MimeUtility.encode and a custom DataSource

Question

I have created a custom DataSource, in which I wrap the OutputStream (returned by getOutputStream()) in a MimeUtility.encode call. When I do this, I seem to be getting an encoded MimeBodyPart, but the 'Content-Transfer-Encoding' is wrong. My DataSource code is:
public class MyDataSource implements DataSource {
    private String _mimeType; // content-type
    private String _encoding;
    private ByteArrayOutputStream _bao;
    /* Create a DataSource from an input stream */
    public MyDataSource(String mimeType, String encoding){
        _mimeType = mimeType;
        _encoding = encoding;
        _bao = new ByteArrayOutputStream();
    * Return an InputStream for the data.
    * Note - a new stream must be returned each time.
    public InputStream getInputStream()
    throws IOException {
        return new
            ByteArrayInputStream(_bao.toByteArray());
    public OutputStream getOutputStream()
    throws IOException {
        OutputStream out = null;
        try {
            out = MimeUtility.encode(_bao, _encoding);
        } catch (MessagingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        return out;
    public String getContentType() {
        return _mimeType;
    public String getName() {
        return "MyDataSource";
}Now, when I invoke the datasource, I call (shortened):
_dataSource = new MyDataSource(mimeType, "base64");
_outStream = _dataSource.getOutputStream();
/* write some data to the output stream here */
DataHandler dh = new DataHandler(_dataSource);
mbp.setDataHandler(dh);
_multiPart.addBodyPart(mbp);
_message.setContent(_multiPart);
_message.setHeader("X-Mailer", "HPDS");
_message.setSentDate(new Date());
_message.writeTo(System.out);
Transport.send(_message);This code was obviously cut down and won't compile, but you get the general idea. When I do this, and I look at the output from the 'writeTo', the data is base64 encoded, but the 'Content-Transfer-Encoding' is set to '7bit'. Now, here's the really wierd thing -- if I change the MyDataSource above to use "7bit" instead of "base64", I get encoded data (I believe that it's base64 encoded, but I'm not positive), and the 'Content-Transfer-Encoding' is set to 'base64'. It looks like the two cases are being swapped?
Has anyone seen something like this? If I set the 'Content-Transfer-Encoding' explicity by calling:
mbp.setHeader("Content-Transfer-Encoding", "base64");it works fine. Is this a bug? Or am I just misunderstanding where the 'Content-Transfer-Encoding' comes from.
TIA,
- Bill

Answer 1

OK, I figured it out. This isn't explicitly spelled out in the FAQ, but it turns out that Javamail performs the encoding for you, based on the "Content-Transfer-Encoding" header. I was trying to encode the data inside my datasource, and was getting twice-encoded data. I set the 'getOutputStream' in MyDataSource to not call MimeUtility.encode(), and set the C-T-E header before calling Transport.send(). Now everything is working fine.
- Bill