Introduce class-level support for @RegisterExtension in Jupiter

Issue: #497

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/JupiterTestDescriptor.java

@@ -14,8 +14,11 @@
 import static java.util.stream.Collectors.toCollection;
 import static java.util.stream.Collectors.toList;
 import static org.apiguardian.api.API.Status.INTERNAL;
+import static org.junit.platform.commons.util.AnnotationUtils.findAnnotatedFields;
 import static org.junit.platform.commons.util.AnnotationUtils.findAnnotation;
 import static org.junit.platform.commons.util.AnnotationUtils.findRepeatableAnnotations;
+import static org.junit.platform.commons.util.ReflectionUtils.isStatic;
+import static org.junit.platform.commons.util.ReflectionUtils.readFieldValue;
 
 import java.lang.reflect.AnnotatedElement;
 import java.util.Arrays;
@@ -32,6 +35,7 @@
 import org.junit.jupiter.api.extension.ConditionEvaluationResult;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.junit.jupiter.api.extension.Extension;
+import org.junit.jupiter.api.extension.RegisterExtension;
 import org.junit.jupiter.api.function.Executable;
 import org.junit.jupiter.engine.execution.ConditionEvaluator;
 import org.junit.jupiter.engine.execution.JupiterEngineExecutionContext;
@@ -142,7 +146,22 @@ protected ExtensionRegistry populateNewExtensionRegistryFromExtendWith(Annotated
 				.flatMap(Arrays::stream)
 				.collect(toList());
 		// @formatter:on
-		return ExtensionRegistry.createRegistryFrom(existingExtensionRegistry, extensionTypes);
+		ExtensionRegistry newRegistry = ExtensionRegistry.createRegistryFrom(existingExtensionRegistry, extensionTypes);
+
+		if (annotatedElement instanceof Class) {
+			Class<?> clazz = (Class<?>) annotatedElement;
+			// @formatter:off
+			findAnnotatedFields(clazz, RegisterExtension.class, field -> isStatic(field)).stream()
+				.map(field -> readFieldValue(clazz, field.getName(), null))
+				.filter(Optional::isPresent)
+				.map(Optional::get)
+				.filter(Extension.class::isInstance)
+				.map(Extension.class::cast)
+				.forEach(ext -> newRegistry.registerExtension(ext, clazz));
+			// @formatter:on
+		}
+
+		return newRegistry;
 	}
 
 	/**

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/ProgrammaticExtensionRegistrationTests.java

@@ -42,12 +42,21 @@ void instanceLevel() {
 		assertOneTestSucceeded(InstanceLevelExtensionRegistrationTestCase.class);
 	}
 
-	@Disabled("Disabled until feature is completed")
 	@Test
 	void classLevel() {
 		assertOneTestSucceeded(ClassLevelExtensionRegistrationTestCase.class);
 	}
 
+	@Test
+	void classLevelFromSuperclass() {
+		assertOneTestSucceeded(SubClassLevelExtensionRegistrationTestCase.class);
+	}
+
+	@Test
+	void classLevelFromInterface() {
+		assertOneTestSucceeded(ExtensionRegistrationFromInterfaceTestCase.class);
+	}
+
 	private void assertOneTestSucceeded(Class<?> testClass) {
 		ExecutionEventRecorder eventRecorder = executeTestsForClass(testClass);
 		assertAll(//
@@ -120,6 +129,52 @@ static void afterAll(String wisdom) {
 
 	}
 
+	static class SubClassLevelExtensionRegistrationTestCase extends ClassLevelExtensionRegistrationTestCase {
+
+		@Test
+		@Override
+		void test(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "Overridden @Test");
+		}
+
+	}
+
+	interface ClassLevelExtensionRegistrationInterface {
+
+		@RegisterExtension
+		static final CrystalBall crystalBall = new CrystalBall("Outlook good");
+
+		@BeforeAll
+		static void beforeAll(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@BeforeAll");
+		}
+
+		@BeforeEach
+		default void beforeEach(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@BeforeEach");
+		}
+
+		@AfterEach
+		default void afterEach(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@AfterEach");
+		}
+
+		@AfterAll
+		static void afterAll(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@AfterAll");
+		}
+
+	}
+
+	static class ExtensionRegistrationFromInterfaceTestCase implements ClassLevelExtensionRegistrationInterface {
+
+		@Test
+		void test(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@Test");
+		}
+
+	}
+
 	private static class CrystalBall implements ParameterResolver {
 
 		private final String wisdom;