annotationProcessor

annotationProcessor refers to a program that runs during compilation to read annotations and generate, modify, or validate code and resources. In Java, processors typically extend AbstractProcessor or implement the Processor interface from the annotation processing API.

Processing occurs in rounds. Each round allows the processor to examine annotated elements, write new source files via the Filer, and report messages through the Messager. If any new files are created, another round runs to process them. The cycle ends when no new files are produced.

Registration uses the Java Service Provider mechanism: a file named META-INF/services/javax.annotation.processing.Processor lists the processor class. A processor declares which annotations it supports and which source version it targets, via getSupportedAnnotationTypes/getSupportedSourceVersion or the corresponding @SupportedAnnotationTypes/@SupportedSourceVersion annotations.

Common uses include code generation (for boilerplate or binding code), compile-time validation, and resource generation. The ProcessingEnvironment provides utilities such as Elements, Types, Messager, and Filer to inspect code, create files, and report diagnostics.

Implementation notes: create a class extending AbstractProcessor and override process(Set<? extends TypeElement>, RoundEnvironment). Use RoundEnvironment to find annotated elements and Filer to write new Java files. To automate service registration, many projects use the AutoService library to generate the META-INF entry.

Build tools support annotation processing. In Maven, configure the maven-compiler-plugin; in Gradle, add dependencies to the annotationProcessor configuration. Incremental processing can improve build times.

Limitations include that processing occurs at compile time and can slow builds. Processors should be deterministic and idempotent to avoid duplicate output, and care should be taken with round semantics and dependency on generated sources.