Log all discovered configuration files to assist in problem resolution

Resolves #3898.

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/LauncherConfigurationParameters.java

@@ -10,6 +10,8 @@
 
 package org.junit.platform.launcher.core;
 
+import static java.util.stream.Collectors.joining;
+
 import java.io.InputStream;
 import java.net.URL;
 import java.net.URLConnection;
@@ -25,6 +27,7 @@
 import java.util.Properties;
 import java.util.Set;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import org.junit.platform.commons.logging.Logger;
 import org.junit.platform.commons.logging.LoggerFactory;
@@ -272,13 +275,21 @@ private static Properties loadClasspathResource(String configFileName) {
 			Set<URL> resources = new LinkedHashSet<>(Collections.list(classLoader.getResources(configFileName)));
 
 			if (!resources.isEmpty()) {
+
+				URL configFileUrl = CollectionUtils.getFirstElement(resources).get();
+
 				if (resources.size() > 1) {
-					logger.warn(() -> String.format(
-						"Discovered %d '%s' configuration files in the classpath; only the first will be used.",
-						resources.size(), configFileName));
+					logger.warn(() -> {
+						String formattedResourceList = Stream.concat( //
+							Stream.of(configFileUrl + " (*)"), //
+							resources.stream().skip(1).map(URL::toString) //
+						).collect(joining("\n- ", "\n- ", ""));
+						return String.format(
+							"Discovered %d '%s' configuration files on the classpath (see below); only the first (*) will be used.%s",
+							resources.size(), configFileName, formattedResourceList);
+					});
 				}
 
-				URL configFileUrl = resources.iterator().next(); // same as List#get(0)
 				logger.config(() -> String.format(
 					"Loading JUnit Platform configuration parameters from classpath resource [%s].", configFileUrl));
 				URLConnection urlConnection = configFileUrl.openConnection();

platform-tests/src/test/java/org/junit/platform/launcher/core/LauncherConfigurationParametersTests.java

@@ -14,16 +14,27 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
 import java.util.Map;
+import java.util.Properties;
+import java.util.logging.Level;
+import java.util.logging.LogRecord;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.TestInstancePostProcessor;
+import org.junit.jupiter.api.fixtures.TrackLogRecords;
+import org.junit.jupiter.api.io.TempDir;
 import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.PreconditionViolationException;
+import org.junit.platform.commons.logging.LogRecordListener;
 import org.junit.platform.engine.ConfigurationParameters;
 import org.junit.platform.engine.discovery.DiscoverySelectors;
 import org.junit.platform.launcher.listeners.SummaryGeneratingListener;
@@ -190,6 +201,40 @@ void ignoresSystemPropertyAndConfigFileWhenImplicitLookupsAreDisabled() {
 		assertThat(configParams.get(KEY)).isEmpty();
 	}
 
+	@Test
+	void warnsOnMultiplePropertyResources(@TempDir Path tempDir, @TrackLogRecords LogRecordListener logRecordListener)
+			throws Exception {
+		Properties properties = new Properties();
+		properties.setProperty(KEY, "from second config file");
+		try (var out = Files.newOutputStream(tempDir.resolve(CONFIG_FILE_NAME))) {
+			properties.store(out, "");
+		}
+		ClassLoader originalClassLoader = Thread.currentThread().getContextClassLoader();
+		try (var customClassLoader = new URLClassLoader(new URL[] { tempDir.toUri().toURL() }, originalClassLoader)) {
+			Thread.currentThread().setContextClassLoader(customClassLoader);
+			ConfigurationParameters configParams = fromMapAndFile(Map.of(), CONFIG_FILE_NAME);
+
+			assertThat(configParams.get(KEY)).contains(CONFIG_FILE);
+
+			List<String> loggedWarnings = logRecordListener.stream(Level.WARNING) //
+					.map(LogRecord::getMessage) //
+					.toList();
+			assertThat(loggedWarnings) //
+					.hasSize(1);
+			assertThat(loggedWarnings.getFirst()) //
+					.contains("Discovered 2 '" + CONFIG_FILE_NAME
+							+ "' configuration files on the classpath (see below); only the first (*) will be used.") //
+					.contains("- "
+							+ Path.of(
+								"build/resources/test/test-junit-platform.properties").toAbsolutePath().toUri().toURL()
+							+ " (*)") //
+					.contains("- " + tempDir.resolve(CONFIG_FILE_NAME).toUri().toURL());
+		}
+		finally {
+			Thread.currentThread().setContextClassLoader(originalClassLoader);
+		}
+	}
+
 	private static LauncherConfigurationParameters fromMap(Map<String, String> map) {
 		return LauncherConfigurationParameters.builder().explicitParameters(map).build();
 	}