Introduce getRequiredTestInstance() in JupiterTestDescriptor

Issue: #419

Author: sbrannen

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

@@ -290,9 +290,7 @@ private AfterEachMethodAdapter synthesizeAfterEachMethodAdapter(Method method) {
 
 	private void invokeMethodInExtensionContext(Method method, ExtensionContext context, ExtensionRegistry registry) {
 
-		Object testInstance = context.getTestInstance().orElseThrow(() -> new JUnitException(
-			"Illegal state: test instance not present for method: " + method.toGenericString()));
-
+		Object testInstance = getRequiredTestInstance(context);
 		testInstance = ReflectionUtils.getOutermostInstance(testInstance, method.getDeclaringClass()).orElseThrow(
 			() -> new JUnitException("Failed to find instance for method: " + method.toGenericString()));
 

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

@@ -30,12 +30,14 @@
 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.ExtensionContext;
 import org.junit.jupiter.api.function.Executable;
 import org.junit.jupiter.engine.execution.ConditionEvaluator;
 import org.junit.jupiter.engine.execution.JupiterEngineExecutionContext;
 import org.junit.jupiter.engine.extension.ExtensionRegistry;
 import org.junit.platform.commons.meta.API;
 import org.junit.platform.commons.util.ExceptionUtils;
+import org.junit.platform.commons.util.Preconditions;
 import org.junit.platform.commons.util.StringUtils;
 import org.junit.platform.engine.TestTag;
 import org.junit.platform.engine.UniqueId;
@@ -134,4 +136,9 @@ protected void executeAndMaskThrowable(Executable executable) {
 		}
 	}
 
+	protected Object getRequiredTestInstance(ExtensionContext extensionContext) {
+		return Preconditions.notNull(extensionContext.getTestInstance().orElse(null),
+			"Illegal state: required test instance is not present");
+	}
+
 }

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

@@ -31,7 +31,6 @@
 import org.junit.jupiter.engine.execution.JupiterEngineExecutionContext;
 import org.junit.jupiter.engine.execution.ThrowableCollector;
 import org.junit.jupiter.engine.extension.ExtensionRegistry;
-import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.meta.API;
 import org.junit.platform.commons.util.ExceptionUtils;
 import org.junit.platform.engine.TestDescriptor;
@@ -164,8 +163,7 @@ protected void invokeTestMethod(JupiterEngineExecutionContext context, DynamicTe
 		throwableCollector.execute(() -> {
 			try {
 				Method testMethod = getTestMethod();
-				Object instance = extensionContext.getTestInstance().orElseThrow(() -> new JUnitException(
-					"Illegal state: test instance not present for method: " + testMethod.toGenericString()));
+				Object instance = getRequiredTestInstance(extensionContext);
 				executableInvoker.invoke(testMethod, instance, extensionContext, context.getExtensionRegistry());
 			}
 			catch (Throwable throwable) {

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

@@ -66,8 +66,7 @@ protected void invokeTestMethod(JupiterEngineExecutionContext context, DynamicTe
 		ExtensionContext extensionContext = context.getExtensionContext();
 
 		context.getThrowableCollector().execute(() -> {
-			Object instance = extensionContext.getTestInstance().orElseThrow(() -> new JUnitException(
-				"Illegal state: test instance not present for method: " + getTestMethod().toGenericString()));
+			Object instance = getRequiredTestInstance(extensionContext);
 			Object testFactoryMethodResult = executableInvoker.invoke(getTestMethod(), instance, extensionContext,
 				context.getExtensionRegistry());
 			TestSource source = getSource().orElseThrow(