿汉式单例类.在类初始化时实例化   
   public class Singleton {  
       private Singleton() {}  
       private static final Singleton single = new Singleton();  
       //静态工厂方法   
       public static Singleton getInstance() {  
           return single;  
       }  
   }  

2. 懒汉式1

   public class Singleton {  
       private static Singleton instance;  
       private Singleton (){}  
   
       public static Singleton getInstance() {  
       if (instance == null) {  
           instance = new Singleton();  
       }  
       return instance;  
       }  
   }  

3. 懒汉式2（同步方法）

   public class Singleton {  
       private static Singleton instance;  
       private Singleton (){}  
       public static synchronized Singleton getInstance() {  
       if (instance == null) {  
           instance = new Singleton();  
       }  
       return instance;  
       }  
   }  

4. 懒加载3（双重校验锁）

   public class Singleton {  
       private static Singleton singleton;  
       private Singleton (){}  
       public static Singleton getSingleton() {  
       if (singleton == null) {  
           synchronized (Singleton.class) {  
           if (singleton == null) {  
               singleton = new Singleton();  
           }  
           }  
       }  
       return singleton;  
       }  
   }  

5. 静态内部类

   public class Singleton {  
       private static class SingletonHolder {  
       private static final Singleton INSTANCE = new Singleton();  
       }  
       private Singleton (){}  
       public static final Singleton getInstance() {  
       return SingletonHolder.INSTANCE;  
       }  
   }  

6. 枚举形式

   public enum Singleton {  
       INSTANCE;  
       //任意方法，调用时使用INSTANCE.方法名
       public void whateverMethod() {  
       }  
   }