如何在Java中制作一个不变的单例? [英] How to make an immutable singleton in Java?

问题描述

不可变对象仅由其构造函数初始化，而单例则由静态方法实例化.如何在Java中制作一个不变的单例?

An immutable object is initialized by its constuctor only, while a singleton is instantiated by a static method. How to make an immutable singleton in Java?

推荐答案

而单例由a实例化 静态方法

while a singleton is instantiated by a static method

虽然这是通常的做法，但这绝不是唯一的方法.

While this is the usual way of doing it, this is by no means the only way.

在Java 1.5中，单例的新版本是枚举单例模式:

In Java 1.5 a new version of Singleton is the enum singleton pattern:

public enum Elvis{

INSTANCE // this is a singleton, no static methods involved

}

由于枚举可以具有构造函数，方法和字段，因此您可以为它们提供所需的所有不可变状态.

And since enums can have constructors, methods and fields, you can give them all the immutable state you want.

参考:

• Java教程:枚举类型
• 有效的Java ，项目3
• Singleton(枚举方式) (WikiPedia)

• Java tutorial: Enum Types
• Effective Java, Item 3
• Singleton (the enum way) (WikiPedia)

此外，单例"一词还留出了一些解释的空间. Singleton表示每个定义的范围只有一个对象，但是范围可以是很多东西:

Also, the term Singleton leaves some room for interpretation. Singleton means that there is exactly one object per defined scope, but the scope can be a number of things:

• Java VM Classloader (感谢@PaŭloEbermann提醒我):在这种情况下，请使用通过静态内部类初始化模式.当然，这通常是单身人士的意思.
  小心:枚举和所有其他单例如果通过多个Classloader加载，则会被破坏.
• 企业应用程序(在这种情况下，您需要一个容器管理的单例，例如 ThreadLocal )
• 请求/会话(同样，您需要一个容器来管理它，接缝，其他几个人也可以为您完成此操作
• 我忘记了什么吗?

• Java VM Classloader (thanks @Paŭlo Ebermann for reminding me): in this case use enums or the initialize-through-static-inner-class pattern. This is of course what is usually meant by a singleton.
  Be Careful: enums and all other singletons are broken if loaded through multiple Classloaders.
• Enterprise Application (in this case you need a container-managed singleton, e.g. a Spring singleton bean). This can be several objects per VM or one object per several VMs (or one Object per VM, of course)
• Thread (use a ThreadLocal)
• Request / Session (again, you'll need a container to manage this, Spring, Seam and several others can do that for you)
• did I forget anything?

可以将上述所有内容设置为不可变的，每种方式各不相同(尽管对于容器管理的组件来说通常并不容易)

All of the above can be made immutable, each in their own way (although it's usually not easy for container-managed components)

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