设计模式之单例模式

懒汉式

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

加锁

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

双重检查加锁

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

饿汉式

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