Nested DTOs in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this nested DTO serialization?

Given the following Spring Boot DTO classes and a controller returning a nested DTO, what JSON will be returned to the client?

Spring Boot

public record AddressDTO(String street, String city) {}

public record UserDTO(String name, AddressDTO address) {}

@RestController
public class UserController {
    @GetMapping("/user")
    public UserDTO getUser() {
        AddressDTO address = new AddressDTO("123 Main St", "Springfield");
        return new UserDTO("John Doe", address);
    }
}

A {"name":"John Doe","address":null}

B {"name":"John Doe","address":"123 Main St, Springfield"}

C {"name":"John Doe","address":{"street":"123 Main St","city":"Springfield"}}

D {"name":"John Doe","address":{"street":"123 Main St"}}

💡 Hint

Think about how nested records are serialized by default in Spring Boot with Jackson.

The nested DTO AddressDTO is serialized as a JSON object inside the address field. So the full nested structure appears in the JSON response.

❓ state_output

intermediate

What is the value of the nested DTO field after mapping?

Consider these DTOs and a mapping method that creates a nested DTO. What is the value of userDTO.address.city after calling mapToUserDTO()?

Spring Boot

public record AddressDTO(String street, String city) {}
public record UserDTO(String name, AddressDTO address) {}

public UserDTO mapToUserDTO() {
    AddressDTO address = new AddressDTO("456 Elm St", "Metropolis");
    return new UserDTO("Alice", address);
}

A "Metropolis"

B "456 Elm St"

C null

D "Alice"

💡 Hint

Remember the nested DTO stores city as a separate field.

The address field inside UserDTO holds an AddressDTO with city "Metropolis".

📝 Syntax

advanced

Which option correctly defines a nested DTO with Lombok in Spring Boot?

You want to create nested DTOs using Lombok annotations. Which code snippet correctly defines UserDTO with a nested AddressDTO?

A

@Data public class UserDTO { private String name; private AddressDTO address; }
@Data public class AddressDTO { private String street; private String city; }

B

@Data public class UserDTO { private String name; private String address; }
@Data public class AddressDTO { private String street; private String city; }

C

@Data public class UserDTO { private String name; private AddressDTO address; }
@Data public class AddressDTO { private String street; }

D

@Data public class UserDTO { private String name; private AddressDTO address; }
@Data public class AddressDTO { private String city; }

💡 Hint

Nested DTOs require matching fields in both classes.

Option A correctly defines both DTOs with all needed fields. Other options miss fields or mismatch types.

🔧 Debug

advanced

Why does this nested DTO cause a JSON serialization error?

Given these DTOs, the controller returns UserDTO but the JSON response fails. What is the cause?

Spring Boot

public class AddressDTO {
    private String street;
    private String city;
    // no getters or setters
}

public class UserDTO {
    private String name;
    private AddressDTO address;
    // getters and setters only for name
}

@RestController
public class UserController {
    @GetMapping("/user")
    public UserDTO getUser() {
        AddressDTO address = new AddressDTO();
        address.street = "789 Oak St";
        address.city = "Gotham";
        UserDTO user = new UserDTO();
        user.setName("Bruce");
        user.setAddress(address);
        return user;
    }
}

A AddressDTO fields are private but should be public for serialization.

B AddressDTO lacks getters, so Jackson cannot serialize its fields.

C UserDTO is missing a no-args constructor causing deserialization failure.

D UserDTO's name field is missing a setter causing serialization error.

💡 Hint

Jackson needs getters to read private fields during serialization.

Jackson uses getters to serialize fields. Without getters in AddressDTO, it cannot read the nested fields, causing serialization failure.

🧠 Conceptual

expert

How does Spring Boot handle nested DTOs with circular references during JSON serialization?

Consider two nested DTOs referencing each other, causing a circular reference. What is the default behavior of Spring Boot's JSON serialization, and how can it be resolved?

A Spring Boot returns null for one of the nested objects to prevent loops.

B Spring Boot automatically detects and ignores circular references without errors.

C Spring Boot serializes both objects fully, duplicating data infinitely.

D Spring Boot throws a StackOverflowError by default; use @JsonManagedReference and @JsonBackReference to fix.

💡 Hint

Think about how Jackson handles circular references by default.

By default, Jackson (used by Spring Boot) throws a StackOverflowError on circular references. Using annotations like @JsonManagedReference and @JsonBackReference helps break the cycle during serialization.