Named fields for another class

I need to access some private fields via reflection. While this is considered bad, it is an external library and I have no other way to access it. The field names (and types) are known in advance (during compile time) and as a precaution for future library updates I want the compiler to throw an error when this field doesn't exist anymore.

Lombok has the annotation @FieldNameConstants which generates all field names as static Strings. When the field doesn't exist anymore, the static variable won't generate and the compiler will throw an error. This would be a great solution, but as this is an external library class I cannot annotate it with lombok.

If you cannot fail during compile time, the second best thing is to add a Unit Test that invokes your reflection code. That way, if you run your tests after every dependency update (you should do that anyway), your test will fail if the implementation details you rely on change.

If you absolutely want to fail compilation, you'll need some kind of preprocessor. You could, for example, build a small, specialized code generator that generates a bridge interface based on the reflection you have in mind, but fails to generate methods for fields it cannot resolve. Then you only use the generated bridge code. I'm not aware of a library that can do this out of the box, but it's not very hard to write in the specific case (it does not need to be flexible).

This could still lead to runtime problems, for example if the compiletime and runtime versions of your dependencies differ, or if old generated code is not replaced for some reason. So make sure to also add Unit Tests.