/*
 * Copyright 2002-2019 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.core.annotation;

import java.lang.annotation.Annotation;
import java.lang.annotation.Repeatable;
import java.lang.reflect.AnnotatedElement;
import java.lang.reflect.Array;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.BridgeMethodResolver;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ConcurrentReferenceHashMap;
import org.springframework.util.ObjectUtils;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

/**
 * General utility methods for working with annotations, handling meta-annotations,
 * bridge methods (which the compiler generates for generic declarations) as well
 * as super methods (for optional <em>annotation inheritance</em>).
 *
 * <p>Note that most of the features of this class are not provided by the
 * JDK's introspection facilities themselves.
 *
 * <p>As a general rule for runtime-retained application annotations (e.g. for
 * transaction control, authorization, or service exposure), always use the
 * lookup methods on this class (e.g. {@link #findAnnotation(Method, Class)} or
 * {@link #getAnnotation(Method, Class)}) instead of the plain annotation lookup
 * methods in the JDK. You can still explicitly choose between a <em>get</em>
 * lookup on the given class level only ({@link #getAnnotation(Method, Class)})
 * and a <em>find</em> lookup in the entire inheritance hierarchy of the given
 * method ({@link #findAnnotation(Method, Class)}).
 *
 * <h3>Terminology</h3>
 * The terms <em>directly present</em>, <em>indirectly present</em>, and
 * <em>present</em> have the same meanings as defined in the class-level
 * javadoc for {@link AnnotatedElement} (in Java 8).
 *
 * <p>An annotation is <em>meta-present</em> on an element if the annotation
 * is declared as a meta-annotation on some other annotation which is
 * <em>present</em> on the element. Annotation {@code A} is <em>meta-present</em>
 * on another annotation if {@code A} is either <em>directly present</em> or
 * <em>meta-present</em> on the other annotation.
 *
 * <h3>Meta-annotation Support</h3>
 * <p>Most {@code find*()} methods and some {@code get*()} methods in this class
 * provide support for finding annotations used as meta-annotations. Consult the
 * javadoc for each method in this class for details. For fine-grained support for
 * meta-annotations with <em>attribute overrides</em> in <em>composed annotations</em>,
 * consider using {@link AnnotatedElementUtils}'s more specific methods instead.
 *
 * <h3>Attribute Aliases</h3>
 * <p>All public methods in this class that return annotations, arrays of
 * annotations, or {@link AnnotationAttributes} transparently support attribute
 * aliases configured via {@link AliasFor @AliasFor}. Consult the various
 * {@code synthesizeAnnotation*(..)} methods for details.
 *
 * <h3>Search Scope</h3>
 * <p>The search algorithms used by methods in this class stop searching for
 * an annotation once the first annotation of the specified type has been
 * found. As a consequence, additional annotations of the specified type will
 * be silently ignored.
 *
 * @author Rob Harrop
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @author Mark Fisher
 * @author Chris Beams
 * @author Phillip Webb
 * @author Oleg Zhurakousky
 * @since 2.0
 * @see AliasFor
 * @see AnnotationAttributes
 * @see AnnotatedElementUtils
 * @see BridgeMethodResolver
 * @see java.lang.reflect.AnnotatedElement#getAnnotations()
 * @see java.lang.reflect.AnnotatedElement#getAnnotation(Class)
 * @see java.lang.reflect.AnnotatedElement#getDeclaredAnnotations()
 */
public abstract class AnnotationUtils {

	/**
	 * The attribute name for annotations with a single element.
	 */
	public static final String VALUE = "value";

	private static final Map<AnnotationCacheKey, Annotation> findAnnotationCache =
			new ConcurrentReferenceHashMap<>(256);

	private static final Map<AnnotationCacheKey, Boolean> metaPresentCache =
			new ConcurrentReferenceHashMap<>(256);

	private static final Map<Class<?>, Set<Method>> annotatedBaseTypeCache =
			new ConcurrentReferenceHashMap<>(256);

