Note: with Maven 4.0.0-rc-3, the tests of the Maven compiler plugin fail with the following stack trace:

java.lang.NullPointerException: Cannot invoke "org.apache.maven.api.services.Lookup.lookupOptional(java.lang.Class)" because the return value of "org.apache.maven.api.Session.getService(java.lang.Class)" is null
	at org.apache.maven.impl.DefaultToolchainManager.retrieveContext(DefaultToolchainManager.java:120)
	at org.apache.maven.impl.DefaultToolchainManager.getToolchainFromBuildContext(DefaultToolchainManager.java:90)
	at org.apache.maven.plugin.compiler.AbstractCompilerMojo.getToolchain(AbstractCompilerMojo.java:1530)
	at org.apache.maven.plugin.compiler.AbstractCompilerMojo.compiler(AbstractCompilerMojo.java:1046)
	at org.apache.maven.plugin.compiler.AbstractCompilerMojo.execute(AbstractCompilerMojo.java:1000)
	at org.apache.maven.plugin.compiler.CompilerMojo.execute(CompilerMojo.java:157)
	at org.apache.maven.plugin.compiler.CompilerMojoTestCase.testCompileSkipTest(CompilerMojoTestCase.java:371)

I do not know how Mockito works (I do not use it myself), but I suspect that the following method of maven-plugin-testing-harness needs modification:

org.apache.maven.api.plugin.testing.stubs.SessionMock.getMockSession(LocalRepository)

The current implementation seems to define the behaviour of getService(Class) for various classes (RepositoryFactory, VersionParser, LocalRepositoryManager, ArtifactInstaller, ArtifactDeployer, ArtifactManager, ProjectManager, ArtifactFactory, ProjectBuilder, ModelXmlFactory), but I see no setting for Lookup.

I don't think it's really a good idea to use the session stub anyway. There's too much needed and a simple stub will almost never work. We can check, but I don't think any plugins we've migrated uses it. So maybe remove it.

Empty lists are fine for project and artifact stubs imho. Or use a fields and a setter to make those easier to use, that may be more reusable.

I can remove the JUnit tests of maven-compiler-plugin that depend on the session stub. It would remove 3 of the 14 tests (not necessarily a big lost, since I find the IT tests more useful anyway). A related question is whether we should also remove the session stub completely from maven-plugin-testing.

Just realized that this is a duplicated of #81, #83 and #67.

Added setter methods for usability. Also added an empty stub for Session.getService(Lookup.class). With the latter addition, the NullPointerException observed in the JUnit tests of maven-compiler-plugin is resolved, and there is no need anymore to delete some tests.

It would be good to modify gha to run tests also with Maven 4. So far all compilation and tests where under Maven 3.