How to write a Java annotation processor?

Question

I may be just looking in the wrong direction but I find the JSE documentation on annotation processing very ... sparse. I want to write an annotation processor which processes annotated String fields and local variables to substitute them with a computed String expression. This should not be too complicated but I'm pretty lost in the Javadoc for javax.annotation.processing.

EDIT: I need to process annotations at compile time because I want to modify the generated code. It should replace annotated constant String expressions with a computed String expression.

Answer 1

This can not be done with a compile time annotation processor. Compile time time annotation processors can only generate new files (and classes) they can not modify existing classes. You can do reflection at runtime but strictly speaking you that is not called annotation processing. Also you won't have access to local variables.

If you're looking on how to write a compile time annotation processor check out https://github.com/pellaton/spring-configuration-validation-processor

Answer 2

Two tools that do this are Project Lombok and DuctileJ. Both of these tools existed at the time the question was originally asked; additional tools now surely exist.

The key idea is to write an annotation processor that traverses and modifies the program's AST (abstract syntax tree) during compilation, before code generation. The compiler won't change the source code on disk, but the generated .class file will reflect the changes that your annotation processor makes.

You may be able to adapt one of these tools to suit your needs, or you could implement your own tool inspired by their implementation techniques.

Compile-time processing has two advantages over class-file processing. One is that the compiler usually has more information than is available from compiled code. Another is that everything happens in one step, during compilation, rather than requiring the developer to run a separate tool to rewrite the .class files after compilation.