Annotation Processing

An annotation by itself does nothing. It's a label attached to code. Something has to read that label and act on it, otherwise @Override would be no different from a comment. Java gives you two completely separate ways to read annotations and react to them: one runs while the compiler is still working, and the other runs while the program is already executing. Both are powerful, both are widely used in real frameworks, and they answer different questions.

This chapter covers both flows. The compile-time path uses the javax.annotation.processing API to plug into javac, inspect annotated code, and either emit compiler errors or generate brand new source files. The runtime path uses reflection on annotations whose retention is RUNTIME to inspect classes, methods, and fields while the application runs. By the end you'll have built one working processor that rejects bad code at compile time and one runtime validator that walks an object's fields and reports violations.