Support @RegisterExtension with @testinstance(PER_CLASS) lifecycle mode

Issue: junit-team#497

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

@@ -120,6 +120,8 @@ public JupiterEngineExecutionContext prepare(JupiterEngineExecutionContext conte
 		ExtensionRegistry registry = populateNewExtensionRegistryFromExtendWith(this.testClass,
 			context.getExtensionRegistry());
 
+		// Register extensions from static fields here, at the class level but
+		// after extensions registered via @ExtendWith.
 		registerExtensionsFromFields(this.testClass, registry, null);
 		registerBeforeEachMethodAdapters(registry);
 		registerAfterEachMethodAdapters(registry);
@@ -180,6 +182,7 @@ private TestInstanceProvider testInstanceProvider(JupiterEngineExecutionContext
 
 		TestInstanceProvider testInstanceProvider = childRegistry -> instantiateAndPostProcessTestInstance(
 			parentExecutionContext, extensionContext, childRegistry.orElse(registry));
+
 		return childRegistry -> extensionContext.getTestInstance().orElseGet(
 			() -> testInstanceProvider.getTestInstance(childRegistry));
 	}
@@ -189,6 +192,10 @@ private Object instantiateAndPostProcessTestInstance(JupiterEngineExecutionConte
 
 		Object instance = instantiateTestClass(context, registry, extensionContext);
 		invokeTestInstancePostProcessors(instance, registry, extensionContext);
+		// In addition, we register extensions from instance fields here since the
+		// best time to do that is immediately following test class instantiation
+		// and post processing.
+		registerExtensionsFromFields(this.testClass, registry, instance);
 		return instance;
 	}
 

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

@@ -79,8 +79,6 @@ public JupiterEngineExecutionContext prepare(JupiterEngineExecutionContext conte
 		ExtensionRegistry registry = populateNewExtensionRegistry(context);
 		Object testInstance = context.getTestInstanceProvider().getTestInstance(Optional.of(registry));
 
-		registerExtensionsFromFields(getTestClass(), registry, testInstance);
-
 		ThrowableCollector throwableCollector = new ThrowableCollector();
 		ExtensionContext extensionContext = new MethodExtensionContext(context.getExtensionContext(),
 			context.getExecutionListener(), this, context.getConfigurationParameters(), testInstance,

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

@@ -13,12 +13,14 @@
 import static org.junit.jupiter.api.Assertions.assertAll;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.TestInstance.Lifecycle.PER_CLASS;
 
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.ParameterContext;
 import org.junit.jupiter.api.extension.ParameterResolver;
@@ -40,6 +42,11 @@ void instanceLevel() {
 		assertOneTestSucceeded(InstanceLevelExtensionRegistrationTestCase.class);
 	}
 
+	@Test
+	void instanceLevelWithTestInstancePerClassLifecycle() {
+		assertOneTestSucceeded(InstanceLevelExtensionRegistrationWithTestInstancePerClassLifecycleTestCase.class);
+	}
+
 	@Test
 	void classLevel() {
 		assertOneTestSucceeded(ClassLevelExtensionRegistrationTestCase.class);
@@ -95,6 +102,39 @@ void afterEach(String wisdom) {
 
 	}
 
+	@TestInstance(PER_CLASS)
+	static class InstanceLevelExtensionRegistrationWithTestInstancePerClassLifecycleTestCase {
+
+		@RegisterExtension
+		final CrystalBall crystalBall = new CrystalBall("Outlook good");
+
+		@BeforeAll
+		void beforeAll(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@BeforeAll");
+		}
+
+		@BeforeEach
+		void beforeEach(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@BeforeEach");
+		}
+
+		@Test
+		void test(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@Test");
+		}
+
+		@AfterEach
+		void afterEach(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@AfterEach");
+		}
+
+		@AfterAll
+		void afterAll(String wisdom) {
+			assertWisdom(crystalBall, wisdom, "@AfterAll");
+		}
+
+	}
+
 	static class ClassLevelExtensionRegistrationTestCase {
 
 		@RegisterExtension