设计模式之单例模式

//饿汉式单例类.在类初始化时，已经自行实例化   
public class Singleton1 {  
    private Singleton1() {}  
    private static final Singleton1 single = new Singleton1();  
    //静态工厂方法   
    public static Singleton1 getInstance() {  
        return single;  
    }  
}  

public class Single {
    //私有化构造函数
    private Single(){}
    //声明本类引用类型变量，但是不要创建对象
    private static Single s;
    //声明一个公共静态的方法获取本类的对象
    public static Single getInstance(){
            if(s==null){
                s=new Single();
            }
            return s;
    }
}

public static Single getInstance(){
            synchronized ("lock") {
                if(s==null){
                    s=new Single();
                }
            }    
            return s;
    }

 public static Single getInstance(){
        if(s==null){
            synchronized ("lock") {
                if(s==null){
                    s=new Single();
                }
            }    
        }
        return s;
 }