0.2.1

[xuechaos]

Please do not create a Pull Request without creating an issue first.

What is the purpose of the change

publish 0.2.1 release version

Brief changelog

1. Update 0.2.1 and refactor some APIs.
2. Fix naming client throws exception when the service contains empty instance list.
3. fix 'mvn -B clean apache-rat:check findbugs:findbugs -e' not work.
4. client jdk support 1.6 and del unuse file.

Verifying this change

Follow this checklist to help us incorporate your contribution quickly and easily:

• [-] Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• [-] Format the pull request title like [ISSUE #123] Fix UnknownException when host config not exist. Each commit in the pull request should have a meaningful subject line and body.
• [-] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• [-] Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.
• [-] Run mvn -B clean apache-rat:check findbugs:findbugs to make sure basic checks pass. Run mvn -Prelease-nacos clean install -U to make sure unit-test pass. Run mvn clean package -Pit-test to make sure integration-test pass.

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
3 out of 5 committers have signed the CLA.

✅ mercyblitz
✅ hxy1991
✅ xuechaos
❌ nkorange
❌ water.lyl

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