用 Java 写一个线程安全的单例模式（Singleton）？

public class Singleton{
 
          private static Singleton singleton;

          private Singleton (){

          }

          public static Singleton getInstance(){
                if(singleton == null){
                      singleton = new Singleton();
                }
                return singleton;
          }


}

　　

public class Singleton{

        private static Singleton singleton;

        private Singleton(){}

        public static synchronized Singleton getInstance(){
                   if(singleton == null){
                          singleton = new Singleton();
                    
                   }
                   return singleton;
        }  

}

　　

public class Singleton{
 
         private static Singleton singleton;

         private Singleton(){}

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

}