我的应用程序必须发送一个文本文件，它首先必须生成一个String。该文本包含非ASCII符号，所以我希望它是UTF-8。我已经尝试了很多变种，但我收到的所有附件都是一些问号。并且，当我发送与邮件正文相同的文本时，它可以正常工作。
以下是生成带附件的MimeBodyPart的代码行：
String attachment = "Привет";
messageBodyPart.setContent(new String(attachment.getBytes("UTF-8"),
"UTF-8"),"text/plain; charset=UTF-8");
我也尝试使用没有任何转换的字符串，只使用字节，现在，如你所见，我试图从字节生成一个字符串...
我究竟做错了什么？ （我确实记得在另一个项目中这样做，但是我不再能够访问它的源代码）。
先感谢您。提莫菲。
UPDATE
阅读完回复后，经过一些不成功的试验后，我认为最好发布我的邮件代码。我有Mailer类，它进行邮件发送，其他类可以调用它的静态sendMessage()方法来发送消息。这一切都在Google App Engine上运行。
public static void sendMessage(String to, String subject, String msgBody,
String attachment) throws AddressException, MessagingException {
Properties props = new Properties();
Session mailSession = Session.getDefaultInstance(props, null);
Message msg = new MimeMessage(mailSession);
String email = "bla-bla-bla"; // userService.getCurrentUser().getEmail();
msg.setFrom(new InternetAddress(email));
msg.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
InternetAddress[] addresses = { new InternetAddress("bla-bla-bla") };
msg.setReplyTo(addresses);
msg.setSubject(subject);
Calendar cal = Calendar.getInstance();
String fileName = cal.get(Calendar.YEAR) + "_"
+ cal.get(Calendar.MONTH) + "_"
+ cal.get(Calendar.DAY_OF_MONTH) + "_"
+ cal.get(Calendar.HOUR_OF_DAY) + "_"
+ cal.get(Calendar.MINUTE) + "_" + cal.get(Calendar.SECOND)
+ "_" + cal.get(Calendar.MILLISECOND) + ".txt";
// create the message part
MimeBodyPart messageBodyPart = new MimeBodyPart();
// fill message
// Here we should have the msgBody.
// Sending attachment contents for debugging only.
messageBodyPart.setText(attachment + " - 4", "UTF-8", "plain");
Multipart multipart = new MimeMultipart();
multipart.addBodyPart(messageBodyPart);
MimeBodyPart att = new MimeBodyPart();
att.setText(attachment, "UTF-8", "plain");
att.addHeader("Content-Type", "text/plain; charset=UTF-8");
att.setFileName(fileName);
multipart.addBodyPart(att);
// Put parts in message
msg.setContent(multipart);
Transport.send(msg);
}
在另一个类中调用此东西的行是：
Mailer.sendMessage("[email protected]", "Test", "No body", "Привет, Я кусок текста");
奇怪的是，邮件的原始来源是（忽略看似无关的标题）：
Message-ID: <[email protected]>
Date: Sat, 12 Feb 2011 11:21:01 +0000
Subject: Pages
From: [email protected]
To: [email protected]
Content-Type: multipart/mixed; boundary=00163662e7107ccbd4049c1402fa
--00163662e7107ccbd4049c1402fa
Content-Type: text/plain; charset=KOI8-R; format=flowed; delsp=yes
Content-Transfer-Encoding: base64
8NLJ18XULCDxIMvV08/LINTFy9PUwSAtIDQNCg==
--00163662e7107ccbd4049c1402fa
Content-Type: text/plain; charset=US-ASCII; name="2011_1_12_11_21_1_691.txt"
Content-Disposition: attachment; filename="2011_1_12_11_21_1_691.txt"
Content-Transfer-Encoding: base64
Pz8/Pz8/LCA/ID8/Pz8/ID8/Pz8/Pw==
--00163662e7107ccbd4049c1402fa--
我只是不明白，为什么charsets与我想要设置的不同，以及它们来自哪里。
将内容类型设置为application/octet-stream：
MimeBodyPart attachmentPart = new MimeBodyPart();
try {
DataSource ds = new ByteArrayDataSource(attachment.getBytes("UTF-8"), "application/octet-stream");
attachmentPart = new MimeBodyPart();
attachmentPart.setDataHandler(new DataHandler(ds));
}
catch (Exception e) {
Logger.getLogger("Blina").log(Level.SEVERE, Misc.getStackTrace(e));
}
attachmentPart.setFileName(fileName);
multipart.addBodyPart(attachmentPart);
// Put parts in message
msg.setContent(multipart);
有类似的情况，下面的代码解决了它：
MimeBodyPart att = new MimeBodyPart();
att.setFileName(MimeUtility.encodeText(fileName));
