Android中Java发送带附件邮件触发UnsupportedDataTypeException错误

问题 #

在Android中通过Java的AsyncTask实现发送带附件的邮件，fileName是通过context.openFileOutput保存的文件路径。运行时触发致命异常，错误信息为javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed。已尝试使用com.sun.mail:android-mail:1.6.7、com.sun.mail:android-activation:1.6.7以及com.sun.mail:jakarta.mail:2.0.1依赖包，相同代码在Windows PC端Java环境可正常运行。测试发现只要添加附件（无论文件还是JSON格式内容）就报错，不添加附件则可正常发送邮件，求解决方法。

代码示例 #

Properties props = new Properties();
props.put("mail.smtp.host", "smtp.gmail.com");
props.put("mail.smtp.port", "587");
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.starttls.enable", "true");

Session session = Session.getInstance(props, new Authenticator() {
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication("youknow@gmail.com", "psw");
    }
});

Message message = new MimeMessage(session);
try {
    message.setFrom(new InternetAddress("youknow@gmail.com@gmail.com"));
    message.setRecipients(Message.RecipientType.TO, InternetAddress.parse("receiver@gmail.com"));
    message.setSubject("Subject line");
    message.setText("Email content");

    MimeBodyPart messageBodyPart = new MimeBodyPart();
    messageBodyPart.setText("test");

    MimeBodyPart attachmentBodyPart = new MimeBodyPart();
    DataSource source = new FileDataSource(fileName);
    attachmentBodyPart.setDataHandler(new DataHandler(source));
    attachmentBodyPart.setFileName(source.getName());

    MimeMultipart multipart = new MimeMultipart();
    multipart.addBodyPart(messageBodyPart);
    multipart.addBodyPart(attachmentBodyPart);
    message.setContent(multipart);

    Transport.send(message);

} catch (MessagingException ex) {
    throw new RuntimeException(ex);
}

错误日志 #

E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
    Process: com.gattolfo.gestionaluber, PID: 6179
    java.lang.RuntimeException: An error occurred while executing doInBackground()
        at android.os.AsyncTask$4.done(AsyncTask.java:415)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
        at java.util.concurrent.FutureTask.run(FutureTask.java:271)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:305)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:923)
     Caused by: java.lang.RuntimeException: javax.mail.MessagingException: IOException while sending message;
      nested exception is:
        javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed; 
        boundary="----=_Part_0_82827468.1680314088984"
        at com.gattolfo.gestionaluber.DataManager.DbRappresentation.Registri.MailSender.sendMailInstant(MailSender.java:296)
        at com.gattolfo.gestionaluber.DataManager.DbRappresentation.Registri.MailSender.doInBackground(MailSender.java:141)
        at com.gattolfo.gestionaluber.DataManager.DbRappresentation.Registri.MailSender.doInBackground(MailSender.java:48)
        at android.os.AsyncTask$3.call(AsyncTask.java:394)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
            ... 4 more
     Caused by: javax.mail.MessagingException: IOException while sending message;
      nested exception is:
        javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed; 
        boundary="----=_Part_0_82827468.1680314088984"
        at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1350)
        at javax.mail.Transport.send0(Transport.java:231)
        at javax.mail.Transport.send(Transport.java:100)
        at com.gattolfo.gestionaluber.DataManager.DbRappresentation.Registri.MailSender.sendMailInstant(MailSender.java:292)
            ... 8 more
     Caused by: javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed; 
        boundary="----=_Part_0_82827468.1680314088984"
        at javax.activation.ObjectDataContentHandler.writeTo(DataHandler.java:878)
        at javax.activation.DataHandler.writeTo(DataHandler.java:300)
        at javax.mail.internet.MimeBodyPart.writeTo(MimeBodyPart.java:1670)
        at javax.mail.internet.MimeMessage.writeTo(MimeMessage.java:1889)
        at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1300)
            ... 11 more

解决方案 #

这个问题是Android环境中JavaMail的DataContentHandler（DCH）未正确注册导致的——Android的类加载机制和PC端不同，默认不会自动加载Multipart类型的处理器，手动注册即可解决。

方法一：手动注册核心DCH类 #

在发送邮件的代码前，添加以下注册逻辑，强制加载对应MIME类型的处理器：

try {
    // 注册multipart/mixed类型处理器
    Class.forName("com.sun.mail.handlers.multipart_mixed");
    // 注册文本内容处理器（确保正文正常解析）
    Class.forName("com.sun.mail.handlers.text_plain");
    // 注册二进制文件处理器（处理附件）
    Class.forName("com.sun.mail.handlers.application_octet_stream");
} catch (ClassNotFoundException e) {
    e.printStackTrace();
}

方法二：通过Activation框架手动绑定处理器 #

如果方法一无效，直接通过Activation的API指定MIME类型对应的处理器：

import javax.activation.DataHandler;
import javax.activation.DataContentHandlerFactory;
import com.sun.mail.handlers.multipart_mixed;

// 这段代码放在Session初始化之前执行
DataHandler.setDataContentHandlerFactory(new DataContentHandlerFactory() {
    @Override
    public DataContentHandler createDataContentHandler(String mimeType) {
        if ("multipart/mixed".equals(mimeType)) {
            return new multipart_mixed();
        }
        // 可根据需要添加其他MIME类型的处理器，比如application/json
        return null;
    }
});

方法三：检查并修正依赖配置 #

确保使用Android专用的JavaMail依赖，不要混用Jakarta Mail和旧版包，保持版本一致：

dependencies {
    implementation 'com.sun.mail:android-mail:1.6.7'
    implementation 'com.sun.mail:android-activation:1.6.7'
}

额外注意事项 #

• 确保邮件发送逻辑在AsyncTask的doInBackground方法中执行，不要在主线程操作；
• 添加附件前检查文件是否存在、是否有读取权限：

File attachmentFile = new File(fileName);
if (!attachmentFile.exists() || !attachmentFile.canRead()) {
    throw new RuntimeException("附件文件不存在或无读取权限：" + fileName);
}

内容的提问来源于stack exchange，提问作者Amedeo Braggio