I just did a quick search through the REST API docs (https://docs.atlassian.com/atlassian-bamboo/REST/6.4.1/) and it doesn't look like there is anything for retriving a plan as java specs.
I can see how that could be challenging to accomplish given that would be a pretty complex object that gets returned.
Sorry I couldn't offer much help here.
Thanks @Jimmy Seddon for looking into this.
Bamboo UI offers the following URL to show the Java Specs. But unfortunately is in HTML format:
I was hoping to find an underlying REST call but there isn't any.
The main reason I was looking for this API was being able to verify encrypted secret variables. I deal with dozens of Bamboo plans which have encrypted variable. The only way to verify if those variables have correct value is by looking at Specs. Specs show the encrypted value but UI masks it.
I was planning to make multiple REST calls, get all the Specs and and verify variables in all plans.