Make ReflectionSupport.findNestedClasses thread-safe

ReflectionSupport.findNestedClasses(..)` is now thread-safe with regard
to cycle detection.

Closes #2715

Author: sbrannen

documentation/src/docs/asciidoc/release-notes/release-notes-5.8.1.adoc

@@ -23,7 +23,8 @@ GitHub.
 
 ==== New Features and Improvements
 
-* ❓
+* `ReflectionSupport.findNestedClasses(..)` is now thread-safe with regard to cycle
+  detection.
 
 
 [[release-notes-5.8.1-junit-jupiter]]

junit-platform-commons/src/main/java/org/junit/platform/commons/util/ReflectionUtils.java

@@ -41,13 +41,13 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.IdentityHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Predicate;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -130,7 +130,7 @@ public enum HierarchyTraversalMode {
 	 * @since 1.6
 	 * @see #detectInnerClassCycle(Class)
 	 */
-	private static final Set<String> noCyclesDetectedCache = new HashSet<>();
+	private static final Set<String> noCyclesDetectedCache = ConcurrentHashMap.newKeySet();
 
 	/**
 	 * Internal cache of common class names mapped to their types.

platform-tests/src/test/java/org/junit/platform/commons/util/ReflectionUtilsTests.java

@@ -45,6 +45,7 @@
 import java.util.function.Predicate;
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
+import java.util.stream.Stream;
 
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
@@ -787,11 +788,14 @@ void findNestedClassesWithSeeminglyRecursiveHierarchies() {
 	 */
 	@Test
 	void findNestedClassesWithRecursiveHierarchies() {
-		assertNestedCycle(OuterClass.class, InnerClass.class, OuterClass.class);
-		assertNestedCycle(StaticNestedClass.class, InnerClass.class, OuterClass.class);
-		assertNestedCycle(RecursiveInnerClass.class, OuterClass.class);
-		assertNestedCycle(RecursiveInnerInnerClass.class, OuterClass.class);
-		assertNestedCycle(InnerClass.class, RecursiveInnerInnerClass.class, OuterClass.class);
+		Runnable runnable1 = () -> assertNestedCycle(OuterClass.class, InnerClass.class, OuterClass.class);
+		Runnable runnable2 = () -> assertNestedCycle(StaticNestedClass.class, InnerClass.class, OuterClass.class);
+		Runnable runnable3 = () -> assertNestedCycle(RecursiveInnerClass.class, OuterClass.class);
+		Runnable runnable4 = () -> assertNestedCycle(RecursiveInnerInnerClass.class, OuterClass.class);
+		Runnable runnable5 = () -> assertNestedCycle(InnerClass.class, RecursiveInnerInnerClass.class,
+			OuterClass.class);
+		Stream.of(runnable1, runnable1, runnable1, runnable2, runnable2, runnable2, runnable3, runnable3, runnable3,
+			runnable4, runnable4, runnable4, runnable5, runnable5, runnable5).parallel().forEach(Runnable::run);
 	}
 
 	private static List<Class<?>> findNestedClasses(Class<?> clazz) {