	@SuppressWarnings("unused")
	@Deprecated  // just here for older tool versions trying to reflectively clear the cache
	private static final Map<Class<?>, ?> annotatedInterfaceCache = annotatedBaseTypeCache;

	private static final Map<Class<? extends Annotation>, Boolean> synthesizableCache =
			new ConcurrentReferenceHashMap<>(256);

	private static final Map<Class<? extends Annotation>, Map<String, List<String>>> attributeAliasesCache =
			new ConcurrentReferenceHashMap<>(256);

	private static final Map<Class<? extends Annotation>, List<Method>> attributeMethodsCache =
			new ConcurrentReferenceHashMap<>(256);

	private static final Map<Method, AliasDescriptor> aliasDescriptorCache =
			new ConcurrentReferenceHashMap<>(256);

	@Nullable
	private static transient Log logger;


	/**
	 * Get a single {@link Annotation} of {@code annotationType} from the supplied
	 * annotation: either the given annotation itself or a direct meta-annotation
	 * thereof.
	 * <p>Note that this method supports only a single level of meta-annotations.
	 * For support for arbitrary levels of meta-annotations, use one of the
	 * {@code find*()} methods instead.
	 * @param annotation the Annotation to check
	 * @param annotationType the annotation type to look for, both locally and as a meta-annotation
	 * @return the first matching annotation, or {@code null} if not found
	 * @since 4.0
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	public static <A extends Annotation> A getAnnotation(Annotation annotation, Class<A> annotationType) {
		if (annotationType.isInstance(annotation)) {
			return synthesizeAnnotation((A) annotation);
		}
		Class<? extends Annotation> annotatedElement = annotation.annotationType();
		try {
			A metaAnn = annotatedElement.getAnnotation(annotationType);
			return (metaAnn != null ? synthesizeAnnotation(metaAnn, annotatedElement) : null);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotatedElement, ex);
			return null;
		}
	}

	/**
	 * Get a single {@link Annotation} of {@code annotationType} from the supplied
	 * {@link AnnotatedElement}, where the annotation is either <em>present</em> or
	 * <em>meta-present</em> on the {@code AnnotatedElement}.
	 * <p>Note that this method supports only a single level of meta-annotations.
	 * For support for arbitrary levels of meta-annotations, use
	 * {@link #findAnnotation(AnnotatedElement, Class)} instead.
	 * @param annotatedElement the {@code AnnotatedElement} from which to get the annotation
	 * @param annotationType the annotation type to look for, both locally and as a meta-annotation
	 * @return the first matching annotation, or {@code null} if not found
	 * @since 3.1
	 */
	@Nullable
	public static <A extends Annotation> A getAnnotation(AnnotatedElement annotatedElement, Class<A> annotationType) {
		try {
			A annotation = annotatedElement.getAnnotation(annotationType);
			if (annotation == null) {
				for (Annotation metaAnn : annotatedElement.getAnnotations()) {
					annotation = metaAnn.annotationType().getAnnotation(annotationType);
					if (annotation != null) {
						break;
					}
				}
			}
			return (annotation != null ? synthesizeAnnotation(annotation, annotatedElement) : null);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotatedElement, ex);
			return null;
		}
	}

	/**
	 * Get a single {@link Annotation} of {@code annotationType} from the
	 * supplied {@link Method}, where the annotation is either <em>present</em>
	 * or <em>meta-present</em> on the method.
	 * <p>Correctly handles bridge {@link Method Methods} generated by the compiler.
	 * <p>Note that this method supports only a single level of meta-annotations.
	 * For support for arbitrary levels of meta-annotations, use
	 * {@link #findAnnotation(Method, Class)} instead.
	 * @param method the method to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @return the first matching annotation, or {@code null} if not found
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod(Method)
	 * @see #getAnnotation(AnnotatedElement, Class)
	 */
	@Nullable
	public static <A extends Annotation> A getAnnotation(Method method, Class<A> annotationType) {
		Method resolvedMethod = BridgeMethodResolver.findBridgedMethod(method);
		return getAnnotation((AnnotatedElement) resolvedMethod, annotationType);
	}

	/**
	 * Get all {@link Annotation Annotations} that are <em>present</em> on the
	 * supplied {@link AnnotatedElement}.
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * @param annotatedElement the Method, Constructor or Field to retrieve annotations from
	 * @return the annotations found, an empty array, or {@code null} if not
	 * resolvable (e.g. because nested Class values in annotation attributes
	 * failed to resolve at runtime)
	 * @since 4.0.8
	 * @see AnnotatedElement#getAnnotations()
	 */
	@Nullable
	public static Annotation[] getAnnotations(AnnotatedElement annotatedElement) {
		try {
			return synthesizeAnnotationArray(annotatedElement.getAnnotations(), annotatedElement);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotatedElement, ex);
			return null;
		}
	}

