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/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);
+		}
+	}
+
+}