@Size for length constraints in Spring Boot - Mini Project: Build & Apply

@Size for length constraints in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage user profiles. Each user has a username that must be between 3 and 15 characters long.

🎯 Goal: Create a User class with a username field that uses the @Size annotation to enforce the length constraint. Then, configure a controller to accept user data and validate it.

📋 What You'll Learn

Create a User class with a username field

Use @Size(min = 3, max = 15) on the username field

Create a controller method to accept a User object

Enable validation on the controller method parameter

💡 Why This Matters

🌍 Real World

Length constraints on user input fields are common in web applications to ensure data quality and prevent errors.

💼 Career

Understanding how to use validation annotations like @Size is essential for backend developers working with Spring Boot to build robust APIs.

1

Create the User class with a username field

Create a class called User with a private String field named username.

Spring Boot

public class User {
    // Your code here
}

Need a hint?

Remember to add getter and setter methods for the username field.

2

Add @Size annotation to username field

Add the @Size(min = 3, max = 15) annotation above the username field in the User class. Also, add the import statement for jakarta.validation.constraints.Size.

Spring Boot

import jakarta.validation.constraints.Size;

public class User {
    // Add @Size annotation here
    private String username;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

Need a hint?

The @Size annotation controls the allowed length of the string.

3

Create a controller method to accept User data

Create a Spring Boot controller class called UserController with a method createUser that accepts a @RequestBody User user parameter. Add @PostMapping("/users") above the method.

Spring Boot

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {
    // Your code here
}

Need a hint?

Use @RestController on the class and @PostMapping on the method.

4

Enable validation on the User parameter

Add the @Valid annotation before the User user parameter in the createUser method. Also, add the import for jakarta.validation.Valid.

Spring Boot

import jakarta.validation.Valid;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {
    // Add @Valid annotation here
    @PostMapping("/users")
    public void createUser(@RequestBody User user) {
        // Method body can be empty for now
    }
}

Need a hint?

The @Valid annotation triggers validation of the User object.