Support global extension registration via ServiceLoader

Prior to this commit, custom extensions could only be registered
declaratively via @ExtendWith.

This commit introduces support for auto-detection of extensions
via Java's ServiceLoader mechanism. Such extensions will be
automatically registered after the default global extensions
in JUnit Jupiter (e.g., support for TestInfo, TestReporter, etc.).

Furthermore, this feature is disabled by default but can be enabled
via a JVM system property or JUnit Platform configuration parameter.
Consult the JavaDoc and User Guide for details.

Issue: #448

documentation/src/docs/asciidoc/extensions.adoc

@@ -21,7 +21,9 @@ class, test method, or custom _<<writing-tests-meta-annotations,composed annotat
 with `@ExtendWith(...)` and supplying class references for the extensions to register.
 
 For example, to register a custom `MockitoExtension` for a particular test method, you
-would annotate the test method as follows.
+would annotate the test method as follows. For special cases there is also the possiblilty
+to use <<extensions-registration-service-loader,service loader based>> extension
+registration.
 
 [source,java,indent=0]
 [subs="verbatim"]
@@ -71,6 +73,21 @@ class MyTestsV2 {
 The execution of tests in both `MyTestsV1` and `MyTestsV2` will be extended by the
 `FooExtension` and `BarExtension`, in exactly that order.
 
+[[extensions-registration-service-loader]]
+==== Service Loader Extension Registration
+
+Besides to the default <<extensions-registration-declarative,extension registration>>
+behaviour using annotations as described before, there is a also an enhanced extension
+registration mechanism using the Java service loader facility.
+
+This feature is not enabled by default and must be activated setting the following system
+property:
+
+`-Djunit.extensions.auto-detect=true`
+
+When being enabled the extension registry will be initialized using the global extensions
+and then all extensions available thru the service loader will be added afterwards.
+
 [[extensions-registration-inheritance]]
 ==== Extension Inheritance
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/Constants.java

@@ -10,6 +10,8 @@
 
 package org.junit.jupiter.engine;
 
+import java.util.ServiceLoader;
+
 /**
  * Collection of constants related to the {@link JupiterTestEngine}.
  *
@@ -59,6 +61,14 @@ public final class Constants {
 	 */
 	public static final String DEACTIVATE_ALL_CONDITIONS_PATTERN = "*";
 
+	/**
+	 * Property name used to enable test extensions to be loaded using the {@link ServiceLoader}  additional
+	 * to the global default ones within {@code ExtensionRegistry.createRegistryWithDefaultExtensions()}.
+	 *
+	 * The default behavior is not to load any extensions using the service loader.
+	 */
+	public static final String EXTENSIONS_AUTODETECT_PROPERTY_NAME = "junit.extensions.auto-detect";
+
 	private Constants() {
 		/* no-op */
 	}

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

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.engine.descriptor;
 
+import static org.junit.jupiter.engine.Constants.EXTENSIONS_AUTODETECT_PROPERTY_NAME;
 import static org.junit.jupiter.engine.extension.ExtensionRegistry.createRegistryWithDefaultExtensions;
 import static org.junit.platform.commons.meta.API.Usage.Internal;
 
@@ -31,7 +32,9 @@ public JupiterEngineDescriptor(UniqueId uniqueId) {
 
 	@Override
 	public JupiterEngineExecutionContext before(JupiterEngineExecutionContext context) {
-		return context.extend().withExtensionRegistry(createRegistryWithDefaultExtensions()).build();
+		boolean autoDetect = Boolean.parseBoolean(
+			context.getConfigurationParameters().get(EXTENSIONS_AUTODETECT_PROPERTY_NAME).orElse("false"));
+		return context.extend().withExtensionRegistry(createRegistryWithDefaultExtensions(autoDetect)).build();
 	}
 
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/ExtensionRegistry.java

@@ -19,6 +19,7 @@
 import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.ServiceLoader;
 import java.util.Set;
 import java.util.logging.Logger;
 import java.util.stream.Stream;
@@ -55,11 +56,17 @@ public class ExtensionRegistry {
 	 * Factory for creating and populating a new root registry with the default
 	 * extensions.
 	 *
+	 * @param autoDetect {@code true} to detect extensions also using
+	 * service loader, {@code false} otherwise
 	 * @return a new {@code ExtensionRegistry}
 	 */
-	public static ExtensionRegistry createRegistryWithDefaultExtensions() {
+	public static ExtensionRegistry createRegistryWithDefaultExtensions(boolean autoDetect) {
 		ExtensionRegistry extensionRegistry = new ExtensionRegistry(null);
 		DEFAULT_EXTENSIONS.forEach(extensionRegistry::registerDefaultExtension);
+		if (autoDetect) {
+			ServiceLoader.load(Extension.class, ReflectionUtils.getDefaultClassLoader()).forEach(
+				extensionRegistry::registerDefaultExtension);
+		}
 		return extensionRegistry;
 	}
 

