JavaMail 實現郵件批量發送：從核心流程到生產級封裝實戰

JavaMail 是 Java 平台發送/接收郵件的標準 API。本文完整演示：核心流程解析、認證器封裝、發送器核心類、工廠模式管理、觀察者模式業務整合，構建可複用的批量郵件發送組件。

核心流程五步驟

1. 認證器：繼承 Authenticator，重寫 getPasswordAuthentication() 提供帳號密碼。
2. Session 配置：Properties 設定 SMTP 主機、端口、認證、加密（SSL/TLS）。
3. 構建郵件：MimeMessage 設定寄件者、收件者、主題、內容（文本/HTML/附件）。
4. 發送：Transport.send(message)。
5. 異常處理：捕獲 MessagingException、AddressException。

1. 認證器封裝

public class MailAuthenticator extends Authenticator {
    private String username;
    private String password;

    public MailAuthenticator(String username, String password) {
        this.username = username;
        this.password = password;
    }

    @Override
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication(username, password);
    }
    // getter/setter 略
}

2. 郵件發送核心類

public class SimpleMailSender {
    private final Properties props = System.getProperties();
    private MailAuthenticator authenticator;
    private Session session;

    // 構造器：自動解析 SMTP 主機 (smtp.域名)
    public SimpleMailSender(String username, String password) {
        String smtpHost = "smtp." + username.split("@")[1];
        init(username, password, smtpHost);
    }
    // 構造器：指定 SMTP 主機
    public SimpleMailSender(String smtpHost, String username, String password) {
        init(username, password, smtpHost);
    }

    private void init(String username, String password, String smtpHost) {
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.host", smtpHost);
        // SSL 支援（465 端口）
        // props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
        // props.put("mail.smtp.socketFactory.port", "465");
        // TLS 支援（587 端口）
        // props.put("mail.smtp.starttls.enable", "true");

        authenticator = new MailAuthenticator(username, password);
        session = Session.getInstance(props, authenticator);
    }

    // 單發
    public void send(String recipient, String subject, Object content) 
            throws AddressException, MessagingException {
        MimeMessage msg = new MimeMessage(session);
        msg.setFrom(new InternetAddress(authenticator.getUsername()));
        msg.setRecipient(Message.RecipientType.TO, new InternetAddress(recipient));
        msg.setSubject(subject);
        msg.setContent(content.toString(), "text/html;charset=utf-8");
        Transport.send(msg);
    }

    // 群發（單郵件多收件人，收件人互見）
    public void send(List recipients, String subject, Object content)
            throws AddressException, MessagingException {
        MimeMessage msg = new MimeMessage(session);
        msg.setFrom(new InternetAddress(authenticator.getUsername()));
        msg.setRecipients(Message.RecipientType.TO, 
            recipients.stream().map(InternetAddress::new).toArray(InternetAddress[]::new));
        msg.setSubject(subject);
        msg.setContent(content.toString(), "text/html;charset=utf-8");
        Transport.send(msg);
    }

    // 重載：接受 SimpleMail POJO
    public void send(String recipient, SimpleMail mail) throws AddressException, MessagingException {
        send(recipient, mail.getSubject(), mail.getContent());
    }
    public void send(List recipients, SimpleMail mail) throws AddressException, MessagingException {
        send(recipients, mail.getSubject(), mail.getContent());
    }
}

3. 郵件 POJO

public class SimpleMail {
    private String subject;
    private String content;
    // getter/setter
}

4. 工廠模式管理發送器單例

public class MailSenderFactory {
    private static SimpleMailSender serviceSender;

    public static SimpleMailSender getSender(MailSenderType type) {
        if (type == MailSenderType.SERVICE) {
            if (serviceSender == null) {
                // 從配置檔讀取帳密
                serviceSender = new SimpleMailSender("service@126.com", "password");
            }
            return serviceSender;
        }
        return null;
    }
}

5. 觀察者模式業務整合：價格變動通知

public class ProductPriceObserver implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        if (!(o instanceof Product)) return;
        Product product = (Product) o;
        if (!(arg instanceof Float)) return;
        Float newPrice = (Float) arg;
        Float decrease = product.getPrice() - newPrice;
        if (decrease > 0) {
            SimpleMailSender sender = MailSenderFactory.getSender(MailSenderType.SERVICE);
            List recipients = Arrays.asList("user1@qq.com", "user2@gmail.com");
            try {
                for (String to : recipients) {
                    sender.send(to, "價格變動通知", 
                        "關注商品 " + product.getName() + " 降價！" +
                        "原價 " + product.getPrice() + " → 現價 " + newPrice +
                        "，降幅 " + decrease + " 元。");
                }
            } catch (MessagingException e) {
                log.error("郵件發送失敗", e);
            }
        }
    }
}

生產環境關鍵增強清單

常見 SMTP 設定參考

開啟「客戶端授權碼」而非登錄密碼，並啟用 SMTP 服務。