To be honest, I would recommend making those changes directly to the existing PublishAMQP processor, as unless I am missing something, that does not seem like a breaking change that would affect other users negatively. I’d suggest opening a Jira for the feature, cloning the GitHub repository (see the instructions in the Contributor Guide [1]), making the changes in your branch, building and testing, and then submitting a PR (or patch) for review and inclusion in the project. This will also require the least amount of code manipulation (example of a commit from Andre adding EL evaluation to a property descriptor [2]). 

If you do not wish to do that, my next recommendation would be to clone the project and create a new processor in the same Maven module (and add it to the MANIFEST) alongside PublishAMQP (call it PublishAMQPWithEL or something) and extend the existing PublishAMQP class and override just the specific behavior you are interested in. 

A possible, but in my opinion unnecessary, process would be to add a completely new NAR with the existing nifi-amqp-bundle/nifi-amqp-processors artifact listed as a dependency and perform your extension in your own NAR. 

Hope this helps. 

[1] https://cwiki.apache.org/confluence/display/NIFI/Contributor+Guide
[2] https://github.com/apache/nifi/commit/84f1fb395963614a2ff78a615c92fc79a52e2942

Andy LoPresto
PGP Fingerprint: 70EC B3E5 98A6 5A3F D3C4  BACE 3C6E F65B 2F7D EF69

On Dec 2, 2017, at 3:43 PM, James McMahon <jsmcmahon3@gmail.com> wrote:

Good evening. I am familiar with NiFi and have developed a number of
workflows using out -of-the-box processors. I now need to develop a custom
version of PublishAMQP that permits the use of attributes in expression
language to set specs for the AMQP target.

I am not very familiar with Maven, but have done some initial research and
hope I can bootstrap my effort following this link:

Lacking any experience developing custom processors, I'd like to understand
how I would establish in my maven project the source code for the existing
PublishAMQP. I'd like to modify that proven code base to allow the use of
expression language to set other configuration properties.

Can anyone help me with an example where source code for an existing
processor is brought in from a github resource or the like to use as my
starting point?

Thank you in advance for any assistance. -Jim