[Java] Using annotation processing (and validating it) to execute logic at runtime

Sample project showcasing how to use annotations to perform runtime logic to log changes in object values.

Remember it is a SAMPLE, so obviously (lazy me) most null-safe checks and so are not included and obviously some logic is a showcase, should be replaced with a real business scenario to implement.

In this SAMPLE, we tag fields to be included in a diff logic to then print to output when those fields values change by comparing two instances of the same object.

Logic can obviously be made much more complex including collections and maps and whatnot (Java Generics are your friends there).

Also worth mentioning JaVers can do most of it for you, unless you have fancy business requirements that force you to write custom code..

Key points:

- How to create an annotation

- How to create an annotation processor to validate the annotation parameters at compile time

- How to register an annotation processor

- How to configure a multi-module Maven project to use a custom annotation processor (also, in the pom of the root project ensure the module containing the processor is built BEFORE everything else)

- How to test an annotation processor by generating classes at runtime and trigger compilation tasks agains them. Includes verifying compilation warnings are properly triggered as expected.

- How to use reflection to get fields annotated with a given annotation (and then execute whatever logic on them)

- How to use reflection to invoke methods (including static methods)

The full project is available on my GitHub repo with commented code: https://github.com/steghio/diff-annotation-processing