Master Lombok: Powerful Annotations (@Constructor, @Delegate, @Cleanup, @Builder) Explained

Preface

This article shares clever Lombok tricks without judging right or wrong, aiming to inspire readers and provide useful code snippets.

@onX

Examples of using onConstructor, onMethod, and onParam to inject custom annotations, such as combining with Spring's @Autowired.

@Service
@RequiredArgsConstructor(onConstructor = @__(@Autowired))
public class MyService {
    private final AnotherService anotherService;
}

Lombok generates a constructor annotated with @Autowired, allowing Spring to automatically inject the required AnotherService bean.

@Delegate

@Delegate

lets a class use methods of another class without writing boilerplate, reducing deep inheritance and tight coupling.

// Class A with a method
public class A {
    public void sayHello() {
        System.out.println("Hello");
    }
}

// Class B delegates A
public class B {
    @Delegate private A a = new A();

    public static void main(String[] args) {
        B b = new B();
        b.sayHello(); // Calls A's method
    }
}

@Cleanup

@Cleanup

automatically manages resources, ensuring that close() or a custom release method is called safely.

@Cleanup InputStream in = new FileInputStream("some/file");
@Cleanup("release") MyResource resource = new MyResource();

Lombok wraps the code in a try‑finally block and invokes in.close() or resource.release() accordingly.

@Singular and @Builder

@Builder

enables fluent object construction, while @Singular simplifies handling of collection fields, allowing both single-element and bulk additions.

@Data
@Builder
public class User {
    private String name;
    private int age;
    @Singular private List<String> hobbies;
}
User user = User.builder()
    .name("练习时长两年半")
    .age(28)
    .hobby("篮球")
    .hobby("唱歌")
    .hobbies(Arrays.asList("跳舞", "其他"))
    .build();

After build(), the collection becomes immutable; clearHobbies() can reset it if needed.

@With

@With

creates a new instance with modified fields, preserving immutability of the original object.

@With
public class Person {
    private String name;
    private int age;
}
Person person = new Person("Alice", 30);
Person updatedPerson = person.withAge(31);

Conclusion

While Lombok provides many convenient features, overusing or misusing them can make code hard to understand and maintain; therefore, apply these annotations judiciously and consider their impact on readability.