	/**
	 * Get all {@link Annotation Annotations} that are <em>present</em> on the
	 * supplied {@link Method}.
	 * <p>Correctly handles bridge {@link Method Methods} generated by the compiler.
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * @param method the Method to retrieve annotations from
	 * @return the annotations found, an empty array, or {@code null} if not
	 * resolvable (e.g. because nested Class values in annotation attributes
	 * failed to resolve at runtime)
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod(Method)
	 * @see AnnotatedElement#getAnnotations()
	 */
	@Nullable
	public static Annotation[] getAnnotations(Method method) {
		try {
			return synthesizeAnnotationArray(BridgeMethodResolver.findBridgedMethod(method).getAnnotations(), method);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(method, ex);
			return null;
		}
	}

	/**
	 * Get the <em>repeatable</em> {@linkplain Annotation annotations} of
	 * {@code annotationType} from the supplied {@link AnnotatedElement}, where
	 * such annotations are either <em>present</em>, <em>indirectly present</em>,
	 * or <em>meta-present</em> on the element.
	 * <p>This method mimics the functionality of Java 8's
	 * {@link java.lang.reflect.AnnotatedElement#getAnnotationsByType(Class)}
	 * with support for automatic detection of a <em>container annotation</em>
	 * declared via @{@link java.lang.annotation.Repeatable} (when running on
	 * Java 8 or higher) and with additional support for meta-annotations.
	 * <p>Handles both single annotations and annotations nested within a
	 * <em>container annotation</em>.
	 * <p>Correctly handles <em>bridge methods</em> generated by the
	 * compiler if the supplied element is a {@link Method}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>present</em> on the supplied element.
	 * @param annotatedElement the element to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @return the annotations found or an empty set (never {@code null})
	 * @since 4.2
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see #getDeclaredRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see AnnotatedElementUtils#getMergedRepeatableAnnotations(AnnotatedElement, Class)
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod
	 * @see java.lang.annotation.Repeatable
	 * @see java.lang.reflect.AnnotatedElement#getAnnotationsByType
	 */
	public static <A extends Annotation> Set<A> getRepeatableAnnotations(AnnotatedElement annotatedElement,
			Class<A> annotationType) {

		return getRepeatableAnnotations(annotatedElement, annotationType, null);
	}

	/**
	 * Get the <em>repeatable</em> {@linkplain Annotation annotations} of
	 * {@code annotationType} from the supplied {@link AnnotatedElement}, where
	 * such annotations are either <em>present</em>, <em>indirectly present</em>,
	 * or <em>meta-present</em> on the element.
	 * <p>This method mimics the functionality of Java 8's
	 * {@link java.lang.reflect.AnnotatedElement#getAnnotationsByType(Class)}
	 * with additional support for meta-annotations.
	 * <p>Handles both single annotations and annotations nested within a
	 * <em>container annotation</em>.
	 * <p>Correctly handles <em>bridge methods</em> generated by the
	 * compiler if the supplied element is a {@link Method}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>present</em> on the supplied element.
	 * @param annotatedElement the element to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @param containerAnnotationType the type of the container that holds
	 * the annotations; may be {@code null} if a container is not supported
	 * or if it should be looked up via @{@link java.lang.annotation.Repeatable}
	 * when running on Java 8 or higher
	 * @return the annotations found or an empty set (never {@code null})
	 * @since 4.2
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class)
	 * @see #getDeclaredRepeatableAnnotations(AnnotatedElement, Class)
	 * @see #getDeclaredRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see AnnotatedElementUtils#getMergedRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod
	 * @see java.lang.annotation.Repeatable
	 * @see java.lang.reflect.AnnotatedElement#getAnnotationsByType
	 */
	public static <A extends Annotation> Set<A> getRepeatableAnnotations(AnnotatedElement annotatedElement,
			Class<A> annotationType, @Nullable Class<? extends Annotation> containerAnnotationType) {