这是我用来发送文件的示例代码（无论编码或数据结构如何）。
BodyPart fileBodyPart = new MimeBodyPart();
fileBodyPart.setDataHandler(new DataHandler(fileDataSource));
fileBodyPart.setFileName(attachment.getName());
fileBodyPart.setHeader("Content-Type", fileDataSource.getContentType());
fileBodyPart.setHeader("Content-ID", attachment.getName());
fileBodyPart.setDisposition(Part.INLINE);
其中fileDataSource是javax.activation.DataSource（文本文件将在这里），fileBodyPart.setDisposition(Part.INLINE);（PART.INLINE表示数据源内联消息体，就像HTML电子邮件一样，PART.ATTACHMENT表示数据源是附件）。
希望这可以帮助。
尝试一下：
String attachment = "Привет";
DataSource ds = new ByteArrayDataSource(attachment, "text/plain; charset=UTF-8");
messageBodyPart.setDataHandler(new DataHandler(ds));
更新:(完整示例）
import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.mail.Session;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;
import javax.mail.util.ByteArrayDataSource;
public class Main {
public static void main(String[] args) throws Exception {
String attachment = "Привет";
DataSource ds = new ByteArrayDataSource(attachment, "text/plain; charset=UTF-8");
MimeBodyPart attachmentPart = new MimeBodyPart();
attachmentPart.setDataHandler(new DataHandler(ds));
MimeBodyPart bodyPart = new MimeBodyPart();
bodyPart.setText("Hello this is some text");
MimeMultipart mp = new MimeMultipart("mixed");
mp.addBodyPart(bodyPart);
mp.addBodyPart(attachmentPart);
MimeMessage msg = new MimeMessage((Session)null);
msg.setContent(mp);
msg.writeTo(System.out);
}
}
输出：
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_Part_0_1579321858.1297366787792"
------=_Part_0_1579321858.1297366787792
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Hello this is some text
------=_Part_0_1579321858.1297366787792
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: base64
0J/RgNC40LLQtdGC
------=_Part_0_1579321858.1297366787792--
如果问题出在文件名中，而不是在正文中，则以下代码在我的（希伯来语）案例中有所帮助：
MimeBodyPart attachment = new MimeBodyPart();
attachment.setFileName(MimeUtility.encodeText(filename, "UTF-8", null));
这有效：
MimeMessage msg = new MimeMessage(session);
msg.setFrom(sendFrom);
msg.setSubject(subject, "utf-8");
msg.setSentDate(new Date());
// create and fill the first message part
MimeBodyPart mbp1 = new MimeBodyPart();
mbp1.setContent(message,"text/plain; charset=UTF-8");
// mbp1.setContent(message,"text/html; charset=UTF-8"); // with this the attachment will fail
// create the Multipart and its parts to it
Multipart mp = new MimeMultipart();
mp.addBodyPart(mbp1);
if (attachment!=null){
// Part two is attachment
MimeBodyPart mbp2 = new MimeBodyPart();
mbp2 = new MimeBodyPart();
DataSource ds = null;
try {
ds = new ByteArrayDataSource(attachment.getBytes("UTF-8"), "application/octet-stream");
} catch (IOException e) {
e.printStackTrace();
}
mbp2.setDataHandler(new DataHandler(ds));
mbp2.addHeader("Content-Type", "text/plain; charset=\"UTF-8\"");
mbp2.addHeader("Content-Transfer-Encoding", "base64");
mbp2.setFileName("attachment.txt");
mbp2.setDisposition(Part.ATTACHMENT);
mp.addBodyPart(mbp2);
}
// add the Multipart to the message
msg.setContent(mp);
msg.saveChanges();
// send the message
Transport.send(msg);
还有一种可能性：
String attachment = "älytöntä";
MimeBodyPart part = new MimeBodyPart();
part.setText(attachment, "UTF-8");
part.setDisposition("attachment");
part.setFileName("attachment.txt");
part.setHeader("Content-Transfer-Encoding", "base64");
part.setHeader("Content-type", "text/plain; charset=utf-8");