Ensure JUnit Jupiter test classes cannot be private

Prior to this commit, if a test class was discovered via package, class
path, or module path scanning, it could NOT be private; however, if the
same test class was selected explicitly (i.e., via one of the
`selectClass()` variants in `DiscoverySelectors`, via the `aClass` or
`classes` element of the `selectors` configuration for the
`junitPlatform` Gradle plugin, or via the `-select-class` or `-c`
command-line options for the `ConsoleLauncher`) it COULD then be
private.

This behavior is inconsistent and dangerous: although a developer could
execute such a test class in an IDE, the same test classes would never
be executed with a typical build configuration that relies on class
path or module path scanning.

This commit ensures that JUnit Jupiter test classes can never be
private, regardless of how they are discovered or selected.

In addition, this commit removes the now obsolete IsScannableTestClass
predicate since scanned test classes are no longer handled differently.

Fixes: #1252

documentation/src/docs/asciidoc/release-notes/release-notes-5.1.0-M2.adoc

@@ -77,7 +77,11 @@ _@API Guardian_ JAR _mandatory_ again.
 
 ==== Bug Fixes
 
-* ❓
+* Test classes selected via one of the `selectClass()` variants in `DiscoverySelectors`,
+  via the `aClass` or `classes` element of the `selectors` configuration for the
+  `junitPlatform` Gradle plugin, or via the `-select-class` or `-c` command-line options
+  for the `ConsoleLauncher` are no longer allowed to be `private`. This aligns with the
+  behavior for test classes discovered via package, class path, and module path scanning.
 
 ==== Deprecations and Breaking Changes
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/DiscoverySelectorResolver.java

@@ -20,7 +20,7 @@
 import java.util.Set;
 
 import org.apiguardian.api.API;
-import org.junit.jupiter.engine.discovery.predicates.IsScannableTestClass;
+import org.junit.jupiter.engine.discovery.predicates.IsTestClassWithTests;
 import org.junit.platform.commons.util.ClassFilter;
 import org.junit.platform.engine.EngineDiscoveryRequest;
 import org.junit.platform.engine.TestDescriptor;
@@ -43,10 +43,10 @@
 @API(status = INTERNAL, since = "5.0")
 public class DiscoverySelectorResolver {
 
-	private static final IsScannableTestClass isScannableTestClass = new IsScannableTestClass();
+	private static final IsTestClassWithTests isTestClassWithTests = new IsTestClassWithTests();
 
 	public void resolveSelectors(EngineDiscoveryRequest request, TestDescriptor engineDescriptor) {
-		ClassFilter classFilter = buildClassFilter(request, isScannableTestClass);
+		ClassFilter classFilter = buildClassFilter(request, isTestClassWithTests);
 		resolve(request, engineDescriptor, classFilter);
 		filter(engineDescriptor, classFilter);
 		pruneTree(engineDescriptor);

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/predicates/IsPotentialTestContainer.java

@@ -12,6 +12,7 @@
 
 import static org.apiguardian.api.API.Status.INTERNAL;
 import static org.junit.platform.commons.util.ReflectionUtils.isAbstract;
+import static org.junit.platform.commons.util.ReflectionUtils.isPrivate;
 import static org.junit.platform.commons.util.ReflectionUtils.isStatic;
 
 import java.util.function.Predicate;
@@ -29,7 +30,10 @@ public class IsPotentialTestContainer implements Predicate<Class<?>> {
 
 	@Override
 	public boolean test(Class<?> candidate) {
-		//please do not collapse into single return
+		// Please do not collapse the following into a single statement.
+		if (isPrivate(candidate)) {
+			return false;
+		}
 		if (isAbstract(candidate)) {
 			return false;
 		}

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/predicates/IsTestClassWithTests.java

@@ -10,18 +10,22 @@
 
 package org.junit.jupiter.engine.discovery.predicates;
 
+import static org.apiguardian.api.API.Status.INTERNAL;
+
 import java.lang.reflect.Method;
 import java.util.function.Predicate;
 
+import org.apiguardian.api.API;
 import org.junit.platform.commons.util.ReflectionUtils;
 
 /**
  * Test if a class is a JUnit Jupiter test class containing executable tests,
- * test factories, or nested tests.
+ * test factories, test templates, or nested tests.
  *
  * @since 5.0
  */
-class IsTestClassWithTests implements Predicate<Class<?>> {
+@API(status = INTERNAL, since = "5.1")
+public class IsTestClassWithTests implements Predicate<Class<?>> {
 
 	private static final IsTestMethod isTestMethod = new IsTestMethod();
 
