Working with Email Services

spring:
  mail:
    host: smtp.gmail.com
    port: 587
    username: ${MAIL_USER}
    password: ${MAIL_PASS}
    properties:
      mail.smtp.auth: true
      mail.smtp.starttls.enable: true
      mail.smtp.connectiontimeout: 5000
      mail.smtp.timeout: 5000

@Service
public class MailService {
  private final JavaMailSender sender;
  public MailService(JavaMailSender sender) { this.sender = sender; }
  public void sendText(String to, String subject, String body) {
    SimpleMailMessage msg = new SimpleMailMessage();
    msg.setTo(to); msg.setSubject(subject); msg.setText(body);
    msg.setFrom("no-reply@example.com");
    sender.send(msg);
  }
}

public void sendHtml(String to, String subject, String html) throws MessagingException {
  MimeMessage mm = sender.createMimeMessage();
  MimeMessageHelper h = new MimeMessageHelper(mm, true, "UTF-8");
  h.setTo(to); h.setSubject(subject); h.setText(html, true);
  h.setFrom("no-reply@example.com");
  sender.send(mm);
}

private final TemplateEngine engine;
public void sendWelcome(User u) throws MessagingException {
  Context ctx = new Context();
  ctx.setVariables(Map.of("name", u.name(), "url", "https://app/verify/" + u.token()));
  String html = engine.process("emails/welcome", ctx);
  sendHtml(u.email(), "Welcome", html);
}

MimeMessageHelper h = new MimeMessageHelper(mm, true);
h.addAttachment("invoice.pdf", new FileSystemResource(path.toFile()));
h.addInline("logo", new ClassPathResource("static/logo.png"), "image/png");
// In HTML: <img src="cid:logo"/>

@Async("emailExecutor")
public CompletableFuture<Void> sendAsync(String to, String subj, String body) {
  sendText(to, subj, body);
  return CompletableFuture.completedFuture(null);
}

try { sender.send(mm); }
catch (MailAuthenticationException e) { /* bad creds */ }
catch (MailSendException e) { e.getFailedMessages().forEach(/*…*/); }
catch (MailException e) { /* generic */ }

@SpringBootTest
class MailServiceTests {
  static GreenMail green;
  @BeforeAll static void up() {
    green = new GreenMail(ServerSetupTest.SMTP);
    green.start();
    green.setUser("user", "pass");
  }
  @AfterAll static void down() { green.stop(); }
  @Autowired MailService svc;
  @Test void sends() throws Exception {
    svc.sendText("to@x", "subj", "body");
    assertTrue(green.waitForIncomingEmail(5000, 1));
  }
}