DTO validation in Spring Boot - Mini Project: Build & Apply

DTO Validation in Spring Boot

📖 Scenario: You are building a simple Spring Boot web application that accepts user registration data. To keep the data clean and safe, you need to validate the input before processing it.

🎯 Goal: Create a Data Transfer Object (DTO) for user registration with validation annotations. Then configure a controller to accept and validate this DTO.

📋 What You'll Learn

Create a UserRegistrationDTO class with fields: username, email, and age

Add validation annotations to ensure username is not empty, email is a valid email, and age is at least 18

Create a controller method that accepts UserRegistrationDTO as a request body

Enable validation so that invalid data returns an error response

💡 Why This Matters

🌍 Real World

Validating user input in web applications is essential to prevent bad data and security issues.

💼 Career

Understanding DTO validation is important for backend developers working with Spring Boot to build robust APIs.

1

Create UserRegistrationDTO with fields

Create a class called UserRegistrationDTO with three private fields: String username, String email, and int age. Add public getters and setters for each field.

Spring Boot

public class UserRegistrationDTO {
    // Your code here
}

Hint

Define the class and fields exactly as named. Use standard getter and setter methods.

2

Add validation annotations to UserRegistrationDTO

Add validation annotations to the UserRegistrationDTO fields: annotate username with @NotEmpty, email with @Email, and age with @Min(18). Import the annotations from jakarta.validation.constraints.

Spring Boot

import jakarta.validation.constraints.*;

public class UserRegistrationDTO {
    // Add validation annotations here
    private String username;
    private String email;
    private int age;

    // getters and setters
}

Hint

Use the correct validation annotations on the fields. Make sure to import them.

3

Create controller method to accept UserRegistrationDTO

Create a Spring Boot controller class called UserController. Inside it, create a method registerUser annotated with @PostMapping("/register") that accepts a @RequestBody UserRegistrationDTO user parameter. Add @Valid before @RequestBody to enable validation.

Spring Boot

import org.springframework.web.bind.annotation.*;
import jakarta.validation.Valid;

@RestController
public class UserController {
    // Add registerUser method here
}

Hint

Use @RestController on the class and @PostMapping on the method. Remember to add @Valid before @RequestBody.

4

Enable validation error handling

Add @Valid to the UserRegistrationDTO parameter in the registerUser method to trigger validation. Also, add @ResponseStatus(HttpStatus.BAD_REQUEST) and @ExceptionHandler(MethodArgumentNotValidException.class) in the controller to handle validation errors and return a simple error message.

Spring Boot

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.*;
import jakarta.validation.Valid;

@RestController
public class UserController {
    @PostMapping("/register")
    public String registerUser(@Valid @RequestBody UserRegistrationDTO user) {
        return "User registered successfully";
    }

    // Add exception handler here
}

Hint

Add an exception handler method in the controller to catch validation errors and respond with a 400 status and message.

Practice

1. What is the main purpose of using DTO validation in a Spring Boot application?

easy

A. To handle user authentication and login

B. To speed up database queries automatically

C. To generate HTML pages from data

D. To check and ensure input data meets rules before processing

Solution

1. Step 1: Understand DTO role

   A DTO (Data Transfer Object) carries data between processes and needs validation to ensure data is correct.

2. Step 2: Purpose of validation

   Validation checks input data early to prevent bad data from reaching business logic or database.

3. Final Answer:

   To check and ensure input data meets rules before processing -> Option D

4. Quick Check:

   DTO validation = input data check [OK]

Hint: Validation means checking input data early [OK]

Common Mistakes:

• Confusing validation with database optimization
• Thinking validation generates UI
• Mixing validation with authentication

2. Which annotation is used on a DTO field to require that it must not be empty or null?

easy

A. @Size(min = 1)

B. @NotEmpty

C. @NotNull

D. @Valid

Solution

1. Step 1: Understand annotations meaning

   @NotNull only checks for null, but allows empty strings. @NotEmpty checks for both null and empty strings.

2. Step 2: Choose correct annotation

   To ensure a field is neither null nor empty, @NotEmpty is the best choice.

3. Final Answer:

   @NotEmpty -> Option B

4. Quick Check:

   @NotEmpty = no null or empty [OK]

Hint: Use @NotEmpty to block null and empty strings [OK]

Common Mistakes:

• Using @NotNull but allowing empty strings
• Confusing @Valid with field validation
• Using @Size without min value

3. Given this DTO class snippet:

public class UserDTO {
  @NotNull
  private String username;

  @Min(18)
  private int age;

  // getters and setters
}

What happens if a request sends username=null and age=16 when validated with @Valid?

medium

A. Validation fails for both username and age fields

B. Validation passes because age is int and can't be null

C. Validation fails only for age field

D. Validation passes because @NotNull is ignored on String

Solution

1. Step 1: Check username validation

   @NotNull on username means null value is invalid, so username=null fails validation.

2. Step 2: Check age validation

   @Min(18) means age must be at least 18. Given age=16, this fails validation.

3. Final Answer:

   Validation fails for both username and age fields -> Option A

4. Quick Check:

   @NotNull + @Min(18) fail for null and age 16 [OK]

Hint: Check each annotation rule against input values [OK]

Common Mistakes:

• Assuming int fields can't fail validation
• Ignoring @NotNull effect on String
• Thinking validation passes if one field is valid

4. Identify the error in this controller method for validating a DTO:

@PostMapping("/users")
public ResponseEntity<String> addUser(UserDTO user) {
  // save user
  return ResponseEntity.ok("User added");
}

medium

A. Missing @Validated annotation on controller class

B. Method should return void instead of ResponseEntity

C. Missing @RequestBody annotation on UserDTO parameter

D. No error, code is correct

Solution

1. Step 1: Check parameter annotations

   To validate JSON input as DTO, @RequestBody is needed to bind request body to UserDTO.

2. Step 2: Check validation annotation

   @Valid is also needed to trigger validation, but missing @RequestBody causes binding failure first.

3. Final Answer:

   Missing @RequestBody annotation on UserDTO parameter -> Option C

4. Quick Check:

   @RequestBody needed for JSON binding [OK]

Hint: Use @RequestBody to bind JSON to DTO [OK]

Common Mistakes:

• Forgetting @RequestBody causes no binding
• Thinking @Valid alone binds JSON
• Assuming return type must be void

5. You want to validate a DTO with a nested object, where the nested object also needs validation. Which is the correct way to enable validation on the nested DTO field?

hard

A. Add @Valid annotation on the nested DTO field inside the parent DTO

B. Add @NotNull on the nested DTO field only

C. Add @Valid on the parent DTO class only

D. No annotation needed, nested DTOs are validated automatically

Solution

1. Step 1: Understand nested validation

   Spring Boot requires @Valid on nested DTO fields to trigger validation of inner objects.

2. Step 2: Why @Valid on nested field

   @NotNull only checks presence, but @Valid triggers validation of nested object's fields.

3. Final Answer:

   Add @Valid annotation on the nested DTO field inside the parent DTO -> Option A

4. Quick Check:

   @Valid on nested field triggers inner validation [OK]

Hint: Use @Valid on nested DTO fields for full validation [OK]

Common Mistakes:

• Using only @NotNull on nested DTO
• Assuming parent @Valid covers nested fields
• Skipping @Valid and expecting automatic nested validation