If you have the source for the dependency, patching the code is simple: Just create a small Maven project that compiles the source with your changes. Since your changes are probably small, you need only a few tests: What’s the point to test the code that you didn’t touch? Also the build will be simple (just compile the sources for your need, no fancy resource processing/filtering).
But what if you don’t have the sources? Jakub Holý has a solution: Hacking A Maven Dependency with Javassist to Fix It
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