Set maven.compiler flags explicitly

[ankitsultana]

Ran into another issue while doing 1.2.0 Release.

What was the error?

The error came on running release:prepare. There were a bunch of errors like the following. These were coming from the. maven-javadoc-plugin, which for some reason was using source Java version as 7:

[INFO] [ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.7.0:jar (attach-javadocs) on project pinot-spi: MavenReportException: Error while generating Javadoc:
[INFO] [ERROR] Exit code: 1
[INFO] [ERROR] /root/pinot-release/pinot/target/checkout/pinot-spi/src/main/java/org/apache/pinot/spi/annotations/metrics/PinotMetricsFactory.java:78: error: lambda expressions are not supported in -source 7
[INFO] [ERROR]       return () -> "noopMetricName";
[INFO] [ERROR]                 ^
[INFO] [ERROR]   (use -source 8 or higher to enable lambda expressions)
...

Why is the Javadoc Plugin using source version as 7

This is because maven-javadoc-plugin is apparently now using the "release" flag for JDK 9 and above. This flag was introduced in maven-javadoc-plugin:3.1.0 (ref). This is by default set to maven.compiler.release, which is set to 7 in Apache's POM (ref)

Why didn't this happen with Pinot 1.1.0 Release?

This is likely because of a version upgrade of maven-javadoc-plugin.

How do we know this change actually fixes the issue?

Verified by cloning the release-1.2.0-rc tag, and running the following. The command fails but when I use the patch in this PR, it works:

$ mvn javadoc:javadoc -pl pinot-spi

[xiangfu0]

Thanks for fixing this!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.01%. Comparing base (59551e4) to head (0bbda16).
Report is 808 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #13710      +/-   ##
============================================
+ Coverage     61.75%   62.01%   +0.25%     
+ Complexity      207      198       -9     
============================================
  Files          2436     2554     +118     
  Lines        133233   140611    +7378     
  Branches      20636    21874    +1238     
============================================
+ Hits          82274    87193    +4919     
- Misses        44911    46789    +1878     
- Partials       6048     6629     +581     

Flag	Coverage Δ
custom-integration1	<0.01% <ø> (-0.01%)	⬇️
integration	<0.01% <ø> (-0.01%)	⬇️
integration1	<0.01% <ø> (-0.01%)	⬇️
integration2	0.00% <ø> (ø)
java-11	35.11% <ø> (-26.60%)	⬇️
java-21	61.90% <ø> (+0.27%)	⬆️
skip-bytebuffers-false	61.99% <ø> (+0.24%)	⬆️
skip-bytebuffers-true	61.86% <ø> (+34.13%)	⬆️
temurin	62.01% <ø> (+0.25%)	⬆️
unittests	62.00% <ø> (+0.25%)	⬆️
unittests1	46.44% <ø> (-0.45%)	⬇️
unittests2	27.78% <ø> (+0.05%)	⬆️

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