Java APIs

I need to generate javadoc with Ant, however I'm just learning to use both. I am compiling with a root build.xml that antcalls to other build.xmls in child directories and so on until the lowest level is tasked to compile the source in its directory. Is there a way to handle this for javadoc? If I use the same methodology for javadoc, javadoc ends up generating the api docs multiple times bit by bit, but the result is that the output is just overwritten as many times as it's generated.

Is there a better way to do this? I can't seem to get all the api into one document.