cglib 2.1：一个开放源码的字节码操作库，需要安装它才能使用easymock类扩展。
Cglib 2.1: an open source byte code manipulation library you'll need to install to use the EasyMock class extension.
还可以添加方法和数据到已有的类中，因为类是开放式的。
You can also add methods and data to existing Ruby classes because classes are open.
在定义时，开放式类并不只限于一组被实现的方法。
An open class isn't limited to the set of methods that were implemented when it was defined.
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