实现该trait的领域对象只需实现getEntity方法即可，该方法的实现仅仅是返回一个自身引用

The domain object implementing this trait only has to provide the getEntity implementation, in which it simply returns a reference to itself

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由于对这个模式使用单操作，对地址或显式地址自身的引用必须作为请求的参数提供。

Since a single operation is used for this pattern, a reference to the address or the explicit address itself must be provided as a parameter on the request.

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为了使这一点更方便，插件类实现单模式（singleton pattern）：它在私有类变量中存储一个自身的引用。

To facilitate this, the plug-in class implements the singleton pattern: It stores a reference to itself in a private class variable.

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