Questions tagged [annotation-processing]

An annotation processor is a plug-in for the Java compiler.

An annotation processor is an instance of javax.annotation.processing.Processor whose process method is invoked during compilation.

Once invoked, the processor is then able to do such things as:

• Analyze parts of the abstract syntax tree, primarily declarations via javax.lang.model.element and types via javax.lang.model.type.
• Cause compilation errors and warnings (similar to how the @Override annotation works) via the Messager.
• Generate new compilation units (i.e. source code files) via the Filer which are subsequently compiled.

Notes on the current limitations of annotation processors

• Annotation processors may only generate new compilation units, not modify existing compilation units. Some annotation processors, such as Project Lombok, achieve the latter via undocumented internal Javac API.
• The existing API in Java Platform SE is limited to only analyzing declarations, such as class declarations and method declarations. The Compiler Tree API exists, which allows one to analyze the full abstract syntax tree, but it's largely undocumented and not yet a part of SE.

Resources

• Code Generation using Annotation Processors in the Java language