Introduce ExtensionContext.getTestInstances() API

The new `getTestInstances()` and `getRequiredTestInstances()` methods of
`ExtensionContext` allow accessing all test instances, including
enclosing ones for `@Nested` tests.

Resolves #1618.

Author: marcphilipp

documentation/src/docs/asciidoc/release-notes/release-notes-5.4.0-M1.adoc

@@ -132,6 +132,8 @@ repository on GitHub.
   methods via the `junit-jupiter-migrationsupport` module.
   - See the <<../user-guide/index.adoc#migrating-from-junit4-ignore-annotation-support,
     User Guide>> for details.
+* New `ExtensionContext` methods to access all test instances, including enclosing ones
+  for `@Nested` tests: `getTestInstances()` and `getRequiredTestInstances()`.
 
 
 [[release-notes-5.4.0-M1-junit-vintage]]

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/ExtensionContext.java

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.api.extension;
 
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
 import static org.apiguardian.api.API.Status.STABLE;
 
 import java.lang.reflect.AnnotatedElement;
@@ -160,6 +161,7 @@ default Class<?> getRequiredTestClass() {
 	 * @return an {@code Optional} containing the test instance; never
 	 * {@code null} but potentially empty
 	 * @see #getRequiredTestInstance()
+	 * @see #getTestInstances()
 	 */
 	Optional<Object> getTestInstance();
 
@@ -173,12 +175,49 @@ default Class<?> getRequiredTestClass() {
 	 * @return the test instance; never {@code null}
 	 * @throws PreconditionViolationException if the test instance is not present
 	 * in this {@code ExtensionContext}
+	 *
+	 * @see #getRequiredTestInstances()
 	 */
 	default Object getRequiredTestInstance() {
 		return Preconditions.notNull(getTestInstance().orElse(null),
 			"Illegal state: required test instance is not present in the current ExtensionContext");
 	}
 
+	/**
+	 * Get the test instances associated with the current test or container,
+	 * if available.
+	 *
+	 * <p>While top-level tests only have a single test instance, nested tests
+	 * have one additional instance for each enclosing test class.
+	 *
+	 * @return an {@code Optional} containing the test instances; never
+	 * {@code null} but potentially empty
+	 * @see #getRequiredTestInstances()
+	 *
+	 * @since 5.4
+	 */
+	@API(status = EXPERIMENTAL, since = "5.4")
+	Optional<TestInstances> getTestInstances();
+
+	/**
+	 * Get the <em>required</em> test instances associated with the current test
+	 * or container.
+	 *
+	 * <p>Use this method as an alternative to {@link #getTestInstances()} for use
+	 * cases in which the test instances are required to be present.
+	 *
+	 * @return the test instances; never {@code null}
+	 * @throws PreconditionViolationException if the test instances are not present
+	 * in this {@code ExtensionContext}
+	 *
+	 * @since 5.4
+	 */
+	@API(status = EXPERIMENTAL, since = "5.4")
+	default TestInstances getRequiredTestInstances() {
+		return Preconditions.notNull(getTestInstances().orElse(null),
+			"Illegal state: required test instances are not present in the current ExtensionContext");
+	}
+
 	/**
 	 * Get the {@link Method} associated with the current test, if available.
 	 *

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/TestInstances.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2015-2018 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * http://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.api.extension;
+
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
+
+import java.util.List;
+import java.util.Optional;
+
+import org.apiguardian.api.API;
+
+/**
+ * {@code TestInstances} encapsulates the <em>test instances</em> of a test.
+ *
+ * <p>While top-level tests only have a single test instance, nested tests
+ * have one additional instance for each enclosing test class.
+ *
+ * @since 5.4
+ * @see ExtensionContext#getTestInstances()
+ * @see ExtensionContext#getRequiredTestInstances()
+ */
+@API(status = EXPERIMENTAL, since = "5.4")
+public interface TestInstances {
+
+	/**
+	 * Get the innermost test instance.
+	 *
+	 * <p>The innermost instance is the one closest to the test method.
+	 *
+	 * @return the innermost test instance; never {@code null}
+	 */
+	Object getInnermostInstance();
+
+	/**
+	 * Get the enclosing test instances, excluding the innermost test instance,
+	 * ordered from outermost to innermost.
+	 *
+	 * @return the enclosing test instances; never {@code null} or containing
+	 * {@code null}, but potentially empty
+	 */
+	List<Object> getEnclosingInstances();
+
+	/**
+	 * Get all test instances, ordered from outermost to innermost.
+	 *
+	 * @return all test instances; never {@code null}, containing {@code null},
+	 * or empty
+	 */
+	List<Object> getAllInstances();
+
+	/**
+	 * Find the first test instance that is an instance of the supplied required
+	 * type, checking from innermost to outermost.
+	 *
+	 * @param requiredType the type to search for
+	 * @return the first test instance of the required type; never {@code null}
+	 * but potentially empty
+	 */
+	<T> Optional<T> findInstance(Class<T> requiredType);
+
+}