@@ -38,11 +42,8 @@ class IsTestClassWithTests implements Predicate<Class<?>> {
 
 	@Override
 	public boolean test(Class<?> candidate) {
-		// please do not collapse into single return
-		if (!isPotentialTestContainer.test(candidate)) {
-			return false;
-		}
-		return hasTestOrTestFactoryOrTestTemplateMethods(candidate) || hasNestedTests(candidate);
+		return isPotentialTestContainer.test(candidate)
+				&& (hasTestOrTestFactoryOrTestTemplateMethods(candidate) || hasNestedTests(candidate));
 	}
 
 	private boolean hasTestOrTestFactoryOrTestTemplateMethods(Class<?> candidate) {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/BeforeAllAndAfterAllComposedAnnotationTests.java

@@ -43,7 +43,7 @@ void beforeAllAndAfterAllAsMetaAnnotations() {
 		assertEquals(asList("beforeAll", "test", "afterAll"), methodsInvoked);
 	}
 
-	private static class TestCase {
+	static class TestCase {
 
 		@CustomBeforeAll
 		static void beforeAll() {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/BeforeEachAndAfterEachComposedAnnotationTests.java

@@ -43,7 +43,7 @@ void beforeEachAndAfterEachAsMetaAnnotations() {
 		assertEquals(asList("beforeEach", "test", "afterEach"), methodsInvoked);
 	}
 
-	private static class TestCase {
+	static class TestCase {
 
 		@CustomBeforeEach
 		void beforeEach() {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/DisabledTests.java

@@ -53,15 +53,15 @@ void executeTestsWithDisabledTestMethods() throws Exception {
 	// -------------------------------------------------------------------
 
 	@Disabled
-	private static class DisabledTestClassTestCase {
+	static class DisabledTestClassTestCase {
 
 		@Test
 		void disabledTest() {
 			fail("this should be @Disabled");
 		}
 	}
 
-	private static class DisabledTestMethodsTestCase {
+	static class DisabledTestMethodsTestCase {
 
 		@Test
 		void enabledTest() {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/ExceptionHandlingTests.java

@@ -190,7 +190,7 @@ public void cleanUpExceptions() {
 		FailureTestCase.exceptionToThrowInAfterEach = Optional.empty();
 	}
 
-	private static class FailureTestCase {
+	static class FailureTestCase {
 
 		static Optional<Throwable> exceptionToThrowInBeforeAll = Optional.empty();
 		static Optional<Throwable> exceptionToThrowInAfterAll = Optional.empty();

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/InvalidLifecycleMethodConfigurationTests.java

@@ -74,14 +74,14 @@ private void assertExecutionResults(Class<?> invalidTestClass) {
 
 	// -------------------------------------------------------------------------
 
-	private static class TestCase {
+	static class TestCase {
 
 		@Test
 		void test() {
 		}
 	}
 
-	private static class TestCaseWithInvalidBeforeAllMethod {
+	static class TestCaseWithInvalidBeforeAllMethod {
 
 		// must be static
 		@BeforeAll
@@ -93,7 +93,7 @@ void test() {
 		}
 	}
 
-	private static class TestCaseWithInvalidAfterAllMethod {
+	static class TestCaseWithInvalidAfterAllMethod {
 
 		// must be static
 		@AfterAll
@@ -105,7 +105,7 @@ void test() {
 		}
 	}
 
-	private static class TestCaseWithInvalidBeforeEachMethod {
+	static class TestCaseWithInvalidBeforeEachMethod {
 
 		// must NOT be static
 		@BeforeEach
@@ -117,7 +117,7 @@ void test() {
 		}
 	}
 
-	private static class TestCaseWithInvalidAfterEachMethod {
+	static class TestCaseWithInvalidAfterEachMethod {
 
 		// must NOT be static
 		@AfterEach

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/MultipleTestableAnnotationsTests.java

@@ -45,7 +45,7 @@ void testAndRepeatedTest(LogRecordListener listener) {
 		// @formatter:on
 	}
 
-	private static class TestCase {
+	static class TestCase {
 
 		@Test
 		@RepeatedTest(1)

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/NestedTestClassesTests.java

@@ -95,7 +95,7 @@ void inheritedNestedTestsAreExecuted() {
 
 	// -------------------------------------------------------------------
 
-	private static class TestCaseWithNesting {
+	static class TestCaseWithNesting {
 
