Log warning if meta-annotation is ignored due to types not present

Prior to this commit, if a meta-annotation could not be loaded because
its attributes referenced types not present in the classpath, the
meta-annotation was silently ignored.

To improve diagnostics for such use cases, this commit introduces WARN
support in IntrospectionFailureLogger and revises AttributeMethods.canLoad()
to log a warning if a meta-annotation is ignored due to an exception
thrown while attempting to load its attributes.

For example, a warning similar to the following is now logged in such
cases.

WARN o.s.c.a.MergedAnnotation -
  Failed to introspect meta-annotation @example.MyAnnotation on class
  example.Config: java.lang.TypeNotPresentException:
  Type example.OptionalDependency not present

This commit also improves log messages in AnnotationTypeMappings.

Closes gh-35927

spring-core/src/main/java/org/springframework/core/annotation/AnnotationTypeMappings.java

@@ -79,7 +79,7 @@ final class AnnotationTypeMappings {
 	private void addAllMappings(Class<? extends Annotation> annotationType,
 			Set<Class<? extends Annotation>> visitedAnnotationTypes) {
 		Deque<AnnotationTypeMapping> queue = new ArrayDeque<>();
-		addIfPossible(queue, null, annotationType, null, visitedAnnotationTypes);
+		addIfPossible(queue, null, annotationType, null, false, visitedAnnotationTypes);
 		while (!queue.isEmpty()) {
 			AnnotationTypeMapping mapping = queue.removeFirst();
 			this.mappings.add(mapping);
@@ -109,12 +109,12 @@ final class AnnotationTypeMappings {
 	}
 
 	private void addIfPossible(Deque<AnnotationTypeMapping> queue, AnnotationTypeMapping source, Annotation ann) {
-		addIfPossible(queue, source, ann.annotationType(), ann, new HashSet<>());
+		addIfPossible(queue, source, ann.annotationType(), ann, true, new HashSet<>());
 	}
 
 	private void addIfPossible(Deque<AnnotationTypeMapping> queue, @Nullable AnnotationTypeMapping source,
 			Class<? extends Annotation> annotationType, @Nullable Annotation ann,
-			Set<Class<? extends Annotation>> visitedAnnotationTypes) {
+			boolean meta, Set<Class<? extends Annotation>> visitedAnnotationTypes) {
 
 		try {
 			queue.addLast(new AnnotationTypeMapping(source, annotationType, ann, visitedAnnotationTypes));
@@ -122,8 +122,8 @@ final class AnnotationTypeMappings {
 		catch (Exception ex) {
 			AnnotationUtils.rethrowAnnotationConfigurationException(ex);
 			if (failureLogger.isEnabled()) {
-				failureLogger.log("Failed to introspect meta-annotation " + annotationType.getName(),
-						(source != null ? source.getAnnotationType() : null), ex);
+				failureLogger.log("Failed to introspect " + (meta ? "meta-annotation @" : "annotation @") +
+						annotationType.getName(), (source != null ? source.getAnnotationType() : null), ex);
 			}
 		}
 	}

spring-core/src/main/java/org/springframework/core/annotation/AnnotationsScanner.java

@@ -443,7 +443,7 @@ abstract class AnnotationsScanner {
 					Annotation annotation = annotations[i];
 					//noinspection DataFlowIssue
 					if (isIgnorable(annotation.annotationType()) ||
-							!AttributeMethods.forAnnotationType(annotation.annotationType()).canLoad(annotation)) {
+							!AttributeMethods.forAnnotationType(annotation.annotationType()).canLoad(annotation, source)) {
 						annotations[i] = null;
 					}
 					else {

spring-core/src/main/java/org/springframework/core/annotation/AttributeMethods.java

@@ -17,6 +17,7 @@
 package org.springframework.core.annotation;
 
 import java.lang.annotation.Annotation;
+import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Comparator;
@@ -38,6 +39,8 @@ import org.springframework.util.ReflectionUtils;
  */
 final class AttributeMethods {
 
+	private static final IntrospectionFailureLogger failureLogger = IntrospectionFailureLogger.WARN;
+
 	static final AttributeMethods NONE = new AttributeMethods(null, new Method[0]);
 
 	static final Map<Class<? extends Annotation>, AttributeMethods> cache = new ConcurrentReferenceHashMap<>();
@@ -91,10 +94,11 @@ final class AttributeMethods {
 	 * exceptions for {@code Class} values (instead of the more typical early
 	 * {@code Class.getAnnotations() failure} on a regular JVM).
 	 * @param annotation the annotation to check
+	 * @param source the element that the supplied annotation is declared on
 	 * @return {@code true} if all values are present
 	 * @see #validate(Annotation)
 	 */
-	boolean canLoad(Annotation annotation) {
+	boolean canLoad(Annotation annotation, AnnotatedElement source) {
 		assertAnnotation(annotation);
 		for (int i = 0; i < size(); i++) {
 			if (canThrowTypeNotPresentException(i)) {
@@ -107,6 +111,10 @@ final class AttributeMethods {
 				}
 				catch (Throwable ex) {
 					// TypeNotPresentException etc. -> annotation type not actually loadable.
+					if (failureLogger.isEnabled()) {
+						failureLogger.log("Failed to introspect meta-annotation @" +
+								getName(annotation.annotationType()), source, ex);
+					}
 					return false;
 				}
 			}

spring-core/src/main/java/org/springframework/core/annotation/IntrospectionFailureLogger.java

@@ -24,9 +24,10 @@ import org.jspecify.annotations.Nullable;
  * Log facade used to handle annotation introspection failures (in particular
  * {@code TypeNotPresentExceptions}). Allows annotation processing to continue,
  * assuming that when Class attribute values are not resolvable the annotation
- * should effectively disappear.
+ * should effectively be ignored.
  *
  * @author Phillip Webb
+ * @author Sam Brannen
  * @since 5.2
  */
 enum IntrospectionFailureLogger {
@@ -51,13 +52,24 @@ enum IntrospectionFailureLogger {
 		public void log(String message) {
 			getLogger().info(message);
 		}
+	},
+
+	WARN {
+		@Override
+		public boolean isEnabled() {
+			return getLogger().isWarnEnabled();
+		}
+		@Override
+		public void log(String message) {
+			getLogger().warn(message);
+		}
 	};
 
 
 	private static @Nullable Log logger;
 
 
-	void log(String message, @Nullable Object source, Exception ex) {
+	void log(String message, @Nullable Object source, Throwable ex) {
 		String on = (source != null ? " on " + source : "");
 		log(message + on + ": " + ex);
 	}

spring-core/src/test/java/org/springframework/core/annotation/AttributeMethodsTests.java

@@ -112,7 +112,7 @@ class AttributeMethodsTests {
 		ClassValue annotation = mockAnnotation(ClassValue.class);
 		given(annotation.value()).willThrow(TypeNotPresentException.class);
 		AttributeMethods attributes = AttributeMethods.forAnnotationType(annotation.annotationType());
-		assertThat(attributes.canLoad(annotation)).isFalse();
+		assertThat(attributes.canLoad(annotation, getClass())).isFalse();
 	}
 
 	@Test
@@ -121,7 +121,7 @@ class AttributeMethodsTests {
 		ClassValue annotation = mock();
 		given(annotation.value()).willReturn((Class) InputStream.class);
 		AttributeMethods attributes = AttributeMethods.forAnnotationType(annotation.annotationType());
-		assertThat(attributes.canLoad(annotation)).isTrue();
+		assertThat(attributes.canLoad(annotation, getClass())).isTrue();
 	}
 
 	@Test