java 单例模式最佳实现方式

class Singleton {
    private static  Singleton SingletonInstance  = null；    
    
    private Singleton() {
    }
    
    public static  Singleton newInstance() {
      if( SingletonInstance == null){  // 无实例，立即返回实例，
       synchronized (Singleton.class){ // 
          if( SingletonInstance == null){
              SingletonInstance = new Singleton();
          }
       }
      }
        return SingletonInstance;
    }
   
}