枚举和单例 - 顶级vs嵌套枚举 [英] enum and singletons - top level vs nested enum

问题描述

现在众所周知，在java中创建单例的推荐方法是通过一个枚举（参见例如）

但是（例如在这个 answer ）似乎被认为是（由@MikeAdler在评论中回复我的）正确的事情有枚举 在 单例类中（例如，参见此处获取完整示例，或下面给出的代码）。我似乎没有真正了解这个需要/使用 - 有人可以详细说明（最好给出这个成语的正确方言）？

  public class包含{
 
 private Enclosing（）{} 
 
 static enum Singleton {
 INSTANCE; 
 
 private static final Enclosing singleton = new Enclosing（）; 
 
 public Enclosing getSingleton（）{
 return singleton; 
} 
} 
} 
  

编辑：单例由 Enclosing.Singleton.INSTANCE.getSingleton（）;

解决方案

当您想执行延迟加载时，您将嵌套一个Singleton，就测试原因而言：

  public class Singleton {
 public Enclosing getInstance（）{
 return SingletonHolder.INSTANCE; 
} 
 
 static enum SingletonHolder {
 INSTANCE; 
} 
} 
  

在此阅读更多信息： http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom

As is now well known the recommended way for creating a singleton in java is via an enum (see for instance here)

But (for example in this answer) it seems to be considered (by @MikeAdler who replied to me in the comments) the right thing to have the enum in the singleton class (see for instance here for a full example, or the code given below). I do not seem to really understand the need/use of this - can someone please elaborate (and preferably give the correct dialect for this idiom) ?

public class Enclosing {

    private  Enclosing() {}

    static enum Singleton {
        INSTANCE;

        private static final Enclosing  singleton = new Enclosing();

        public Enclosing getSingleton() {
            return singleton;
        }
    }
}

EDIT : one would get the singleton by Enclosing.Singleton.INSTANCE.getSingleton();

解决方案

You would nest a Singleton when you wanted to perform lazy-loading of it, say for testing reasons:

public class Singleton {
    public Enclosing getInstance() {
        return SingletonHolder.INSTANCE;
    }

    static enum SingletonHolder {
        INSTANCE;
    }
}

Read more about it here: http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom

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