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

@@ -19,6 +19,7 @@
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.TestInstance.Lifecycle;
 import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.TestInstances;
 import org.junit.jupiter.engine.config.JupiterConfiguration;
 import org.junit.platform.engine.EngineExecutionListener;
 import org.junit.platform.engine.support.hierarchical.ThrowableCollector;
@@ -33,7 +34,7 @@ public final class ClassExtensionContext extends AbstractExtensionContext<ClassT
 
 	private final ThrowableCollector throwableCollector;
 
-	private Object testInstance;
+	private TestInstances testInstances;
 
 	/**
 	 * Create a new {@code ClassExtensionContext} with {@link Lifecycle#PER_METHOD}.
@@ -72,13 +73,18 @@ public Optional<Lifecycle> getTestInstanceLifecycle() {
 		return Optional.of(this.lifecycle);
 	}
 
-	void setTestInstance(Object testInstance) {
-		this.testInstance = testInstance;
+	@Override
+	public Optional<Object> getTestInstance() {
+		return getTestInstances().map(TestInstances::getInnermostInstance);
 	}
 
 	@Override
-	public Optional<Object> getTestInstance() {
-		return Optional.ofNullable(this.testInstance);
+	public Optional<TestInstances> getTestInstances() {
+		return Optional.ofNullable(testInstances);
+	}
+
+	void setTestInstances(TestInstances testInstances) {
+		this.testInstances = testInstances;
 	}
 
 	@Override

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

@@ -42,13 +42,15 @@
 import org.junit.jupiter.api.extension.ExtensionContext;
 import org.junit.jupiter.api.extension.TestInstanceFactory;
 import org.junit.jupiter.api.extension.TestInstancePostProcessor;
+import org.junit.jupiter.api.extension.TestInstances;
 import org.junit.jupiter.api.extension.TestInstantiationException;
 import org.junit.jupiter.engine.config.JupiterConfiguration;
 import org.junit.jupiter.engine.execution.AfterEachMethodAdapter;
 import org.junit.jupiter.engine.execution.BeforeEachMethodAdapter;
+import org.junit.jupiter.engine.execution.DefaultTestInstances;
 import org.junit.jupiter.engine.execution.ExecutableInvoker;
 import org.junit.jupiter.engine.execution.JupiterEngineExecutionContext;
-import org.junit.jupiter.engine.execution.TestInstanceProvider;
+import org.junit.jupiter.engine.execution.TestInstancesProvider;
 import org.junit.jupiter.engine.extension.ExtensionRegistry;
 import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.util.BlacklistedExceptions;
@@ -169,7 +171,7 @@ public JupiterEngineExecutionContext prepare(JupiterEngineExecutionContext conte
 
