SpringBoot Email Sending with @Async Optimization#

Basic Configuration#

1. Add the Dependency#

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

2. Configure Mail Properties#

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spring:
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  mail:
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    username: [email protected]
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    password: your-auth-code
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    host: smtp.163.com

The password field requires an authorization code, not your email login password. For 163 mail, enable POP3/SMTP/IMAP in settings and generate an auth code.

3. Send a Simple Email#

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@RestController
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@RequiredArgsConstructor
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public class EmailController {
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    private final JavaMailSender javaMailSender;
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    @GetMapping("/send-email")
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    public String sendEmail(String email) {
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        SimpleMailMessage message = new SimpleMailMessage();
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        message.setFrom("your-name<[email protected]>");
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        message.setTo(email);
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        message.setSubject("Verification Code");
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        message.setText("Your code is: 123456");
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        javaMailSender.send(message);
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        return "ok";
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    }
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}

Async Sending with @Async#

Synchronous email sending blocks the request thread. For high-concurrency scenarios, async is recommended.

1. Enable Async Support#

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@SpringBootApplication
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@EnableAsync
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public class Application {
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    public static void main(String[] args) {
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        SpringApplication.run(Application.class, args);
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    }
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}

2. Async Email Service#

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@Component
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@Slf4j
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public class EmailService {
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    @Autowired
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    private JavaMailSender javaMailSender;
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    @Autowired
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    private StringRedisTemplate stringRedisTemplate;
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    @Async
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    public void sendVerificationCode(String email) {
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        String code = UUID.randomUUID().toString().substring(0, 6);
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        SimpleMailMessage message = new SimpleMailMessage();
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        message.setFrom("your-name<[email protected]>");
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        message.setTo(email);
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        message.setSubject("Verification Code");
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        message.setText(code);
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        stringRedisTemplate.boundValueOps("code:" + email)
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            .set(code, 5, TimeUnit.MINUTES);
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        javaMailSender.send(message);
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        log.info("Verification code sent to: {}", email);
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    }
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}

3. Custom Thread Pool#

The default async thread pool has a core pool size of 8. Adjust it as needed:

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spring:
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  task:
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    execution:
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      pool:
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        core-size: 50

Or via Java configuration:

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@Bean
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public ThreadPoolTaskExecutor threadPoolExecutor() {
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    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
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    executor.setCorePoolSize(20);
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    executor.setMaxPoolSize(50);
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    executor.setQueueCapacity(200);
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    executor.setThreadNamePrefix("mail-executor-");
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    executor.initialize();
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    return executor;
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}

Common @Async Pitfalls#

@Async won’t work in these cases:

The method is not public
Return type is not void or Future
The method is static
Spring cannot scan the class (missing @Component/@Service etc.)
Caller and callee are in the same class — Spring AOP proxies can’t intercept internal calls
Creating the object with new instead of dependency injection
Adding @Transactional directly on an @Async method — put @Transactional on the called method instead