Apply p3c pmd plugin/rules and fix/ignore all violations of priority 1

[blindpirate]

Describe what this PR does / why we need it

This PR applies alibaba p3c pmd plugin/rules and fixes all violations of priority 1.

Does this pull request fix one issue?

Fixes #573

Describe how you did it

Just fix all violations.

Describe how to verify it

mvn clean install -DskipTests -DminimumPriority=1

Special notes for reviews

[codecov-io]

Codecov Report

Merging #574 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #574      +/-   ##
============================================
+ Coverage     38.74%   39.16%   +0.41%     
- Complexity     1216     1225       +9     
============================================
  Files           275      275              
  Lines          8702     8702              
  Branches       1163     1163              
============================================
+ Hits           3372     3408      +36     
+ Misses         4900     4853      -47     
- Partials        430      441      +11

Impacted Files	Coverage Δ	Complexity Δ
.../sentinel/cluster/client/NettyTransportClient.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...inel/cluster/server/connection/ConnectionPool.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...a/csp/sentinel/slots/system/SystemRuleManager.java	43.79% <ø> (ø)	8 <0> (ø)	⬇️
.../csp/sentinel/slots/block/degrade/DegradeRule.java	55.55% <ø> (ø)	17 <0> (ø)	⬇️
...csp/sentinel/slots/block/flow/FlowRuleManager.java	60.86% <ø> (ø)	7 <0> (ø)	⬇️
...a/csp/sentinel/slots/statistic/base/LongAdder.java	31.91% <0%> (+14.89%)	10% <0%> (+6%)	⬆️
...a/csp/sentinel/slots/statistic/base/Striped64.java	56.25% <0%> (+30.2%)	8% <0%> (+3%)	⬆️

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[blindpirate]

Seems like some tests are behaving flaky: WarmUpRateLimiterControllerTest and OccupiableBucketLeapArrayTest. With nearly same code, 1st build and 2nd build fail but 3rd build succeeds.

Are there any known issues? Please let me know if you need me to investigate.

[sczyh30]

Seems like some tests are behaving flaky: WarmUpRateLimiterControllerTest and OccupiableBucketLeapArrayTest. With nearly same code, 1st build and 2nd build fail but 3rd build succeeds.

Are there any known issues? Please let me know if you need me to investigate.

It's a known issue that some sleep-based test cases are not stable. See #541 for discussion.

[blindpirate]

Thanks! @sczyh30

[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!

[blindpirate]

Thanks!