Add modular compilation test

This commit adds an extra compilation using
only `javac` and its multi-module compilation
feature in order to detect missing directives
in JUnit's module descriptors.

---------

Co-authored-by: Marc Philipp <[email protected]>

Author: sormuras

platform-tooling-support-tests/platform-tooling-support-tests.gradle.kts

@@ -1,9 +1,9 @@
 import com.gradle.develocity.agent.gradle.internal.test.TestDistributionConfigurationInternal
 import junitbuild.extensions.capitalized
 import junitbuild.extensions.dependencyProject
+import junitbuild.extensions.javaModuleName
 import net.ltgt.gradle.errorprone.errorprone
 import org.gradle.api.tasks.PathSensitivity.RELATIVE
-import org.gradle.jvm.toolchain.JvmVendorSpec.GRAAL_VM
 import org.gradle.kotlin.dsl.support.listFilesOrdered
 import java.time.Duration
 
@@ -45,6 +45,8 @@ val mavenDistributionClasspath = configurations.resolvable("mavenDistributionCla
 	extendsFrom(mavenDistribution.get())
 }
 
+val modularProjects: List<Project> by rootProject
+
 dependencies {
 	implementation(libs.commons.io) {
 		because("moving/deleting directory trees")
@@ -63,11 +65,17 @@ dependencies {
 	}
 	thirdPartyJars(libs.assertj)
 	thirdPartyJars(libs.apiguardian)
+	thirdPartyJars(libs.fastcsv)
 	thirdPartyJars(libs.hamcrest)
+	thirdPartyJars(libs.jimfs)
 	thirdPartyJars(libs.jspecify)
+	thirdPartyJars(kotlin("stdlib"))
+	thirdPartyJars(kotlin("reflect"))
+	thirdPartyJars(libs.kotlinx.coroutines)
 	thirdPartyJars(libs.opentest4j)
+	thirdPartyJars(libs.openTestReporting.events)
 	thirdPartyJars(libs.openTestReporting.tooling.spi)
-	thirdPartyJars(libs.jimfs)
+	thirdPartyJars(libs.picocli)
 
 	antJars(platform(projects.junitBom))
 	antJars(libs.bundles.ant)
@@ -130,7 +138,6 @@ val archUnit by testing.suites.registering(JvmTestSuite::class) {
 		}
 		implementation(libs.assertj)
 		runtimeOnly.bundle(libs.bundles.log4j)
-		val modularProjects: List<Project> by rootProject
 		modularProjects.forEach {
 			runtimeOnly(project(it.path))
 		}
@@ -207,6 +214,12 @@ val test by testing.suites.getting(JvmTestSuite::class) {
 				jvmArgumentProviders += JarPath(project, antJarsClasspath.get(), "antJars")
 				jvmArgumentProviders += MavenDistribution(project, unzipMavenDistribution, mavenDistributionDir)
 
+				systemProperty("junit.modules", modularProjects.map { it.javaModuleName }.joinToString(","))
+
+				jvmArgumentProviders += CommandLineArgumentProvider {
+					modularProjects.map { "-Djunit.moduleSourcePath.${it.javaModuleName}=${it.sourceSets["main"].allJava.sourceDirectories.filter { it.exists() }.asPath}" }
+				}
+
 				inputs.apply {
 					dir("projects").withPathSensitivity(RELATIVE)
 					file("${rootDir}/gradle.properties").withPathSensitivity(RELATIVE)

platform-tooling-support-tests/src/test/java/platform/tooling/support/tests/ModularCompilationTests.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package platform.tooling.support.tests;
+
+import static java.util.Objects.requireNonNull;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.junit.platform.tests.process.OutputFiles;
+
+import platform.tooling.support.ProcessStarters;
+import platform.tooling.support.ThirdPartyJars;
+
+class ModularCompilationTests {
+
+	@Test
+	void compileAllJUnitModules(@TempDir Path workspace, @FilePrefix("javac") OutputFiles javacOutputFiles)
+			throws Exception {
+		var lib = Files.createDirectories(workspace.resolve("lib"));
+		ThirdPartyJars.copyAll(lib);
+
+		var moduleNames = Arrays.asList(System.getProperty("junit.modules").split(","));
+
+		var outputDir = workspace.resolve("classes").toAbsolutePath();
+		var processStarter = ProcessStarters.javaCommand("javac") //
+				.workingDir(workspace) //
+				.addArguments("-d", outputDir.toString()) //
+				.addArguments("-Xlint:all", "-Werror") //
+				.addArguments("-Xlint:-requires-automatic,-requires-transitive-automatic") // JUnit 4
+				// external modules
+				.addArguments("--module-path", lib.toAbsolutePath().toString());
+
+		// source locations in module-specific form
+		moduleNames.forEach(
+			moduleName -> processStarter.addArguments("--module-source-path", moduleSourcePath(moduleName)));
+
+		var result = processStarter
+				// un-shadow
+				.addArguments("--add-modules", "info.picocli") //
+				.addArguments("--add-reads", "org.junit.platform.console=info.picocli") //
+				.addArguments("--add-modules", "org.opentest4j.reporting.events") //
+				.addArguments("--add-reads", "org.junit.platform.reporting=org.opentest4j.reporting.events") //
+				.addArguments("--add-modules", "de.siegmar.fastcsv") //
+				.addArguments("--add-reads", "org.junit.jupiter.params=de.siegmar.fastcsv")
+				// modules to compile
+				.addArguments("--module", String.join(",", moduleNames)) //
+				.redirectOutput(javacOutputFiles) //
+				.startAndWait();
+
+		assertEquals(0, result.exitCode());
+		assertThat(outputDir).isNotEmptyDirectory();
+	}
+
+	static String moduleSourcePath(String moduleName) {
+		return "%s=%s".formatted(moduleName,
+			requireNonNull(System.getProperty("junit.moduleSourcePath." + moduleName)));
+	}
+}

platform-tooling-support-tests/src/test/java/platform/tooling/support/tests/ModularUserGuideTests.java

@@ -156,7 +156,6 @@ void runTestsFromUserGuideWithinModularBoundaries(@TempDir Path temp,
 		assertLinesMatch(List.of( //
 			"destination", //
 			">> CLASSES AND JARS >>", //
-			"lib/opentest4j-.+\\.jar", //
 			"src", //
 			"src/documentation", //
 			"src/documentation/module-info.java" //