处理必须进行子类化的单例 [英] Dealing with Singletons which have to subclass

问题描述

在问题在Java中实现单例模式的有效方法是什么？最大的升值的答案是使用枚举来实现单例。

这很好，我理解论据，分别是语言的优点。

然而，我有一组我定义单例但需要扩展其他类的类，这是不可能的枚举方法，因为枚举不能子类。

Joshua Bloch在幻灯片中说：

• 但有一件事丢失 - 您不能扩展枚举类型
  
  
  
  
  • 在大多数情况下，您不应该
  
  
  • 一个引人注目的用例操作代码
  
  

在大多数情况下，您不应该：可以有人详细说明了？我已经实现了几个servlet，它们扩展了 HttpServlet ，为什么这些servlet不是单例？我只需要我的应用程序中的一个实例。

解决方案

Singleton类可以扩展其他类;实际上默认情况下在Java中将会扩展Object。但是Josh所指的是你不应该扩展一个Singleton类，因为一旦你扩展它，就有超过1个实例存在。

回答评论： / p>

实际上，实现Singleton的最佳方式是：

从有效的Java

  //具有静态工厂的Singleton 
 public class Elvis {
 private static final Elvis INSTANCE = new Elvis（）; 
 private Elvis（）{...} 
 public static Elvis getInstance（）{return INSTANCE; } 
 public void leaveTheBuilding（）{...} 
} 
  

这里Elvis可以扩展任何其他类。

In the question What is an efficient way to implement a singleton pattern in Java? the answer with the most upvotes says, to use a Enum for implementing a singleton.

That is fine and I understand the arguments, respectively the language advantages.

I have, however, a set of classes which I define singleton but which need to extend other classes, this is not possible with the enum approach, since enums cannot subclass.

Joshua Bloch says in his slides:

• But one thing is missing—you can’t extend an enum type
  • In most cases, you shouldn’t
  • One compelling use case—operation codes

In most cases you shouldn't: could someone elaborate on that? I have implemented several servlets and they extend HttpServlet, why shouldn't these be singletons? I only want one instance of them in my application.

解决方案

A Singleton class can extend other classes; actually by default in Java it would anyway extend Object. However what Josh is referring to is that you shouldn't extend a Singleton class because once you extend it, there is more than 1 instance present.

Answering the comment:

Actually the best way to implement the Singleton is:

From Effective Java

// Singleton with static factory
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public static Elvis getInstance() { return INSTANCE; }
public void leaveTheBuilding() { ... }
}

Here Elvis can extend any other class.

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