JavaMail: get size of a MimeMessage

Question

I'm trying to get the size of a MimeMessage. The method getSize() simply always returns -1.

This is my code:

MimeMessage m = new MimeMessage(session);
m.setFrom(new InternetAddress(fromAddress, true));
m.setRecipient(RecipientType.TO, new InternetAddress(toAddress, true));
m.setSubject(subject);

MimeBodyPart bodyPart = new MimeBodyPart();
bodyPart.setContent(body, "text/html");
Multipart mp = new MimeMultipart();
mp.addBodyPart(bodyPart);
m.setContent(mp);

m.getSize(); // -1 is returned

THIS IS THE ANSWER TO MY QUESTION:

ByteArrayOutputStream os = new ByteArrayOutputStream();
m.writeTo(os);
int bytes = os.size();

Answer 1

A more efficient solution, but requiring an external library, is the following one:

public static long getReliableSize(MimeMessage m) throws IOException, MessagingException {
    try (CountingOutputStream out = new CountingOutputStream(new NullOutputStream())) {
        m.writeTo(out);
        return out.getByteCount();
    }
}

Both CountingOutputStream and NullOutputStream are available in Apache Common IO. That solution doesn't require to work with a temporary byte buffer (write, allocate, re-allocate, etc.)