jdk9+ profile replaces a child's source and target with release 8

[slachiewicz]

Since 36 (#543, #551) the jdk9+ profile sets maven.compiler.release to ${javaVersion}, which defaults to 8. Parents 32 to 35 derived it from maven.compiler.target instead, so overriding target in a child still took effect there. 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 (Empty release value fails with StringIndexOutOfBoundsException maven-javadoc-plugin#1376).
• 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.

This issue was created with AI assistance.