Annotaties tijdens runtime verwerken

@Service
@RequestMapping("/api")
public class UserController {}

RequestMapping rm = UserController.class.getAnnotation(RequestMapping.class);
if (rm != null) System.out.println(Arrays.toString(rm.value())); // ["/api"]

for (Method m : UserController.class.getDeclaredMethods()) {
    GetMapping gm = m.getAnnotation(GetMapping.class);
    if (gm != null) System.out.println(m.getName() + " -> " + Arrays.toString(gm.value()));
}

for (Field f : User.class.getDeclaredFields()) {
    Column col = f.getAnnotation(Column.class);
    if (col != null) System.out.println(f.getName() + " -> column: " + col.name());
}

Method m = UserController.class.getDeclaredMethod("create", UserDto.class);
Annotation[][] pa = m.getParameterAnnotations();
for (Annotation a : pa[0]) System.out.println(a.annotationType().getSimpleName()); // e.g. Valid

if (m.isAnnotationPresent(Transactional.class)) {
    beginTransaction();
    m.invoke(target, args);
    commitTransaction();
}

Map<Class<?>, Object> beans = new HashMap<>();
for (Class<?> cls : scanPackage("com.example")) {
    if (cls.isAnnotationPresent(Component.class)) {
        beans.put(cls, cls.getDeclaredConstructor().newInstance());
    }
}

for (Object bean : beans.values()) {
    for (Field f : bean.getClass().getDeclaredFields()) {
        if (f.isAnnotationPresent(Inject.class)) {
            f.setAccessible(true);
            f.set(bean, beans.get(f.getType()));
        }
    }
}

for (Field f : dto.getClass().getDeclaredFields()) {
    Range range = f.getAnnotation(Range.class);
    if (range != null) {
        f.setAccessible(true);
        int val = (int) f.get(dto);
        if (val < range.min() || val > range.max())
            throw new ValidationException(f.getName() + " out of range");
    }
}

Tag[] tags = MyClass.class.getAnnotationsByType(Tag.class);
for (Tag t : tags) System.out.println(t.value());

Reflections reflections = new Reflections("com.example.service");
Set<Class<?>> services = reflections.getTypesAnnotatedWith(Service.class);
services.forEach(System.out::println);