单例设计模式

　　懒汉式  先判断是否已经实例化,如果没有则实例化,有则直接返回.

　　public class Single {

    private static Single single=null;
    private Single(){}

    public static synchronized Single getSingleton (){
        if(single!=null){
            return new Single();
        }
        return single;
    }
}

   饿汉式  一开始就实例化

public class Singleton2 {
    private final static Singleton2 singleton2=new Singleton2();
    private Singleton2(){
    }
    public static  Singleton2 getSingleton2(){
        return singleton2;
    }
}

优化版懒汉式(线程安全的懒汉式，性能比懒汉式好)

public class Single {

    private static Single single=null;
    private Single(){}

    public static  Single getSingleton (){
        if(single!=null){

　　　　　　synchronized(Single.class){
　　　　if(single!=null){

　　　　　　return new Single();

　　　　}

}

        }
        return single;
    }
}