how to generate files only if the spec is changed?

[kingluo]

Description

The maven plugin seems to re-generate files each time you re-run maven, no matter whether the input sepc file is changed (i.e. the last modification time is newer than any output files) or not, then it would make maven re-compile the generated files each time.

How to improve it? Just like what checkStaleness of protobuf-maven-plugin does.