I have several pieces of code in one of our company's projects that dynamically reads the classpath and scans for classes that have certain annotations and values in those annotations. For the longest time, I was able to unit test these successfully. Then one day, I started getting strange ClassNotFoundExceptions when trying to execute the tests, and I could never figure out why. The project still ran just fine on our servers, so I let the issue slide at the time because I was pressed for time and needed to get other shit done. Overtime, I continued to let the problem bounce up and down over rocks as I tried to unit test certain things and couldn't because of this problem. Today I finally got fed up and wound up investing 3 hours troubleshooting the problem, and here's what happened:
Right around the same time I started getting this problem, I had upgraded my version of the M2Eclipse plugin (which I use to integrate the Maven build system and Eclipse) to a new version. As it turns out, in the intervening versions, the Maven developers changed how Maven integrates with the Eclipse classpath, and if you got the plugin to update your project's Eclipse configuration, it would specify separated output folders for your projects code and your project's test classes, and that's what caused the problem. Separating the two meant I could no longer dynamically resolve my project's model classes in a test environment, because the classpath visitor wouldn't receive the correct source folder from the test environment's classloader. So, the solution as it turned out, was to have Eclipse's output go into the same folder for source and test classes.