Sending Async Emails in Spring

I have a student taking my course that sent me the following question

It would be really cool to show how to send this email asynchronously. Because it's never a good idea to send any sort of notification synchronously.

I do agree with him in that there is no reason for us block program execution for something like sending an email. I am going to take this chance to walk you through sending email asynchronously using Spring Boot. Start off by creating a basic new Spring Boot project using the Spring Initializr from the web or your favorite IDE. We are going to select Web & Mail as dependencies.

By selecting web and mail as dependencies we should have these two starter dependencies included included in our build file.

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

With the starter mail dependency in place we need to configure our mail server properties.

spring.mail.host = smtp.gmail.com
spring.mail.username = [email protected]
spring.mail.password = xxxxxxxxxxxxxxxxxx
spring.mail.port=587
spring.mail.properties.mail.smtp.starttls.enable = true

The first thing we are going to do is create a controller. This controller is going to have a request mapping of signup-success. This method is going to create a user (just a simple POJO) and then try to send a notification using our notification service.

@RestController
public class RegistrationController {

    private Logger logger = LoggerFactory.getLogger(RegistrationController.class);

    @Autowired
    private NotificationService notificationService;

    @RequestMapping("/signup-success")
    public String signupSuccess(){

        // create user
        User user = new User();
        user.setFirstName("Dan");
        user.setLastName("Vega");
        user.setEmailAddress("[email protected]");

        // send a notification
        try {
            notificationService.sendNotificaitoin(user);
        }catch( Exception e ){
            // catch error
            logger.info("Error Sending Email: " + e.getMessage());
        }

        return "Thank you for registering with us.";
    }

}

The Notification Service is where the real magic happens. At the start of our send notification method we are going to simulate a long process by using the sleep method. Notice that we have add the @Async annotation to the method that marks a method as a candidate for asynchronous execution.

@Service
public class NotificationService {

    private JavaMailSender javaMailSender;

    @Autowired
    public NotificationService(JavaMailSender javaMailSender){
        this.javaMailSender = javaMailSender;
    }

    @Async
    public void sendNotificaitoin(User user) throws MailException, InterruptedException {

        System.out.println("Sleeping now...");
        Thread.sleep(10000);

        System.out.println("Sending email...");

        SimpleMailMessage mail = new SimpleMailMessage();
        mail.setTo(user.getEmailAddress());
        mail.setFrom("[email protected]");
        mail.setSubject("Spring Boot is awesome!");
        mail.setText("Why aren't you using Spring Boot?");
        javaMailSender.send(mail);

        System.out.println("Email Sent!");
    }

}

We have one final task to make all of this work. Go to the main application class and add the @EnableAsync annotation.

package com.therealdanvega;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableAsync;

@SpringBootApplication
@EnableAsync
public class SendingEmailAsyncApplication {

    public static void main(String\[\] args) {
        SpringApplication.run(SendingEmailAsyncApplication.class, args);
    }
}

Start the application and go to http://localhost:8080/signup-success. You will see the message "Thank you for registering with us." right away and in the console you will see the Sleeping/Sending/Sent messages printed out to the console. If you aren't familiar with Async this is where the real power comes in. The program is not blocking a task like sending an email and returns execution to the user while it performs that task in the background.

You can grab the source for this application below.

https://github.com/danvega/spring-boot-intro/tree/master/guides/sending-email-async