		Set<A> annotations = getDeclaredRepeatableAnnotations(annotatedElement, annotationType, containerAnnotationType);
		if (!annotations.isEmpty()) {
			return annotations;
		}

		if (annotatedElement instanceof Class) {
			Class<?> superclass = ((Class<?>) annotatedElement).getSuperclass();
			if (superclass != null && superclass != Object.class) {
				return getRepeatableAnnotations(superclass, annotationType, containerAnnotationType);
			}
		}

		return getRepeatableAnnotations(annotatedElement, annotationType, containerAnnotationType, false);
	}

	/**
	 * Get the declared <em>repeatable</em> {@linkplain Annotation annotations}
	 * of {@code annotationType} from the supplied {@link AnnotatedElement},
	 * where such annotations are either <em>directly present</em>,
	 * <em>indirectly present</em>, or <em>meta-present</em> on the element.
	 * <p>This method mimics the functionality of Java 8's
	 * {@link java.lang.reflect.AnnotatedElement#getDeclaredAnnotationsByType(Class)}
	 * with support for automatic detection of a <em>container annotation</em>
	 * declared via @{@link java.lang.annotation.Repeatable} (when running on
	 * Java 8 or higher) and with additional support for meta-annotations.
	 * <p>Handles both single annotations and annotations nested within a
	 * <em>container annotation</em>.
	 * <p>Correctly handles <em>bridge methods</em> generated by the
	 * compiler if the supplied element is a {@link Method}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>present</em> on the supplied element.
	 * @param annotatedElement the element to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @return the annotations found or an empty set (never {@code null})
	 * @since 4.2
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class)
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see #getDeclaredRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see AnnotatedElementUtils#getMergedRepeatableAnnotations(AnnotatedElement, Class)
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod
	 * @see java.lang.annotation.Repeatable
	 * @see java.lang.reflect.AnnotatedElement#getDeclaredAnnotationsByType
	 */
	public static <A extends Annotation> Set<A> getDeclaredRepeatableAnnotations(AnnotatedElement annotatedElement,
			Class<A> annotationType) {

		return getDeclaredRepeatableAnnotations(annotatedElement, annotationType, null);
	}

	/**
	 * Get the declared <em>repeatable</em> {@linkplain Annotation annotations}
	 * of {@code annotationType} from the supplied {@link AnnotatedElement},
	 * where such annotations are either <em>directly present</em>,
	 * <em>indirectly present</em>, or <em>meta-present</em> on the element.
	 * <p>This method mimics the functionality of Java 8's
	 * {@link java.lang.reflect.AnnotatedElement#getDeclaredAnnotationsByType(Class)}
	 * with additional support for meta-annotations.
	 * <p>Handles both single annotations and annotations nested within a
	 * <em>container annotation</em>.
	 * <p>Correctly handles <em>bridge methods</em> generated by the
	 * compiler if the supplied element is a {@link Method}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>present</em> on the supplied element.
	 * @param annotatedElement the element to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @param containerAnnotationType the type of the container that holds
	 * the annotations; may be {@code null} if a container is not supported
	 * or if it should be looked up via @{@link java.lang.annotation.Repeatable}
	 * when running on Java 8 or higher
	 * @return the annotations found or an empty set (never {@code null})
	 * @since 4.2
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class)
	 * @see #getRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see #getDeclaredRepeatableAnnotations(AnnotatedElement, Class)
	 * @see AnnotatedElementUtils#getMergedRepeatableAnnotations(AnnotatedElement, Class, Class)
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod
	 * @see java.lang.annotation.Repeatable
	 * @see java.lang.reflect.AnnotatedElement#getDeclaredAnnotationsByType
	 */
	public static <A extends Annotation> Set<A> getDeclaredRepeatableAnnotations(AnnotatedElement annotatedElement,
			Class<A> annotationType, @Nullable Class<? extends Annotation> containerAnnotationType) {

		return getRepeatableAnnotations(annotatedElement, annotationType, containerAnnotationType, true);
	}

