Perhaps something like: http://www.prodigyproductionsllc.com/articles/programming/decompile-java-using-c/
Maybe one of those potential ideas will help you.
Also if I remember correctly jar files are really just zip files. You can "extract" the jar file and look at the class files inside of it... I don't know if you will be able to do much with the class files but if you extract the jar file to a temp directory and then iterate through the files you should be able to determine the names of things potentially.