XD-3703 : Add SSL and attachments to mail sink

[fmarchand]

I have signed and agree to the terms of the SpringSource Individual
Contributor License Agreement.

This PR takes in consideration XD-2076, XD-2498 but needs discussion (test coverage is not perfect and I have some doubts on where to put packages and classes).

[garyrussell]

@fmarchand

First of all; thanks for the contribution - our sincere apologies for it taking so long to look at it; I have some initial observations:

1. The spring standard is to use leading tabs, not spaces

2. Curly braces are always required:

   if(attachmentFilename.contains(";"))
       attachmentFilenames = StringUtils.split(attachmentFilename, ";");
   else attachmentFilenames = new String[] { attachmentFilename };
   

   should be

     if(attachmentFilename.contains(";")) {
         attachmentFilenames = StringUtils.split(attachmentFilename, ";");
     }
     else {
         attachmentFilenames = new String[] { attachmentFilename };
     }
   

3. With try {...} catch {...} the catch should start on a new line.

4. MailTransformer: Using a <transformer/> in a sink that routes to nullChannel is not appropriate. In fact, this "transformer" transforms nothing; it returns the message unchanged. If we were to use it this way, the class should be called something like MailSender; the method should return void and be invoked using a <service-activator/>. However, I think it should be a Transformer but return the assembled MimeMessage which we then route to the standard Spring Integration MailSendingMesageHandler - <int-mail:outbound-channel-adapter/> (which you removed).

5. I like the idea of supporting multiple attachments (the standard Spring Integration MailSendingMessageHandler only supports one). However, I don't know how useful it is to only support getting the attachments from the file system in an XD environment. We should support other mechanisms for the attachments (such as a Map payload).

A fully featured transformer like this would certainly be a candidate to be promoted to Spring Integration itself (or at least add the logic directly into the conversion logic within the adapter).

That's all for this pass - thanks again for the contribution.

[fmarchand]

@garyrussell

That's fine :) I guess you have a lot of work with spring-cloud-data-flow.

First : thanks for your review !

I'm gonna fix what you mentioned : remark 1 to 3. I changed my IDE and I guess I forgot to use the right formater and code cleaner.

For the point 4. You're right : a transformer that routes to nullChannel is a bit weird. I'll fix it too if you want, with a transformer that will return a MimeMessage to a <int-mail:outbound-channel-adapter/>

For the point 5. I need to think a bit more about it :)

For the last thing you said, when this PR will be finished, I can propose a pull request to Spring Integration base on this contribution.

Thx again Gary for your review.