不管您相信与否，Log4j的记录器的层次结构保留在一个称为Hierarchy的对象中。
Its hierarchy of loggers is kept in an object called, believe it or not, hierarchy.
这是必需的，这样，在插件停止时，就可以关闭该插件的所有层次结构，并正确删除 appender 和记录器，如清单 5 所示。
This is needed so that when the plug-in stops, it will be possible to shut down all the hierarchies and allow the appenders and loggers to be correctly removed as you can see in Listing 5.
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