DocumentBuilder 返回文档的一些实现。你不必担心实现 Document 界面，有人已经为你做了这个。返回的文档将代表XML文档，这就是您想要的。

DocumentBuilder returns some implementation of Document. You don't have to worry about implementing the Document interface, somebody has already done that for you. The returned document will represent the XML document, which is what you want.