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/execution/ExecutableInvokerTests.java

@@ -46,7 +46,7 @@ class ExecutableInvokerTests {
 	private Method method;
 
 	private final ExtensionContext extensionContext = mock(ExtensionContext.class);
-	private ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions();
+	private ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions(false);
 
 	@Test
 	void constructorInjection() throws Exception {

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/execution/JUnit5EngineExecutionContextTests.java

@@ -48,7 +48,7 @@ void executionListenerIsHandedOnWhenContextIsExtended() {
 	void extendWithAllAttributes() {
 		ClassBasedContainerExtensionContext extensionContext = new ClassBasedContainerExtensionContext(null, null,
 			null);
-		ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions();
+		ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions(false);
 		TestInstanceProvider testInstanceProvider = mock(TestInstanceProvider.class);
 		JupiterEngineExecutionContext newContext = originalContext.extend() //
 				.withExtensionContext(extensionContext) //
@@ -65,7 +65,7 @@ void extendWithAllAttributes() {
 	void canOverrideAttributeWhenContextIsExtended() {
 		ClassBasedContainerExtensionContext extensionContext = new ClassBasedContainerExtensionContext(null, null,
 			null);
-		ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions();
+		ExtensionRegistry extensionRegistry = ExtensionRegistry.createRegistryWithDefaultExtensions(false);
 		TestInstanceProvider testInstanceProvider = mock(TestInstanceProvider.class);
 		ClassBasedContainerExtensionContext newExtensionContext = new ClassBasedContainerExtensionContext(
 			extensionContext, null, null);

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

@@ -23,6 +23,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.BeforeAllCallback;
 import org.junit.jupiter.api.extension.ContainerExecutionCondition;
 import org.junit.jupiter.api.extension.Extension;
 import org.junit.jupiter.api.extension.ParameterResolver;
@@ -34,7 +35,26 @@
  */
 public class ExtensionRegistryTests {
 
-	private final ExtensionRegistry registry = createRegistryWithDefaultExtensions();
+	private final ExtensionRegistry registry = createRegistryWithDefaultExtensions(false);
+
+	@Test
+	void newRegistryWithoutParentHasDefaultExtensionsUsingServiceLocator() {
+		ExtensionRegistry registry = createRegistryWithDefaultExtensions(true);
+		List<Extension> extensions = registry.getExtensions(Extension.class);
+
+		assertEquals(5, extensions.size());
+		assertExtensionRegistered(registry, DisabledCondition.class);
+		assertExtensionRegistered(registry, RepeatedTestExtension.class);
+		assertExtensionRegistered(registry, TestInfoParameterResolver.class);
+		assertExtensionRegistered(registry, TestReporterParameterResolver.class);
+		assertExtensionRegistered(registry, ServiceLoaderExtension.class);
+
+		assertEquals(2, countExtensions(registry, ParameterResolver.class));
+		assertEquals(1, countExtensions(registry, ContainerExecutionCondition.class));
+		assertEquals(1, countExtensions(registry, TestExecutionCondition.class));
+		assertEquals(1, countExtensions(registry, TestTemplateInvocationContextProvider.class));
+		assertEquals(1, countExtensions(registry, BeforeAllCallback.class));
+	}
 
 	@Test
 	void newRegistryWithoutParentHasDefaultExtensions() {

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

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2015-2017 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 v1.0 which
+ * accompanies this distribution and is available at
+ *
+ * http://www.eclipse.org/legal/epl-v10.html
+ */
+
+package org.junit.jupiter.engine.extension;
+
+import org.junit.jupiter.api.extension.BeforeAllCallback;
+import org.junit.jupiter.api.extension.ContainerExtensionContext;
+
+public class ServiceLoaderExtension implements BeforeAllCallback {
+
+	@Override
+	public void beforeAll(ContainerExtensionContext context) throws Exception {
+	}
+
+}

junit-jupiter-engine/src/test/resources/META-INF/services/org.junit.jupiter.api.extension.Extension

@@ -0,0 +1 @@
+org.junit.jupiter.engine.extension.ServiceLoaderExtension