Check deps only once in MicrometerObservationRegistryTestExecutionListener

Prior to this commit, the required runtime dependencies were checked
via reflection each time an attempt was made to instantiate
MicrometerObservationRegistryTestExecutionListener.

Since it's sufficient to check for the presence of required runtime
dependencies only once, this commit caches the results of the
dependency checks in a static field.

This commit also introduces automated tests for the runtime dependency
checks in MicrometerObservationRegistryTestExecutionListener.

See gh-30747

spring-test/src/main/java/org/springframework/test/context/observation/MicrometerObservationRegistryTestExecutionListener.java

@@ -16,8 +16,6 @@
 
 package org.springframework.test.context.observation;
 
-import java.lang.reflect.Method;
-
 import io.micrometer.observation.ObservationRegistry;
 import io.micrometer.observation.contextpropagation.ObservationThreadLocalAccessor;
 import org.apache.commons.logging.Log;
@@ -27,7 +25,6 @@ import org.springframework.context.ApplicationContext;
 import org.springframework.core.Conventions;
 import org.springframework.test.context.TestContext;
 import org.springframework.test.context.support.AbstractTestExecutionListener;
-import org.springframework.util.ReflectionUtils;
 
 /**
  * {@code TestExecutionListener} which provides support for Micrometer's
@@ -45,11 +42,6 @@ class MicrometerObservationRegistryTestExecutionListener extends AbstractTestExe
 
 	private static final Log logger = LogFactory.getLog(MicrometerObservationRegistryTestExecutionListener.class);
 
-	private static final String THREAD_LOCAL_ACCESSOR_CLASS_NAME = "io.micrometer.context.ThreadLocalAccessor";
-
-	private static final String OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME =
-			"io.micrometer.observation.contextpropagation.ObservationThreadLocalAccessor";
-
 	/**
 	 * Attribute name for a {@link TestContext} attribute which contains the
 	 * {@link ObservationRegistry} that was previously stored in the
@@ -62,41 +54,46 @@ class MicrometerObservationRegistryTestExecutionListener extends AbstractTestExe
 	private static final String PREVIOUS_OBSERVATION_REGISTRY = Conventions.getQualifiedAttributeName(
 			MicrometerObservationRegistryTestExecutionListener.class, "previousObservationRegistry");
 
+	static final String DEPENDENCIES_ERROR_MESSAGE = """
+			MicrometerObservationRegistryTestExecutionListener requires \
+			io.micrometer:micrometer-observation:1.10.8 or higher and \
+			io.micrometer:context-propagation:1.0.3 or higher.""";
 
-	public MicrometerObservationRegistryTestExecutionListener() {
+	static final String THREAD_LOCAL_ACCESSOR_CLASS_NAME = "io.micrometer.context.ThreadLocalAccessor";
+
+	static final String OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME =
+			"io.micrometer.observation.contextpropagation.ObservationThreadLocalAccessor";
+
+	private static final String ERROR_MESSAGE;
+
+	static {
 		// Trigger eager resolution of Micrometer Observation types to ensure that
 		// this listener can be properly skipped when SpringFactoriesLoader attempts
 		// to load it -- for example, if context-propagation and micrometer-observation
 		// are not in the classpath or if the version of ObservationThreadLocalAccessor
 		// present does not include the getObservationRegistry() method.
-		ClassLoader classLoader = getClass().getClassLoader();
-		String errorMessage = """
-				MicrometerObservationRegistryTestExecutionListener requires \
-				io.micrometer:micrometer-observation:1.10.8 or higher and \
-				io.micrometer:context-propagation:1.0.3 or higher.""";
 
+		String errorMessage = null;
+		ClassLoader classLoader = MicrometerObservationRegistryTestExecutionListener.class.getClassLoader();
+		String classToCheck = THREAD_LOCAL_ACCESSOR_CLASS_NAME;
 		try {
-			Class.forName(THREAD_LOCAL_ACCESSOR_CLASS_NAME, false, classLoader);
-			Class<?> clazz = Class.forName(OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME, false, classLoader);
-			Method method = ReflectionUtils.findMethod(clazz, "getObservationRegistry");
-			if (method == null) {
-				// We simulate "class not found" even though it's "method not found", since
-				// the ClassNotFoundException will be processed in the subsequent catch-block.
-				throw new ClassNotFoundException(OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME);
-			}
+			Class.forName(classToCheck, false, classLoader);
+			classToCheck = OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME;
+			Class<?> clazz = Class.forName(classToCheck, false, classLoader);
+			clazz.getMethod("getObservationRegistry");
 		}
 		catch (Throwable ex) {
-			// Ensure that we throw a NoClassDefFoundError so that the exception will be properly
-			// handled in TestContextFailureHandler.
-			// If the original exception was a ClassNotFoundException or NoClassDefFoundError,
-			// we throw a NoClassDefFoundError with an augmented error message and omit the cause.
-			if (ex instanceof ClassNotFoundException || ex instanceof NoClassDefFoundError) {
-				throw new NoClassDefFoundError(ex.getMessage() + ". " + errorMessage);
-			}
-			// Otherwise, we throw a NoClassDefFoundError with the cause initialized.
-			Error error = new NoClassDefFoundError(errorMessage);
-			error.initCause(ex);
-			throw error;
+			errorMessage = classToCheck + ". " + DEPENDENCIES_ERROR_MESSAGE;
+		}
+		ERROR_MESSAGE = errorMessage;
+	}
+
+
+	public MicrometerObservationRegistryTestExecutionListener() {
+		// If required dependencies are missing, throw a NoClassDefFoundError so
+		// that this listener will be properly skipped in TestContextFailureHandler.
+		if (ERROR_MESSAGE != null) {
+			throw new NoClassDefFoundError(ERROR_MESSAGE);
 		}
 	}
 

