Hey,
I have created a cookiecutter template to ease the writing of own processors. Cookiecutter is a templating tool written in python. Using this all you need to do is to run
cookiecutter gh:spinscale/cookiecutter-elasticsearch-ingest-processor
and answer four questions, and you get the basic java classes, plus test classes and sample rest tests in your projects.
Here is the github project.
Hope it helps a bit.
--Alex