	/**
	 * Perform the actual work for {@link #getRepeatableAnnotations(AnnotatedElement, Class, Class)}
	 * and {@link #getDeclaredRepeatableAnnotations(AnnotatedElement, Class, Class)}.
	 * <p>Correctly handles <em>bridge methods</em> generated by the
	 * compiler if the supplied element is a {@link Method}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>present</em> on the supplied element.
	 * @param annotatedElement the element to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @param containerAnnotationType the type of the container that holds
	 * the annotations; may be {@code null} if a container is not supported
	 * or if it should be looked up via @{@link java.lang.annotation.Repeatable}
	 * when running on Java 8 or higher
	 * @param declaredMode {@code true} if only declared annotations (i.e.,
	 * directly or indirectly present) should be considered
	 * @return the annotations found or an empty set (never {@code null})
	 * @since 4.2
	 * @see org.springframework.core.BridgeMethodResolver#findBridgedMethod
	 * @see java.lang.annotation.Repeatable
	 */
	private static <A extends Annotation> Set<A> getRepeatableAnnotations(AnnotatedElement annotatedElement,
			Class<A> annotationType, @Nullable Class<? extends Annotation> containerAnnotationType, boolean declaredMode) {

		try {
			if (annotatedElement instanceof Method) {
				annotatedElement = BridgeMethodResolver.findBridgedMethod((Method) annotatedElement);
			}
			return new AnnotationCollector<>(annotationType, containerAnnotationType, declaredMode).getResult(annotatedElement);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotatedElement, ex);
			return Collections.emptySet();
		}
	}

	/**
	 * Find a single {@link Annotation} of {@code annotationType} on the
	 * supplied {@link AnnotatedElement}.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>directly present</em> on the supplied element.
	 * <p><strong>Warning</strong>: this method operates generically on
	 * annotated elements. In other words, this method does not execute
	 * specialized search algorithms for classes or methods. If you require
	 * the more specific semantics of {@link #findAnnotation(Class, Class)}
	 * or {@link #findAnnotation(Method, Class)}, invoke one of those methods
	 * instead.
	 * @param annotatedElement the {@code AnnotatedElement} on which to find the annotation
	 * @param annotationType the annotation type to look for, both locally and as a meta-annotation
	 * @return the first matching annotation, or {@code null} if not found
	 * @since 4.2
	 */
	@Nullable
	public static <A extends Annotation> A findAnnotation(
			AnnotatedElement annotatedElement, @Nullable Class<A> annotationType) {

		if (annotationType == null) {
			return null;
		}

		// Do NOT store result in the findAnnotationCache since doing so could break
		// findAnnotation(Class, Class) and findAnnotation(Method, Class).
		A ann = findAnnotation(annotatedElement, annotationType, new HashSet<>());
		return (ann != null ? synthesizeAnnotation(ann, annotatedElement) : null);
	}

	/**
	 * Perform the search algorithm for {@link #findAnnotation(AnnotatedElement, Class)}
	 * avoiding endless recursion by tracking which annotations have already
	 * been <em>visited</em>.
	 * @param annotatedElement the {@code AnnotatedElement} on which to find the annotation
	 * @param annotationType the annotation type to look for, both locally and as a meta-annotation
	 * @param visited the set of annotations that have already been visited
	 * @return the first matching annotation, or {@code null} if not found
	 * @since 4.2
	 */
	@Nullable
	private static <A extends Annotation> A findAnnotation(
			AnnotatedElement annotatedElement, Class<A> annotationType, Set<Annotation> visited) {
		try {
			A annotation = annotatedElement.getDeclaredAnnotation(annotationType);
			if (annotation != null) {
				return annotation;
			}
			for (Annotation declaredAnn : annotatedElement.getDeclaredAnnotations()) {
				Class<? extends Annotation> declaredType = declaredAnn.annotationType();
				if (!isInJavaLangAnnotationPackage(declaredType) && visited.add(declaredAnn)) {
					annotation = findAnnotation((AnnotatedElement) declaredType, annotationType, visited);
					if (annotation != null) {
						return annotation;
					}
				}
			}
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotatedElement, ex);
		}
		return null;
	}

	/**
	 * Find a single {@link Annotation} of {@code annotationType} on the supplied
	 * {@link Method}, traversing its super methods (i.e. from superclasses and
	 * interfaces) if the annotation is not <em>directly present</em> on the given
	 * method itself.
	 * <p>Correctly handles bridge {@link Method Methods} generated by the compiler.
	 * <p>Meta-annotations will be searched if the annotation is not
	 * <em>directly present</em> on the method.
	 * <p>Annotations on methods are not inherited by default, so we need to handle
	 * this explicitly.
	 * @param method the method to look for annotations on
	 * @param annotationType the annotation type to look for
	 * @return the first matching annotation, or {@code null} if not found
	 * @see #getAnnotation(Method, Class)
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	public static <A extends Annotation> A findAnnotation(Method method, @Nullable Class<A> annotationType) {
		Assert.notNull(method, "Method must not be null");
		if (annotationType == null) {
			return null;
		}

