单例模式最通用的代码，注意final

public class Singleton{
     // 修饰词 static、final
     private static final Singleton singleton = new Singleton();

     private Singleton(){
     }
     //方法尽量使静态方法
     public static Singleton getSingletonInstance(){
             return singleton ;
     }
  
     //方法尽量使静态方法
      public static doSomething(){
         //doSomethind()
     }

}