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
}

Need a 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
}

Need a 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
}

Need a 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
}

Need a hint?

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