Procesory adnotacji w czasie kompilacji

Czym są procesory adnotacji

AbstractProcessor: klasa bazowa

import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.*;

@SupportedAnnotationTypes("com.example.GenerateBuilder")
@SupportedSourceVersion(SourceVersion.RELEASE_17)
public class BuilderProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations,
                           RoundEnvironment roundEnv) {
        for (Element e : roundEnv.getElementsAnnotatedWith(GenerateBuilder.class)) {
            generateBuilder((TypeElement) e);
        }
        return true;
    }
    private void generateBuilder(TypeElement cls) { /* generate source */ }
}

Rejestrowanie procesora

# META-INF/services/javax.annotation.processing.Processor
com.example.BuilderProcessor

Generowanie plików źródłowych

JavaFileObject file = processingEnv.getFiler()
    .createSourceFile("com.example.UserBuilder");
try (Writer w = file.openWriter()) {
    w.write("package com.example;\n");
    w.write("public class UserBuilder {\n");
    w.write("  // generated builder\n");
    w.write("}\n");
}

Analizowanie modelu elementów

TypeElement classElement = (TypeElement) element;
String className = classElement.getSimpleName().toString();
List<VariableElement> fields = classElement.getEnclosedElements().stream()
    .filter(e -> e.getKind() == ElementKind.FIELD)
    .map(e -> (VariableElement) e)
    .collect(Collectors.toList());

Zgłaszanie błędów kompilatora

processingEnv.getMessager().printMessage(
    Diagnostic.Kind.ERROR,
    "@GenerateBuilder requires at least one field",
    element
);

Przetwarzanie wielorundowe

if (roundEnv.processingOver()) {
    // last round — write summary files or do cleanup
}

Lombok od kuchni

Generowanie kodu przez MapStruct

@Mapper
public interface UserMapper {
    UserDto toDto(User user);
    User toEntity(UserDto dto);
}
// MapStruct generates: UserMapperImpl.java at compile time

Testowanie procesorów adnotacji

JavaFileObject src = JavaFileObjects.forSourceString("Test",
    "@GenerateBuilder public class Test { private String name; }");
Compilation c = Compiler.javac().withProcessors(new BuilderProcessor()).compile(src);
CompilationSubject.assertThat(c).succeeded();
CompilationSubject.assertThat(c).generatedSourceFile("TestBuilder");

Pakowanie procesora

// build.gradle:
dependencies {
    implementation "com.example:my-annotations:1.0"
    annotationProcessor "com.example:my-processor:1.0"
}

Szybkie sprawdzenie

Podsumowanie