[Java] maven plugin to clean-up output before generation

[borsch]

#14928

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
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• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date.sh
  

  Commit all changed files.
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@wing328 (2015/07) ❤️
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[wing328]

The CI tests (maven plugin) failed. Can you please take a look when you've time?

[borsch]

@wing328 issue was due to this change 564939a
Now jersey2 use diamond operator here 564939a#diff-e9ec55c3c14a0cce87cfa478732f202228aa39c395d1bae2c2f9dd7ea5074175R100 which is not supported inside maven plugin samples because java 1.7 was configured there

Is there a way to trigger maven/gradle plugin build once related libraries are updated? Like in this case to trigger jersey2 sample generation with maven plugin

[wing328]

Is there a way to trigger maven/gradle plugin build once related libraries are updated? Like in this case to trigger jersey2 sample generation with maven plugin

it's possible. i'll see what i can do after merging this PR.