The javadoc tool excels at extracting documentation and other source code metadata from Java classes. The default output is, unfortunately, restricted to a myriad of HTML files. Luckily, the javadoc tool supports a "plugin mechanism" called "doclets". A doclet receives metadata, such as class members and source code comments, and produces output documents. The default doclet produces said myriad of HTML files.
The doclet module includes replacement doclets for the default one. By using these alternative doclets you can harness the wealth of data which javadoc extracts from the source code without limiting yourself to the default HTML output.
ChainDoclet - Save Time In Large Multi-Report Projects
The ChainDoclet is useful when producing documentation in a variety of formats (possibly using multiple doclets) in very large projects (i.e. projects where it takes a lot of time for javadoc to analyze and process the source code). Using the ChainDoclet allows you to save time by only having to analyze the source code once and then sending the metadata to multiple doclets without re-analyzing the source code between each doclet invokation.
XmlDoclet - XML Documents From Source Code
The XmlDoclet outputs XML documents instead of HTML documents. The XML documents can then be further processed, using other tools, in order to produce reports or as input to other applications.