 		// @formatter:off
 		return context.extend()
-				.withTestInstanceProvider(testInstanceProvider(context, registry, extensionContext))
+				.withTestInstancesProvider(testInstancesProvider(context, registry, extensionContext))
 				.withExtensionRegistry(registry)
 				.withExtensionContext(extensionContext)
 				.withThrowableCollector(throwableCollector)
@@ -186,8 +188,8 @@ public JupiterEngineExecutionContext before(JupiterEngineExecutionContext contex
 			// Eagerly load test instance for BeforeAllCallbacks, if necessary,
 			// and store the instance in the ExtensionContext.
 			ClassExtensionContext extensionContext = (ClassExtensionContext) context.getExtensionContext();
-			throwableCollector.execute(() -> extensionContext.setTestInstance(
-				context.getTestInstanceProvider().getTestInstance(Optional.empty())));
+			throwableCollector.execute(() -> extensionContext.setTestInstances(
+				context.getTestInstancesProvider().getTestInstances(Optional.empty())));
 		}
 
 		if (throwableCollector.isEmpty()) {
@@ -251,40 +253,43 @@ private TestInstanceFactory resolveTestInstanceFactory(ExtensionRegistry registr
 		return null;
 	}
 
-	private TestInstanceProvider testInstanceProvider(JupiterEngineExecutionContext parentExecutionContext,
+	private TestInstancesProvider testInstancesProvider(JupiterEngineExecutionContext parentExecutionContext,
 			ExtensionRegistry registry, ClassExtensionContext extensionContext) {
 
-		TestInstanceProvider testInstanceProvider = childRegistry -> instantiateAndPostProcessTestInstance(
+		TestInstancesProvider testInstancesProvider = childRegistry -> instantiateAndPostProcessTestInstance(
 			parentExecutionContext, extensionContext, childRegistry.orElse(registry));
 
-		return childRegistry -> extensionContext.getTestInstance().orElseGet(
-			() -> testInstanceProvider.getTestInstance(childRegistry));
+		return childRegistry -> extensionContext.getTestInstances().orElseGet(
+			() -> testInstancesProvider.getTestInstances(childRegistry));
 	}
 
-	private Object instantiateAndPostProcessTestInstance(JupiterEngineExecutionContext parentExecutionContext,
+	private TestInstances instantiateAndPostProcessTestInstance(JupiterEngineExecutionContext parentExecutionContext,
 			ExtensionContext extensionContext, ExtensionRegistry registry) {
 
-		Object instance = instantiateTestClass(parentExecutionContext, registry, extensionContext);
-		invokeTestInstancePostProcessors(instance, registry, extensionContext);
+		TestInstances instances = instantiateTestClass(parentExecutionContext, registry, extensionContext);
+		invokeTestInstancePostProcessors(instances.getInnermostInstance(), 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(registry, this.testClass, instance);
-		return instance;
+		registerExtensionsFromFields(registry, this.testClass, instances.getInnermostInstance());
+		return instances;
 	}
 
-	protected Object instantiateTestClass(JupiterEngineExecutionContext parentExecutionContext,
+	protected TestInstances instantiateTestClass(JupiterEngineExecutionContext parentExecutionContext,
 			ExtensionRegistry registry, ExtensionContext extensionContext) {
 
 		return instantiateTestClass(Optional.empty(), registry, extensionContext);
 	}
 
-	protected Object instantiateTestClass(Optional<Object> outerInstance, ExtensionRegistry registry,
+	protected TestInstances instantiateTestClass(Optional<TestInstances> outerInstances, ExtensionRegistry registry,
 			ExtensionContext extensionContext) {
 
-		return this.testInstanceFactory != null //
+		Optional<Object> outerInstance = outerInstances.map(TestInstances::getInnermostInstance);
+		Object instance = this.testInstanceFactory != null //
 				? invokeTestInstanceFactory(outerInstance, extensionContext) //
 				: invokeTestClassConstructor(outerInstance, registry, extensionContext);
+		return outerInstances.map(instances -> DefaultTestInstances.of(instances, instance)).orElse(
+			DefaultTestInstances.of(instance));
 	}
 
 	private Object invokeTestInstanceFactory(Optional<Object> outerInstance, ExtensionContext extensionContext) {
@@ -427,11 +432,11 @@ private AfterEachMethodAdapter synthesizeAfterEachMethodAdapter(Method method) {
 	}
 
 	private void invokeMethodInExtensionContext(Method method, ExtensionContext context, ExtensionRegistry registry) {
-		Object testInstance = context.getRequiredTestInstance();
-		testInstance = ReflectionUtils.getOutermostInstance(testInstance, method.getDeclaringClass()).orElseThrow(
+		TestInstances testInstances = context.getRequiredTestInstances();
+		Object target = testInstances.findInstance(method.getDeclaringClass()).orElseThrow(
 			() -> new JUnitException("Failed to find instance for method: " + method.toGenericString()));
 
