Introduce @ForkedClassPath for testing unmodified class path

See gh-22710

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/classpath/ForkedClassPath.java

+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.testsupport.classpath;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.junit.jupiter.api.extension.ExtendWith;
+
+/**
+ * Annotation used to fork the classpath. This can be helpful were neither
+ * {@link ClassPathExclusions} or {@link ClassPathOverrides} are needed, but just a copy
+ * of the classpath.
+ *
+ * @author Christoph Dreis
+ * @since 2.4.0
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+@Documented
+@ExtendWith(ModifiedClassPathExtension.class)
+public @interface ForkedClassPath {
+
+}

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/classpath/ModifiedClassPathClassLoader.java

@@ -89,7 +89,14 @@ final class ModifiedClassPathClassLoader extends URLClassLoader {
 
 	private static ModifiedClassPathClassLoader compute(Class<?> testClass) {
 		ClassLoader classLoader = testClass.getClassLoader();
-		return new ModifiedClassPathClassLoader(processUrls(extractUrls(classLoader), testClass),
+		MergedAnnotations annotations = MergedAnnotations.from(testClass,
+				MergedAnnotations.SearchStrategy.TYPE_HIERARCHY);
+		if (annotations.isPresent(ForkedClassPath.class) && (annotations.isPresent(ClassPathOverrides.class)
+				|| annotations.isPresent(ClassPathExclusions.class))) {
+			throw new IllegalStateException("@ForkedClassPath is redundant in combination with either "
+					+ "@ClassPathOverrides or @ClassPathExclusions");
+		}
+		return new ModifiedClassPathClassLoader(processUrls(extractUrls(classLoader), annotations),
 				classLoader.getParent(), classLoader);
 	}
 
@@ -170,9 +177,7 @@ final class ModifiedClassPathClassLoader extends URLClassLoader {
 		}
 	}
 
-	private static URL[] processUrls(URL[] urls, Class<?> testClass) {
-		MergedAnnotations annotations = MergedAnnotations.from(testClass,
-				MergedAnnotations.SearchStrategy.TYPE_HIERARCHY);
+	private static URL[] processUrls(URL[] urls, MergedAnnotations annotations) {
 		ClassPathEntryFilter filter = new ClassPathEntryFilter(annotations.get(ClassPathExclusions.class));
 		List<URL> additionalUrls = getAdditionalUrls(annotations.get(ClassPathOverrides.class));
 		List<URL> processedUrls = new ArrayList<>(additionalUrls);

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/classpath/ModifiedClassPathExtension.java

@@ -35,14 +35,14 @@ import org.springframework.util.ReflectionUtils;
 /**
  * A custom {@link Extension} that runs tests using a modified class path. Entries are
  * excluded from the class path using {@link ClassPathExclusions @ClassPathExclusions} and
- * overridden using {@link ClassPathOverrides @ClassPathOverrides} on the test class. A
- * class loader is created with the customized class path and is used both to load the
- * test class and as the thread context class loader while the test is being run.
+ * overridden using {@link ClassPathOverrides @ClassPathOverrides} on the test class. For
+ * an unchanged copy of the class path {@link ForkedClassPath @ForkedClassPath} can be
+ * used. A class loader is created with the customized class path and is used both to load
+ * the test class and as the thread context class loader while the test is being run.
  *
  * @author Christoph Dreis
- * @since 2.4.0
  */
-public class ModifiedClassPathExtension implements InvocationInterceptor {
+class ModifiedClassPathExtension implements InvocationInterceptor {
 
 	@Override
 	public void interceptBeforeAllMethod(Invocation<Void> invocation,

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/test/java/org/springframework/boot/testsupport/classpath/ModifiedClassPathExtensionForkTests.java

+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.testsupport.classpath;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ForkedClassPath @ForkedClassPath}.
+ *
+ * @author Christoph Dreis
+ */
+@ForkedClassPath
+class ModifiedClassPathExtensionForkTests {
+
+	@Test
+	void modifiedClassLoaderIsUsed() {
+		ClassLoader classLoader = getClass().getClassLoader();
+		assertThat(classLoader.getClass().getName()).isEqualTo(ModifiedClassPathClassLoader.class.getName());
+	}
+
+}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/IgnoringXmlBeanDefinitionLoaderTests.java

@@ -19,15 +19,14 @@ package org.springframework.boot;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.extension.ExtendWith;
 
 import org.springframework.beans.factory.BeanDefinitionStoreException;
-import org.springframework.boot.testsupport.classpath.ModifiedClassPathExtension;
+import org.springframework.boot.testsupport.classpath.ForkedClassPath;
 import org.springframework.context.support.StaticApplicationContext;
 
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
-@ExtendWith(ModifiedClassPathExtension.class)
+@ForkedClassPath
 class IgnoringXmlBeanDefinitionLoaderTests {
 
 	@BeforeAll