set "start" attribute in content-type for mime multipart/related message in javax.mail api

Question

How can I set start attribute in the content-type for mime multipart/related message? To be more precise, I want to know how I can set the start attribute for the root attachment in the below example taken from https://www.rfc-editor.org/rfc/rfc6362:

--OUTER-BOUNDARY
Content-type: multipart/related; boundary="INNER-BOUNDARY";
   start="<root.attachment>"; type="application/xml"

--INNER-BOUNDARY
Content-type: application/xml
Content-ID: <root.attachment>

[XML DOCUMENT]

--INNER-BOUNDARY
Content-type: application/pdf
Content-ID: <2nd.attachment>

[PDF DOCUMENT]

--INNER-BOUNDARY--

--OUTER-BOUNDARY

I am not able to find it in the javax.mail api. Please help.

Answer 1

I've been struggling with that recently, the code below is the best I could come up with (actually nothing else worked for me):

    public class MultipartGenerator {

      //Let's assume the static members below 
      //hold our message parts content 
      //an the instances of arrays of byte

      private static final byte [] ROOT_BYTES = new byte[]{/* ... bytes ... */};

      private static final byte [] ATTCH_1_BYTES = new byte[]{/* ... bytes ... */};

      private static final byte [] ATTCH_2_BYTES = new byte[]{/* ... bytes ... */};

      /**
       * Generate multipart with headers
       *
       * @return javax.mail.Multipart instance
       */
       public static Multipart generateMultipart() {

         //This is our root MimeBodyPart, 
         //content-id equals 'rootcid'
         //content-type equals 'roottype/rootsubtype'

         InternetHeaders ih0 = new InternetHeaders();
         ih0.addHeader("Content-Type", "roottype/rootsubtype");
         ih0.addHeader("Content-Transfer-Encoding", "binary");
         ih0.addHeader("Content-ID", "rootcid");
         MimeBodyPart rootBodyPart = new MimeBodyPart(ih0, ROOT_BYTES);

         //This is a body part wrapping first message attachment
         InternetHeaders ih1 = new InternetHeaders();
         ih1.addHeader("Content-Type", "text/plain; name=attachment1.txt");
         ih1.addHeader("Content-Transfer-Encoding", "binary");
         ih1.addHeader("Content-Location", "attachment1.txt");
         ih1.addHeader("Content-ID", "a00");
         MimeBodyPart attch1BodyPart = new MimeBodyPart(ih1, ATTCH_1_BYTES);

         //This is a body part wrapping second message attachment
         InternetHeaders ih2 = new InternetHeaders();
         ih2.addHeader("Content-Type", "text/plain; name=attachment2.txt");
         ih2.addHeader("Content-Transfer-Encoding", "binary");
         ih2.addHeader("Content-Location", "attachment2.txt");
         ih2.addHeader("Content-ID", "a01");        
         MimeBodyPart attch2BodyPart = new MimeBodyPart(ih2, ATTCH_2_BYTES);

         //This is our desired multipart, this is where things turn a bit dirty
         //No success with setting the parameters in a different way

         Multipart multipart = new MimeMultipart("related;start=\"<rootcid>\";type=\"roottype/rootsubtype\"");

         multipart.addBodyPart(rootBodyPart,0);
         multipart.addBodyPart(attch1BodyPart);
         multipart.addBodyPart(attch2BodyPart);

         return multipart;
       }
    }

There's probably a better way to handle this task, however I am not able to find one.