-		executableInvoker.invoke(method, testInstance, context, registry);
+		executableInvoker.invoke(method, target, context, registry);
 	}
 
 }

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

@@ -18,6 +18,7 @@
 
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.TestInstance.Lifecycle;
+import org.junit.jupiter.api.extension.TestInstances;
 import org.junit.jupiter.engine.config.JupiterConfiguration;
 import org.junit.platform.engine.EngineExecutionListener;
 
@@ -53,6 +54,11 @@ public Optional<Object> getTestInstance() {
 		return Optional.empty();
 	}
 
+	@Override
+	public Optional<TestInstances> getTestInstances() {
+		return Optional.empty();
+	}
+
 	@Override
 	public Optional<Method> getTestMethod() {
 		return Optional.empty();

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

@@ -19,6 +19,7 @@
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.TestInstance.Lifecycle;
 import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.TestInstances;
 import org.junit.jupiter.engine.config.JupiterConfiguration;
 import org.junit.platform.engine.EngineExecutionListener;
 import org.junit.platform.engine.support.hierarchical.ThrowableCollector;
@@ -29,17 +30,17 @@
 @API(status = INTERNAL, since = "5.0")
 public final class MethodExtensionContext extends AbstractExtensionContext<TestMethodTestDescriptor> {
 
-	private final Object testInstance;
+	private final TestInstances testInstances;
 
 	private final ThrowableCollector throwableCollector;
 
 	public MethodExtensionContext(ExtensionContext parent, EngineExecutionListener engineExecutionListener,
-			TestMethodTestDescriptor testDescriptor, JupiterConfiguration configuration, Object testInstance,
+			TestMethodTestDescriptor testDescriptor, JupiterConfiguration configuration, TestInstances testInstances,
 			ThrowableCollector throwableCollector) {
 
 		super(parent, engineExecutionListener, testDescriptor, configuration);
 
-		this.testInstance = testInstance;
+		this.testInstances = testInstances;
 		this.throwableCollector = throwableCollector;
 	}
 
@@ -60,7 +61,12 @@ public Optional<Lifecycle> getTestInstanceLifecycle() {
 
 	@Override
 	public Optional<Object> getTestInstance() {
-		return Optional.of(this.testInstance);
+		return Optional.of(this.testInstances.getInnermostInstance());
+	}
+
+	@Override
+	public Optional<TestInstances> getTestInstances() {
+		return Optional.of(this.testInstances);
 	}
 
 	@Override

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

@@ -19,6 +19,7 @@
 
 import org.apiguardian.api.API;
 import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.TestInstances;
 import org.junit.jupiter.engine.config.JupiterConfiguration;
 import org.junit.jupiter.engine.execution.JupiterEngineExecutionContext;
 import org.junit.jupiter.engine.extension.ExtensionRegistry;
@@ -63,14 +64,14 @@ public final Set<TestTag> getTags() {
 	// --- Node ----------------------------------------------------------------
 
 	@Override
-	protected Object instantiateTestClass(JupiterEngineExecutionContext parentExecutionContext,
+	protected TestInstances instantiateTestClass(JupiterEngineExecutionContext parentExecutionContext,
 			ExtensionRegistry registry, ExtensionContext extensionContext) {
 
 		// Extensions registered for nested classes and below are not to be used for instantiating outer classes
 		Optional<ExtensionRegistry> childExtensionRegistryForOuterInstance = Optional.empty();
-		Object outerInstance = parentExecutionContext.getTestInstanceProvider().getTestInstance(
+		TestInstances outerInstances = parentExecutionContext.getTestInstancesProvider().getTestInstances(
 			childExtensionRegistryForOuterInstance);
-		return instantiateTestClass(Optional.of(outerInstance), registry, extensionContext);
+		return instantiateTestClass(Optional.of(outerInstances), registry, extensionContext);
 	}
 
 }