[WIP][SPARK-30194][BUILD][test-maven] Re-enable checkstyle for Java

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to prevent Java checkstyle violation.

Why are the changes needed?

Historically, we disabled this because there was no way to check this in PR Builder (9b98aef).
However, now we have GitHub Action which do this every PR.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Manually.

• https://github.com/apache/spark/pull/26817/files#r355571891

[srowen]

Sure, but, heh does it actually pass right now?

[dongjoon-hyun]

Yes. lint-java failure was fixed at the last commit. The following job also was recovered together.

• spark-master-test-sbt-hadoop-3.2/

[dongjoon-hyun]

Oh, let me check the build-error part once more.

[SparkQA]

Test build #115047 has finished for PR 26820 at commit 4b8d0f1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Retest this please.

[HeartSaVioR]

Btw, I'm feeling that it might be too strict to throw error on unused imports; as it could happen when multiple PRs modify the same file. Scala checkstyle doesn't throw error on unused imports, right?

[dongjoon-hyun]

Ya. That's a good point. Let me think about that, too.

[HyukjinKwon]

We enabled checkstyle in PR builder via #21399. So does this PR only apply when PR has the title [test-maven]?

[HyukjinKwon]

Alternatively, we can just completely switch linter-java to sbt-checkstyle completely so that we don't suffer from the differences anymore.

I have seen some differences so far, e.g. #26818 and #21801

[SparkQA]

Test build #115050 has finished for PR 26820 at commit 4b8d0f1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Hi, All. Thank you for review.
Originally, I made this PR because one Jenkins job died because of lint-java failure.
However, I'll drop this approach since this is more risky than I thought.

We had better remove the lint-java from that Jenkins job instead because GitHub Action is cheap and finished faster than Jenkins jobs.