No AOP? Achieve Zero‑Intrusion Parameter Auto‑Filling in Spring Boot

Overview

Using Spring Boot 3.5.0, the article shows how to exploit Spring's powerful type‑conversion and formatting framework to create a zero‑intrusion, annotation‑driven parameter auto‑filling mechanism. By defining custom @interface annotations and corresponding AnnotationFormatterFactory implementations, developers can automatically convert request parameters into complex objects without writing boilerplate code or using AOP.

Case 1 – Basic Conversion

Define Annotation

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
public @interface ToUser {}

Formatter Factory

public class ToUserAnnotationFormatterFactory implements AnnotationFormatterFactory<ToUser> {
  @Override
  public Set<Class<?>> getFieldTypes() {
    return Set.of(User.class);
  }
  @Override
  public Printer<User> getPrinter(ToUser annotation, Class<?> fieldType) {
    return (value, locale) -> "【id = %s, name = %s, age = %s]".formatted(
        value.getId(), value.getName(), value.getAge());
  }
  @Override
  public Parser<User> getParser(ToUser annotation, Class<?> fieldType) {
    return new Parser<User>() {
      @Override
      public User parse(String text, Locale locale) throws ParseException {
        if (!StringUtils.hasLength(text)) return null;
        String[] data = text.split(",");
        if (data.length != 3) return null;
        return new User(Long.valueOf(data[0]), data[1], Integer.valueOf(data[2]));
      }
    };
  }
}

Register Formatter

@Configuration
public class WebConfig implements WebMvcConfigurer {
  @Override
  public void addFormatters(FormatterRegistry registry) {
    registry.addFormatterForFieldAnnotation(new ToUserAnnotationFormatterFactory());
  }
}

Test Controllers

@GetMapping
public User getUser(@ToUser User user) { return user; }

@GetMapping("/profile")
public Profile getProfile(Profile profile) { return profile; }

public class User { private Long id; private String name; private Integer age; }
public class Profile { @ToUser private User user; }

Running the endpoints with a request like GET /getUser?user=1,zhangsan,30 returns a populated User object, as shown in the accompanying screenshots.

Case 2 – Automatic Remote Service Invocation

In micro‑service architectures, DTOs often contain only an ID (e.g., createdById). Manually calling a remote API to fetch the full object is error‑prone. The article extends the same concept to resolve IDs via a remote service.

Define Annotation

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
public @interface ResolveRemoteUser { boolean required() default true; }

Remote Formatter Factory

@Component
public class RemoteUserAnnotationFormatterFactory implements AnnotationFormatterFactory<ResolveRemoteUser> {
  private final RemoteUserService remoteUserService;
  public RemoteUserAnnotationFormatterFactory(RemoteUserService remoteUserService) {
    this.remoteUserService = remoteUserService;
  }
  @Override
  public Set<Class<?>> getFieldTypes() { return Set.of(RemoteUser.class); }
  @Override
  public Printer<RemoteUser> getPrinter(ResolveRemoteUser annotation, Class<?> fieldType) {
    return (user, locale) -> user != null ? user.getId() : "";
  }
  @Override
  public Parser<RemoteUser> getParser(ResolveRemoteUser annotation, Class<?> fieldType) {
    return new Parser<RemoteUser>() {
      @Override
      public RemoteUser parse(String text, Locale locale) throws ParseException {
        if (!StringUtils.hasLength(text)) return null;
        RemoteUser resolved = remoteUserService.findUserById(text);
        if (resolved == null) {
          if (annotation.required()) {
            throw new ParseException("Could not resolve RemoteUser with ID: " + text, 0);
          } else {
            RemoteUser placeholder = new RemoteUser();
            placeholder.setId(text);
            return placeholder;
          }
        }
        return resolved;
      }
    };
  }
}

Register Remote Formatter

private final RemoteUserAnnotationFormatterFactory userAnnotationFormatterFactory;
public void addFormatters(FormatterRegistry registry) {
  registry.addFormatterForFieldAnnotation(userAnnotationFormatterFactory);
}

Test Controller

@GetMapping("/review")
public ReviewerRequest getProjectReviewer(ReviewerRequest request) { return request; }

public class RemoteUser { private String id; private String username; private String email; }
public class ReviewerRequest {
  @ResolveRemoteUser(required = true)
  private RemoteUser reviewer;
  private String title;
}

When the endpoint receives a request with reviewer=123, Spring automatically calls RemoteUserService.findUserById("123") and injects the full RemoteUser object. The result screenshot confirms the behavior.

Conclusion

By harnessing Spring's AnnotationFormatterFactory, developers can achieve clean, zero‑intrusion parameter auto‑filling for both simple string parsing and complex remote lookups, eliminating repetitive boilerplate and keeping business logic untouched.