🚀 Sending OTP via Email using Spring Boot & Spring Mail

Email verification is a very common feature in web applications — whether it’s signing up for a new account, resetting a password, or enabling two-factor authentication. In this tutorial, we’ll learn how to send a One-Time Password (OTP) to a user’s email using Spring Boot and Spring Mail.

1️⃣ Add Dependencies

Add this dependency in your pom.xml:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-mail</artifactId>
</dependency>

2️⃣ Configure Mail Properties

In application.properties (or application.yml), configure your SMTP server.
(Here I’ll use Gmail SMTP as an example):

spring.mail.host=smtp.gmail.com
spring.mail.port=587
spring.mail.username=your-email@gmail.com
spring.mail.password=your-app-password
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true

👉 Note: If you’re using Gmail, you’ll need to generate an App Password instead of using your Gmail password.

3️⃣ Create an OTP Generator

A simple utility to generate numeric OTPs:

import java.security.SecureRandom;

public class OtpUtil {
    private static final SecureRandom random = new SecureRandom();
    
    public static String generateOtp(int length) {
        StringBuilder otp = new StringBuilder();
        for (int i = 0; i < length; i++) {
            otp.append(random.nextInt(10));
        }
        return otp.toString();
    }
}

4️⃣ Service to Send Email

Now, let’s create a service that will generate the OTP and send it via email.

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import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.mail.SimpleMailMessage;
import org.springframework.mail.javamail.JavaMailSender;
import org.springframework.stereotype.Service;

@Service
public class EmailService {

    @Autowired
    private JavaMailSender mailSender;

    public String sendOtpEmail(String toEmail) {
        String otp = OtpUtil.generateOtp(6); // 6-digit OTP
        
        SimpleMailMessage message = new SimpleMailMessage();
        message.setTo(toEmail);
        message.setSubject("Your OTP Code");
        message.setText("Your OTP is: " + otp + "\nIt will expire in 5 minutes.");
        
        mailSender.send(message);
        
        return otp;
    }
}

5️⃣ Create a REST API to Trigger OTP

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/api/otp")
public class OtpController {

    @Autowired
    private EmailService emailService;

    @PostMapping("/send")
    public String sendOtp(@RequestParam String email) {
        String otp = emailService.sendOtpEmail(email);
        return "OTP sent to " + email + " (for demo, OTP: " + otp + ")";
    }
}

6️⃣ Test the API

• Start your Spring Boot application
• Use Postman or cURL to send a request:

POST http://localhost:8080/api/otp/send?email=test@example.com

You should receive an email with the OTP. 🎉

7️⃣ What’s Next?

This is a basic implementation. In a real application, you should also:

• Store OTPs in DB or cache (e.g., Redis) with expiry
• Verify OTP on another API endpoint
• Limit OTP requests to avoid spam

We just built a simple OTP email sender with Spring Boot and Spring Mail.
This feature can be extended for signup verification, password reset, or two-factor authentication.