This method returns a message context property if one has been set, otherwise it returns the value of the defined mediation parameter.
如果设置了消息上下文属性，此方法会返回此属性，否则它将返回定义的中介参数的值。
A defined set of messages, each including a related set of the previously defined types, elements and attributes structured to provide a business document with a specific semantic meaning and context.
一组定义好的消息，每条消息包括一组相关的之前定义的类型、元素和属性，它们的结构可提供一个具有特定语义和上下文的业务文档。
The callback URL is set to this servlet's context as it was defined in web.xml.
这个回调url会被保存在这个Servlet的上下文中，就像它在web . xml定义一样。
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