[I] jdk9+ profile replaces a child's source and target with release 8 [maven-apache-parent]

["slachiewicz (via GitHub)" <gi...@apache.org>]

slachiewicz opened a new issue, #608:
URL: https://github.com/apache/maven-apache-parent/issues/608

   Since 32 (MPOM-264, #479) the `jdk9+` profile sets `maven.compiler.release` to `${javaVersion}`, which defaults to 8. maven-compiler-plugin gives `release` precedence over `source`/`target` (plexus-compiler emits `--release` whenever the value is non-empty), so a child POM that declares `maven.compiler.source` and `maven.compiler.target` but no `release`, which is every child written against a parent before 32, is compiled for Java 8 on JDK 9+ regardless of its own settings.
   
   Observed on flink-connector-parent moving from `org.apache:apache:20` to 39 with source 11 / target 17:
   
   ```
   [INFO] Compiling 130 source files with javac [debug release 8] to target/classes
   [ERROR] TransactionOwnership.java:[74,29] method orElseThrow in class java.util.Optional<T> cannot be applied to given types
   ```
   
   The compile error is the good outcome. A child whose sources happen to be Java 8 compatible is silently downgraded to bytecode 52 instead.
   
   Each available workaround has a cost:
   
   - Setting `javaVersion` fixes `release`, but it is a single value. A child with different source and target levels, or with per-profile targets, cannot express that through `--release`.
   - Blanking `maven.compiler.release` restores `-source`/`-target` for javac, but maven-javadoc-plugin fails on the empty string (apache/maven-javadoc-plugin issue linked below).
   - Overriding the compiler plugin's `release` parameter in the child works, but nothing points a user at it.
   
   A POM profile cannot see whether the child set `maven.compiler.source`, so the profile itself cannot be made conditional. What would help:
   
   1. Call this out in the release notes and on the parent's site: a child that sets `maven.compiler.source`/`target` must also set `javaVersion` or override `maven.compiler.release`, or its levels are ignored.
   2. Consider having the parent's enforcer execution fail when `maven.compiler.source` or `maven.compiler.target` differs from `javaVersion` on JDK 9+, so the mismatch fails loudly instead of downgrading silently.
   
   Context: [FLINK-40630](https://issues.apache.org/jira/browse/FLINK-40630).
   
   *This issue was created with AI assistance.*


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