 		@Test
 		void someTest() {
@@ -115,7 +115,7 @@ void failing() {
 		}
 	}
 
-	private static class TestCaseWithDoubleNesting {
+	static class TestCaseWithDoubleNesting {
 
 		static int beforeTopCount = 0;
 		static int beforeNestedCount = 0;

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/OverloadedTestMethodTests.java

@@ -86,7 +86,7 @@ void executeTestCaseWithOverloadedMethodsWithSingleMethodThatAcceptsArgumentsSel
 		assertTrue(first.isPresent());
 	}
 
-	private static class TestCase {
+	static class TestCase {
 
 		@Test
 		void test() {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/StandardTestClassTests.java

@@ -122,7 +122,7 @@ void testsFailWhenAfterEachFails() {
 		assertTrue(TestCaseWithFailingAfter.testExecuted, "test executed?");
 	}
 
-	private static class MyStandardTestCase {
+	static class MyStandardTestCase {
 
 		static int countBefore1 = 0;
 		static int countBefore2 = 0;
@@ -165,7 +165,7 @@ void abortedTest() {
 
 	}
 
-	private static class FirstOfTwoTestCases {
+	static class FirstOfTwoTestCases {
 
 		@Test
 		void succeedingTest1() {
@@ -184,7 +184,7 @@ void failingTest() {
 
 	}
 
-	private static class SecondOfTwoTestCases {
+	static class SecondOfTwoTestCases {
 
 		@Test
 		void succeedingTest1() {
@@ -203,7 +203,7 @@ void succeedingTest3() {
 
 	}
 
-	private static class TestCaseWithFailingBefore {
+	static class TestCaseWithFailingBefore {
 
 		static int countBefore = 0;
 
@@ -223,7 +223,7 @@ void test2() {
 
 	}
 
-	private static class TestCaseWithFailingAfter {
+	static class TestCaseWithFailingAfter {
 
 		static boolean testExecuted = false;
 

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestCaseWithInheritanceTests.java

@@ -166,7 +166,7 @@ void superTest() {
 		}
 	}
 
-	private static class LocalTestCase extends AbstractTestCase {
+	static class LocalTestCase extends AbstractTestCase {
 
 		boolean throwExceptionInAfterMethod = false;
 
@@ -215,7 +215,7 @@ void alwaysFails() {
 		}
 	}
 
-	private static class TestCase1 {
+	static class TestCase1 {
 
 		@BeforeAll
 		static void beforeAll1() {
@@ -238,7 +238,7 @@ static void afterAll1() {
 		}
 	}
 
-	private static class TestCase2 extends TestCase1 {
+	static class TestCase2 extends TestCase1 {
 
 		@BeforeAll
 		static void beforeAll2() {
@@ -261,7 +261,7 @@ static void afterAll2() {
 		}
 	}
 
-	private static class TestCase3 extends TestCase2 {
+	static class TestCase3 extends TestCase2 {
 
 		@BeforeAll
 		static void beforeAll3() {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestInstanceLifecycleConfigurationTests.java

@@ -147,7 +147,7 @@ private void performAssertions(Class<?> testClass, Map<String, String> configPar
 	// -------------------------------------------------------------------------
 
 	@TestInstance(PER_METHOD)
-	private static class ExplicitInstancePerTestMethodTestCase {
+	static class ExplicitInstancePerTestMethodTestCase {
 
 		@BeforeAll
 		static void beforeAll() {
@@ -171,7 +171,7 @@ static void afterAllStatic() {
 	 * {@code @AfterAll} methods are static, even though there is no explicit
 	 * {@code @TestInstance} declaration.
 	 */
-	private static class AssumedInstancePerTestMethodTestCase {
+	static class AssumedInstancePerTestMethodTestCase {
 
 		@BeforeAll
 		static void beforeAll() {
@@ -191,7 +191,7 @@ static void afterAllStatic() {
 	}
 
 	@TestInstance(PER_CLASS)
-	private static class ExplicitInstancePerClassTestCase {
+	static class ExplicitInstancePerClassTestCase {
 
 		@BeforeAll
 		void beforeAll(TestInfo testInfo) {
@@ -215,7 +215,7 @@ void afterAll(TestInfo testInfo) {
 	 * {@code @AfterAll} methods are non-static, even though there is no
 	 * explicit {@code @TestInstance} declaration.
 	 */
-	private static class AssumedInstancePerClassTestCase {
+	static class AssumedInstancePerClassTestCase {
 
 		@BeforeAll
 		void beforeAll(TestInfo testInfo) {