Improve diagnostics in failing tests

Issue: #910

Author: sbrannen

Diff for: junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestInstanceLifecycleTests.java

@@ -878,7 +878,11 @@ public ConditionEvaluationResult evaluateExecutionCondition(ExtensionContext con
 
 		@Override
 		public void postProcessTestInstance(ExtensionContext context) {
-			Object testInstance = context.getTestInstance().orElse(null);
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			instanceMap.put(postProcessTestInstanceKey(context.getTestClass().get()), testInstance);
 		}
 
@@ -894,15 +898,25 @@ public void afterAll(ExtensionContext context) {
 
 		@Override
 		public void beforeEach(ExtensionContext context) {
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			instanceMap.put(
 				beforeEachCallbackKey(context.getTestClass().get(), context.getTestMethod().get().getName()),
-				context.getTestInstance().orElse(null));
+				testInstance);
 		}
 
 		@Override
 		public void afterEach(ExtensionContext context) {
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			instanceMap.put(afterEachCallbackKey(context.getTestClass().get(), context.getTestMethod().get().getName()),
-				context.getTestInstance().orElse(null));
+				testInstance);
 		}
 
 		@Override
@@ -912,9 +926,13 @@ public boolean supportsTestTemplate(ExtensionContext context) {
 
 		@Override
 		public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContexts(ExtensionContext context) {
-
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			instanceMap.put(testTemplateKey(context.getTestClass().get(), context.getTestMethod().get().getName()),
-				context.getTestInstance().orElse(null));
+				testInstance);
 
 			return Stream.of(new TestTemplateInvocationContext() {
 			});

Diff for: junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestTemplateInvocationTests.java

@@ -648,7 +648,12 @@ public String getDisplayName(int invocationIndex) {
 				@Override
 				public List<Extension> getAdditionalExtensions() {
 					return singletonList((TestInstancePostProcessor) (context) -> {
-						Object testInstance = context.getTestInstance().orElse(null);
+						Object testInstance = context.getTestInstance().orElseThrow(() -> {
+							IllegalStateException exception = new IllegalStateException(
+								"test instance must not be null");
+							exception.printStackTrace(System.err);
+							return exception;
+						});
 						Field field = testInstance.getClass().getDeclaredField("parameterInstanceVariable");
 						field.setAccessible(true);
 						field.set(testInstance, argument);

Diff for: junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/TestInstancePostProcessorTests.java

@@ -160,7 +160,11 @@ private static class FooInstancePostProcessor implements TestInstancePostProcess
 
 		@Override
 		public void postProcessTestInstance(ExtensionContext context) throws Exception {
-			Object testInstance = context.getTestInstance().orElse(null);
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			if (testInstance instanceof Named) {
 				((Named) testInstance).setName("foo");
 			}
@@ -172,7 +176,11 @@ private static class BarInstancePostProcessor implements TestInstancePostProcess
 
 		@Override
 		public void postProcessTestInstance(ExtensionContext context) throws Exception {
-			Object testInstance = context.getTestInstance().orElse(null);
+			Object testInstance = context.getTestInstance().orElseThrow(() -> {
+				IllegalStateException exception = new IllegalStateException("test instance must not be null");
+				exception.printStackTrace(System.err);
+				return exception;
+			});
 			if (testInstance instanceof Named) {
 				((Named) testInstance).setName("bar");
 			}