		AnnotationCacheKey cacheKey = new AnnotationCacheKey(method, annotationType);
		A result = (A) findAnnotationCache.get(cacheKey);

		if (result == null) {
			Method resolvedMethod = BridgeMethodResolver.findBridgedMethod(method);
			result = findAnnotation((AnnotatedElement) resolvedMethod, annotationType);
			if (result == null) {
				result = searchOnInterfaces(method, annotationType, method.getDeclaringClass().getInterfaces());
			}

			Class<?> clazz = method.getDeclaringClass();
			while (result == null) {
				clazz = clazz.getSuperclass();
				if (clazz == null || clazz == Object.class) {
					break;
				}
				Set<Method> annotatedMethods = getAnnotatedMethodsInBaseType(clazz);
				if (!annotatedMethods.isEmpty()) {
					for (Method annotatedMethod : annotatedMethods) {
						if (isOverride(method, annotatedMethod)) {
							Method resolvedSuperMethod = BridgeMethodResolver.findBridgedMethod(annotatedMethod);
							result = findAnnotation((AnnotatedElement) resolvedSuperMethod, annotationType);
							if (result != null) {
								break;
							}
						}
					}
				}
				if (result == null) {
					result = searchOnInterfaces(method, annotationType, clazz.getInterfaces());
				}
			}

			if (result != null) {
				result = synthesizeAnnotation(result, method);
				findAnnotationCache.put(cacheKey, result);
			}
		}

		return result;
	}

	@Nullable
	private static <A extends Annotation> A searchOnInterfaces(Method method, Class<A> annotationType, Class<?>... ifcs) {
		for (Class<?> ifc : ifcs) {
			Set<Method> annotatedMethods = getAnnotatedMethodsInBaseType(ifc);
			if (!annotatedMethods.isEmpty()) {
				for (Method annotatedMethod : annotatedMethods) {
					if (isOverride(method, annotatedMethod)) {
						A annotation = getAnnotation(annotatedMethod, annotationType);
						if (annotation != null) {
							return annotation;
						}
					}
				}
			}
		}
		return null;
	}

	/**
	 * Does the given method override the given candidate method?
	 * @param method the overriding method
	 * @param candidate the potentially overridden method
	 * @since 5.0.8
	 */
	static boolean isOverride(Method method, Method candidate) {
		if (!candidate.getName().equals(method.getName()) ||
				candidate.getParameterCount() != method.getParameterCount()) {
			return false;
		}
		Class<?>[] paramTypes = method.getParameterTypes();
		if (Arrays.equals(candidate.getParameterTypes(), paramTypes)) {
			return true;
		}
		for (int i = 0; i < paramTypes.length; i++) {
			if (paramTypes[i] != ResolvableType.forMethodParameter(candidate, i, method.getDeclaringClass()).resolve()) {
				return false;
			}
		}
		return true;
	}

	/**
	 * Determine the methods on the given type with searchable annotations on them.
	 * @param baseType the superclass or interface to search
	 * @return the cached set of annotated methods
	 * @since 5.0.5
	 */
	static Set<Method> getAnnotatedMethodsInBaseType(Class<?> baseType) {
		boolean ifcCheck = baseType.isInterface();
		if (ifcCheck && ClassUtils.isJavaLanguageInterface(baseType)) {
			return Collections.emptySet();
		}

		Set<Method> annotatedMethods = annotatedBaseTypeCache.get(baseType);
		if (annotatedMethods != null) {
			return annotatedMethods;
		}
		Method[] methods = (ifcCheck ? baseType.getMethods() : baseType.getDeclaredMethods());
		for (Method baseMethod : methods) {
			try {
				// Public methods on interfaces (including interface hierarchy),
				// non-private (and therefore overridable) methods on base classes
				if ((ifcCheck || !Modifier.isPrivate(baseMethod.getModifiers())) &&
						hasSearchableAnnotations(baseMethod)) {
					if (annotatedMethods == null) {
						annotatedMethods = new HashSet<>();
					}
					annotatedMethods.add(baseMethod);
				}
			}
			catch (Throwable ex) {
				handleIntrospectionFailure(baseMethod, ex);
			}
		}
		if (annotatedMethods == null) {
			annotatedMethods = Collections.emptySet();
		}
		annotatedBaseTypeCache.put(baseType, annotatedMethods);
		return annotatedMethods;
	}

