Awhile ago, I put together a package for doing what you want, and more. (I needed it for a utility I was writing). It uses the ASM library. You can use reflection, but ASM turned out to perform better.
I put my package in an open source library I have on my web site. The library is here: http://software.clapper.org/javautil/. You want to start with the with ClassFinder class.
The utility I wrote it for is an RSS reader that I still use every day, so the code does tend to get exercised. I use ClassFinder to support a plug-in API in the RSS reader; on startup, it looks in a couple directory trees for jars and class files containing classes that implement a certain interface. It's a lot faster than you might expect.
The library is BSD-licensed, so you can safely bundle it with your code. Source is available.
If that's useful to you, help yourself.
Update: If you're using Scala, you might find this library to be more Scala-friendly.
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