单例模式实现

1、饿汉式

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

2、懒汉式

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

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

3、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;  
   }  
}

4、枚举

public enum Singleton {  
   INSTANCE;  
   public void whateverMethod() {}  
}

5、双重校验锁

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