	/**
	 * Determine whether the specified method has searchable annotations,
	 * i.e. not just {@code java.lang} or {@code org.springframework.lang}
	 * annotations such as {@link Deprecated} and {@link Nullable}.
	 * @param ifcMethod the interface method to check
	 * @@since 5.0.5
	 */
	private static boolean hasSearchableAnnotations(Method ifcMethod) {
		Annotation[] anns = ifcMethod.getAnnotations();
		if (anns.length == 0) {
			return false;
		}
		for (Annotation ann : anns) {
			Class<?> annType = ann.annotationType();
			if (annType != Nullable.class && annType != Deprecated.class) {
				return true;
			}
		}
		return false;
	}

	/**
	 * Find a single {@link Annotation} of {@code annotationType} on the
	 * supplied {@link Class}, traversing its interfaces, annotations, and
	 * superclasses if the annotation is not <em>directly present</em> on
	 * the given class itself.
	 * <p>This method explicitly handles class-level annotations which are not
	 * declared as {@link java.lang.annotation.Inherited inherited} <em>as well
	 * as meta-annotations and annotations on interfaces</em>.
	 * <p>The algorithm operates as follows:
	 * <ol>
	 * <li>Search for the annotation on the given class and return it if found.
	 * <li>Recursively search through all annotations that the given class declares.
	 * <li>Recursively search through all interfaces that the given class declares.
	 * <li>Recursively search through the superclass hierarchy of the given class.
	 * </ol>
	 * <p>Note: in this context, the term <em>recursively</em> means that the search
	 * process continues by returning to step #1 with the current interface,
	 * annotation, or superclass as the class to look for annotations on.
	 * @param clazz the class to look for annotations on
	 * @param annotationType the type of annotation to look for
	 * @return the first matching annotation, or {@code null} if not found
	 */
	@Nullable
	public static <A extends Annotation> A findAnnotation(Class<?> clazz, @Nullable Class<A> annotationType) {
		return findAnnotation(clazz, annotationType, true);
	}

	/**
	 * 查找指定class对象有没有指定类型的注解信息
	 * @param clazz 待查询的class对象
	 * @param annotationType 注解类型
	 * @param synthesize {@code true} if the result should be
	 * {@linkplain #synthesizeAnnotation(Annotation) synthesized}
	 * @return the first matching annotation, or {@code null} if not found
	 * @since 4.2.1
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	private static <A extends Annotation> A findAnnotation(
			Class<?> clazz, @Nullable Class<A> annotationType, boolean synthesize) {

		Assert.notNull(clazz, "Class must not be null");
		if (annotationType == null) {
			return null;
		}
		//构建注解信息的缓存key
		AnnotationCacheKey cacheKey = new AnnotationCacheKey(clazz, annotationType);
		//先去缓存中获取我们的注解信息
		A result = (A) findAnnotationCache.get(cacheKey);
		//缓存中没有获取到我们的指定的注解信息
		if (result == null) {
			//解析我们的注解信息
			result = findAnnotation(clazz, annotationType, new HashSet<>());
			if (result != null && synthesize) {
				result = synthesizeAnnotation(result, clazz);
				//获取到了加入到缓存中
				findAnnotationCache.put(cacheKey, result);
			}
		}
		return result;
	}

