Is there a standard mapping between JSON and Protocol Buffers?
From what I have seen, Protostuff is the project to use for any PB work on Java, including serializing it as JSON, based on protocol definition. I have not used it myself, just heard good things.
Yes, since Protocol Buffers version 3.0.0 (released July 28, 2016) thereis "A well-defined encoding in JSON as an alternative to binary protoencoding" as mentioned in the release notes