spring-test/src/test/java/org/springframework/test/context/observation/MicrometerObservationRegistryTestExecutionListenerDependencyTests.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2002-2023 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.test.context.observation;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+import java.util.function.Predicate;
+import java.util.stream.IntStream;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.core.OverridingClassLoader;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatNoException;
+import static org.springframework.test.context.observation.MicrometerObservationRegistryTestExecutionListener.DEPENDENCIES_ERROR_MESSAGE;
+import static org.springframework.test.context.observation.MicrometerObservationRegistryTestExecutionListener.OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME;
+import static org.springframework.test.context.observation.MicrometerObservationRegistryTestExecutionListener.THREAD_LOCAL_ACCESSOR_CLASS_NAME;
+
+/**
+ * Unit tests for {@link MicrometerObservationRegistryTestExecutionListener}
+ * behavior regarding required dependencies.
+ *
+ * @author Sam Brannen
+ * @since 6.0.11
+ */
+class MicrometerObservationRegistryTestExecutionListenerDependencyTests {
+
+	@Test
+	void allDependenciesArePresent() throws Exception {
+		FilteringClassLoader classLoader = new FilteringClassLoader(getClass().getClassLoader(), name -> false);
+		Class<?> listenerClass = classLoader.loadClass(MicrometerObservationRegistryTestExecutionListener.class.getName());
+		// Invoke multiple times to ensure consistency.
+		IntStream.rangeClosed(1, 5).forEach(n -> assertThatNoException().isThrownBy(() -> instantiateListener(listenerClass)));
+	}
+
+	@Test
+	void threadLocalAccessorIsNotPresent() throws Exception {
+		assertNoClassDefFoundErrorIsThrown(THREAD_LOCAL_ACCESSOR_CLASS_NAME);
+	}
+
+	@Test
+	void observationThreadLocalAccessorIsNotPresent() throws Exception {
+		assertNoClassDefFoundErrorIsThrown(OBSERVATION_THREAD_LOCAL_ACCESSOR_CLASS_NAME);
+	}
+
+	private void assertNoClassDefFoundErrorIsThrown(String missingClassName) throws Exception {
+		FilteringClassLoader classLoader = new FilteringClassLoader(getClass().getClassLoader(), missingClassName::equals);
+		Class<?> listenerClass = classLoader.loadClass(MicrometerObservationRegistryTestExecutionListener.class.getName());
+		// Invoke multiple times to ensure the same error message is generated every time.
+		IntStream.rangeClosed(1, 5).forEach(n -> assertExceptionThrown(missingClassName, listenerClass));
+	}
+
+	private void assertExceptionThrown(String missingClassName, Class<?> listenerClass) {
+		assertThatExceptionOfType(InvocationTargetException.class)
+				.isThrownBy(() -> instantiateListener(listenerClass))
+				.havingCause()
+					.isInstanceOf(NoClassDefFoundError.class)
+					.withMessage(missingClassName + ". " + DEPENDENCIES_ERROR_MESSAGE);
+	}
+
+	private void instantiateListener(Class<?> listenerClass) throws Exception {
+		assertThat(listenerClass).isNotNull();
+		Constructor<?> constructor = listenerClass.getDeclaredConstructor();
+		constructor.setAccessible(true);
+		constructor.newInstance();
+	}
+
+
+	static class FilteringClassLoader extends OverridingClassLoader {
+
+		private static final Predicate<? super String> isListenerClass =
+				MicrometerObservationRegistryTestExecutionListener.class.getName()::equals;
+
+		private final Predicate<String> classNameFilter;
+
+
+		FilteringClassLoader(ClassLoader parent, Predicate<String> classNameFilter) {
+			super(parent);
+			this.classNameFilter = classNameFilter;
+		}
+
+		@Override
+		protected boolean isEligibleForOverriding(String className) {
+			return this.classNameFilter.or(isListenerClass).test(className);
+		}
+
+		@Override
+		protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
+			if (this.classNameFilter.test(name)) {
+				throw new ClassNotFoundException(name);
+			}
+			return super.loadClass(name, resolve);
+		}
+	}
+
+}