Fix #2520: default maven.mainClass in Java instead of classworlds config

[gnodet]

Summary

Fixes #2520 — Maven Wrapper (and IDEs like IntelliJ) broken with Maven 4.0.0-beta-5+ because maven.mainClass is not set when launching through external tools.

Root cause: m2.conf used main is ${maven.mainClass} which required classworlds property resolution. When external tools bypassed the mvn scripts, -Dmaven.mainClass was never set, causing ConfigurationException: No such property: maven.mainClass. The previous fix (PR #10998, rc-5) added set maven.mainClass default in m2.conf, but this kept the defaulting in classworlds config rather than Java — as noted by @gnodet in the issue discussion.

Fix: Move the main class defaulting from classworlds property resolution to Java code:

• m2.conf: Hardcode main is org.apache.maven.cling.MavenCling from plexus.core — no variable resolution needed, no possibility of classworlds ConfigurationException
• MavenCling.java: Check maven.mainClass system property in the ClassWorld Launcher entry point and delegate to the specified class via reflection when set (e.g., MavenEncCling, MavenShellCling, MavenUpCling)

This makes MavenCling the Java-level default entry point that all external tools can rely on without any special configuration.

Behavior

Scenario	Before	After
mvn script (normal)	-Dmaven.mainClass=MavenCling → classworlds resolves	-Dmaven.mainClass=MavenCling → classworlds calls MavenCling → no dispatch
mvn --enc	-Dmaven.mainClass=MavenEncCling → classworlds resolves	-Dmaven.mainClass=MavenEncCling → MavenCling delegates to MavenEncCling
Maven Wrapper (no -Dmaven.mainClass)	m2.conf default resolves	classworlds calls MavenCling directly — no resolution needed
IDE (no -Dmaven.mainClass)	m2.conf default resolves	classworlds calls MavenCling directly — no resolution needed

Test plan

• New MavenClingTest with 3 tests: delegation to valid class, unknown class error, missing method error
• All new tests pass
• mvn verify -pl impl/maven-cli -DskipTests — build + format checks pass
• Pre-existing test failures are unrelated (MimirInfuser version mismatch)

🤖 Generated with Claude Code

[gnodet]

Clean, well-motivated fix that moves the maven.mainClass dispatch from classworlds property resolution to Java-level reflection in MavenCling. The approach eliminates the root cause of Maven Wrapper and IDE failures and follows the design suggested in the original issue discussion.

All four known delegation targets (MavenCling, MavenEncCling, MavenShellCling, MavenUpCling) have the expected public static int main(String[], ClassWorld) signature, and the error handling in delegateMain is thorough.

The PR description and behavior matrix are excellent. The approach of hardcoding MavenCling in m2.conf and delegating in Java is strictly more robust than the previous classworlds-level default (PR #10998), since it eliminates classworlds property resolution from the critical path entirely.

Minor observations:

• The instanceof checks in delegateMain use old-style casts rather than pattern matching instanceof, which is used elsewhere in the codebase (Java 17 target). Minor style inconsistency.
• Tests cover delegation scenarios well but miss the two no-delegation paths: (1) when maven.mainClass is not set (null), and (2) when it is set to MavenCling itself. While these paths are simple, explicit tests would document the contract.
• Tests mutate the global maven.mainClass system property — the try/finally cleanup is correct, but this could cause flaky failures if tests run in parallel within the same JVM.

📋 PR Metadata

Aspect	Current	Suggested
Labels	(none)	bug

This review was generated by an AI agent and may contain inaccuracies. Please verify all suggestions before applying.

Claude Code on behalf of Guillaume Nodet

[elharo]

I don't know the syntax here but is this right? Is this supposed to be a comment?