Advanced Lombok Annotations: @onX, @Delegate, @Cleanup, @Builder/@Singular, and @With

The article introduces a series of Lombok annotations that enable concise and expressive Java code, especially when used together with Spring components.

@onX allows custom annotations to be injected into generated code, such as onConstructor , oMethod , and onParam . A typical use case is combining it with Spring's @Autowired :

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

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

@Delegate lets a class expose methods of another class without writing boilerplate delegation code. Example:

public class A { public void sayHello() { System.out.println("Hello"); } } public class B { @Delegate // delegates A's methods private A a = new A(); public static void main(String[] args) { B b = new B(); b.sayHello(); // calls A.sayHello() } }

This reduces deep inheritance hierarchies and tight coupling, improving readability and maintainability.

@Cleanup automatically manages resources that need to be closed. Simple usage:

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

Lombok wraps the code in a try‑finally block and calls in.close() . A custom cleanup method can be specified:

@Cleanup("release") MyResource resource = new MyResource();

Here Lombok will invoke resource.release() in the finally clause.

@Builder together with @Singular provides fluent builders and convenient handling of collection fields. Example class:

@Data @Builder public class User { private String name; private int age; @Singular private List hobbies; }

Using the builder:

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();

The @Singular annotation generates methods to add single elements ( hobby() ) and whole collections ( hobbies() ), and the resulting collection becomes immutable after build() . The builder can also clear a collection:

User user = User.builder() .name("签") .age(28) .hobby("说唱") .hobby("跳舞") .clearHobbies() .hobby("踩缝纫机") .build();

Note that if the class extends a parent, @Builder only includes fields declared in the subclass.

@With creates a copy of an object with modified fields, supporting immutable patterns. Example:

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

The article concludes by cautioning that while Lombok greatly reduces boilerplate, excessive or inappropriate use can make code harder to understand and maintain, so developers should apply these annotations judiciously.