Disallow duplicate registration via @RegisterExtension and @ExtendWith

Prior to this commit, @ExtendWith and @RegisterExtension could be
declared on a field to register duplicate extensions of the same time,
but that scenario likely does not make sense.

This commit ensures that @RegisterExtension and @ExtendWith cannot be
used to register an extension of the same type for a given field, by
throwing a PreconditionViolationException if the user attempts to do so.

See #2680

Author: sbrannen

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

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.engine.descriptor;
 
+import static java.util.stream.Collectors.toList;
 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.AnnotationUtils.isAnnotated;
@@ -95,17 +96,29 @@ static void registerExtensionsFromFields(ExtensionRegistrar registrar, Class<?>
 		findFields(clazz, predicate, TOP_DOWN).stream()//
 				.sorted(orderComparator)//
 				.forEach(field -> {
-					List<ExtendWith> extendWithAnnotations = findRepeatableAnnotations(field, ExtendWith.class);
-					boolean isExtendWithPresent = !extendWithAnnotations.isEmpty();
+					List<Class<? extends Extension>> extensionTypes = streamExtensionTypes(field).collect(toList());
+					boolean isExtendWithPresent = !extensionTypes.isEmpty();
 					boolean isRegisterExtensionPresent = isAnnotated(field, RegisterExtension.class);
 					if (isExtendWithPresent) {
-						streamExtensionTypes(extendWithAnnotations).forEach(registrar::registerExtension);
+						extensionTypes.forEach(registrar::registerExtension);
 					}
 					if (isRegisterExtensionPresent) {
 						tryToReadFieldValue(field, instance).ifSuccess(value -> {
 							Preconditions.condition(value instanceof Extension, () -> String.format(
 								"Failed to register extension via @RegisterExtension field [%s]: field value's type [%s] must implement an [%s] API.",
 								field, (value != null ? value.getClass().getName() : null), Extension.class.getName()));
+
+							if (isExtendWithPresent) {
+								Class<?> valueType = value.getClass();
+								extensionTypes.forEach(extensionType -> {
+									Preconditions.condition(!extensionType.equals(valueType),
+										() -> String.format("Failed to register extension via field [%s]. "
+												+ "The field registers an extension of type [%s] via @RegisterExtension and @ExtendWith, "
+												+ "but only one registration of a given extension type is permitted.",
+											field, valueType.getName()));
+								});
+							}
+
 							registrar.registerExtension((Extension) value, field);
 						});
 					}

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

@@ -16,6 +16,9 @@
 import static org.junit.jupiter.api.DynamicTest.dynamicTest;
 import static org.junit.platform.commons.util.AnnotationUtils.findAnnotatedFields;
 import static org.junit.platform.commons.util.ReflectionUtils.makeAccessible;
+import static org.junit.platform.testkit.engine.EventConditions.finishedWithFailure;
+import static org.junit.platform.testkit.engine.TestExecutionResultConditions.instanceOf;
+import static org.junit.platform.testkit.engine.TestExecutionResultConditions.message;
 
 import java.lang.annotation.Annotation;
 import java.lang.annotation.ElementType;
@@ -61,13 +64,14 @@
 import org.junit.jupiter.api.fixtures.TrackLogRecords;
 import org.junit.jupiter.engine.AbstractJupiterTestEngineTests;
 import org.junit.jupiter.engine.execution.injection.sample.LongParameterResolver;
+import org.junit.platform.commons.PreconditionViolationException;
 import org.junit.platform.commons.logging.LogRecordListener;
 import org.junit.platform.commons.util.ExceptionUtils;
 import org.junit.platform.commons.util.ReflectionUtils;
 
 /**
  * Integration tests that verify support for extension registration via
- * {@link ExtendWith @ExtendWith} on annotations on parameters and fields.
+ * {@link ExtendWith @ExtendWith} on parameters and fields.
  *
  * @since 5.8
  */
@@ -133,6 +137,26 @@ void fieldsWithTestInstancePerClass() {
 		assertOneTestSucceeded(TestInstancePerClassFieldTestCase.class);
 	}
 
+	@Test
+	@TrackLogRecords
+	void multipleRegistrationsViaField(LogRecordListener listener) {
+		assertOneTestSucceeded(MultipleRegistrationsViaFieldTestCase.class);
+		assertThat(getRegisteredLocalExtensions(listener)).containsExactly("LongParameterResolver", "DummyExtension");
+	}
+
+	@Test
+	void duplicateRegistrationViaField() {
+		Class<?> testClass = DuplicateRegistrationViaFieldTestCase.class;
+		String expectedMessage = "Failed to register extension via field "
+				+ "[org.junit.jupiter.api.extension.Extension "
+				+ "org.junit.jupiter.engine.extension.ExtensionRegistrationViaParametersAndFieldsTests$DuplicateRegistrationViaFieldTestCase.dummy]. "
+				+ "The field registers an extension of type [org.junit.jupiter.engine.extension.DummyExtension] "
+				+ "via @RegisterExtension and @ExtendWith, but only one registration of a given extension type is permitted.";
+
+		executeTestsForClass(testClass).testEvents().assertThatEvents().haveExactly(1,
+			finishedWithFailure(instanceOf(PreconditionViolationException.class), message(expectedMessage)));
+	}
+
 	@Test
 	@TrackLogRecords
 	void registrationOrder(LogRecordListener listener) {
@@ -531,6 +555,28 @@ private static TestTemplateInvocationContext emptyTestTemplateInvocationContext(
 		}
 	}
 
+	static class MultipleRegistrationsViaFieldTestCase {
+
+		@ExtendWith(LongParameterResolver.class)
+		@RegisterExtension
+		Extension dummy = new DummyExtension();
+
+		@Test
+		void test() {
+		}
+	}
+
+	static class DuplicateRegistrationViaFieldTestCase {
+
+		@ExtendWith(DummyExtension.class)
+		@RegisterExtension
+		Extension dummy = new DummyExtension();
+
+		@Test
+		void test() {
+		}
+	}
+
 	/**
 	 * The {@link MagicField.Extension} is registered via a static field.
 	 */
@@ -791,6 +837,9 @@ class Extension extends BaseParameterExtension<TestParameter> {
 	}
 }
 
+class DummyExtension implements Extension {
+}
+
 class BaseFieldExtension<T extends Annotation> implements BeforeAllCallback, BeforeEachCallback {
 
 	private final Class<T> annotationType;