	/**
	 * Perform the search algorithm for {@link #findAnnotation(Class, Class)},
	 * avoiding endless recursion by tracking which annotations have already
	 * been <em>visited</em>.
	 * @param clazz the class to look for annotations on
	 * @param annotationType the type of annotation to look for
	 * @param visited the set of annotations that have already been visited
	 * @return the first matching annotation, or {@code null} if not found
	 */
	@Nullable
	private static <A extends Annotation> A findAnnotation(Class<?> clazz, Class<A> annotationType, Set<Annotation> visited) {
		try {
			//获取我们类上所有的声明的注解信息
			A annotation = clazz.getDeclaredAnnotation(annotationType);
			//在类上获取到了我们的注解信息
			if (annotation != null) {
				return annotation;
			}
			//循环判断我们的组件信息
			for (Annotation declaredAnn : clazz.getDeclaredAnnotations()) {
				//获取组件的类型
				Class<? extends Annotation> declaredType = declaredAnn.annotationType();
				if (!isInJavaLangAnnotationPackage(declaredType) && visited.add(declaredAnn)) {
					//进行递归解析
					annotation = findAnnotation(declaredType, annotationType, visited);
					if (annotation != null) {
						return annotation;
					}
				}
			}
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(clazz, ex);
			return null;
		}

		for (Class<?> ifc : clazz.getInterfaces()) {
			A annotation = findAnnotation(ifc, annotationType, visited);
			if (annotation != null) {
				return annotation;
			}
		}

		Class<?> superclass = clazz.getSuperclass();
		if (superclass == null || superclass == Object.class) {
			return null;
		}
		return findAnnotation(superclass, annotationType, visited);
	}

	/**
	 * Find the first {@link Class} in the inheritance hierarchy of the
	 * specified {@code clazz} (including the specified {@code clazz} itself)
	 * on which an annotation of the specified {@code annotationType} is
	 * <em>directly present</em>.
	 * <p>If the supplied {@code clazz} is an interface, only the interface
	 * itself will be checked; the inheritance hierarchy for interfaces will
	 * not be traversed.
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * <p>The standard {@link Class} API does not provide a mechanism for
	 * determining which class in an inheritance hierarchy actually declares
	 * an {@link Annotation}, so we need to handle this explicitly.
	 * @param annotationType the annotation type to look for
	 * @param clazz the class to check for the annotation on (may be {@code null})
	 * @return the first {@link Class} in the inheritance hierarchy that
	 * declares an annotation of the specified {@code annotationType},
	 * or {@code null} if not found
	 * @see Class#isAnnotationPresent(Class)
	 * @see Class#getDeclaredAnnotations()
	 * @see #findAnnotationDeclaringClassForTypes(List, Class)
	 * @see #isAnnotationDeclaredLocally(Class, Class)
	 */
	@Nullable
	public static Class<?> findAnnotationDeclaringClass(Class<? extends Annotation> annotationType, @Nullable Class<?> clazz) {
		if (clazz == null || clazz == Object.class) {
			return null;
		}
		if (isAnnotationDeclaredLocally(annotationType, clazz)) {
			return clazz;
		}
		return findAnnotationDeclaringClass(annotationType, clazz.getSuperclass());
	}

	/**
	 * Find the first {@link Class} in the inheritance hierarchy of the
	 * specified {@code clazz} (including the specified {@code clazz} itself)
	 * on which at least one of the specified {@code annotationTypes} is
	 * <em>directly present</em>.
	 * <p>If the supplied {@code clazz} is an interface, only the interface
	 * itself will be checked; the inheritance hierarchy for interfaces will
	 * not be traversed.
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * <p>The standard {@link Class} API does not provide a mechanism for
	 * determining which class in an inheritance hierarchy actually declares
	 * one of several candidate {@linkplain Annotation annotations}, so we
	 * need to handle this explicitly.
	 * @param annotationTypes the annotation types to look for
	 * @param clazz the class to check for the annotation on (may be {@code null})
	 * @return the first {@link Class} in the inheritance hierarchy that
	 * declares an annotation of at least one of the specified
	 * {@code annotationTypes}, or {@code null} if not found
	 * @since 3.2.2
	 * @see Class#isAnnotationPresent(Class)
	 * @see Class#getDeclaredAnnotations()
	 * @see #findAnnotationDeclaringClass(Class, Class)
	 * @see #isAnnotationDeclaredLocally(Class, Class)
	 */
	@Nullable
	public static Class<?> findAnnotationDeclaringClassForTypes(
			List<Class<? extends Annotation>> annotationTypes, @Nullable Class<?> clazz) {

		if (clazz == null || clazz == Object.class) {
			return null;
		}
		for (Class<? extends Annotation> annotationType : annotationTypes) {
			if (isAnnotationDeclaredLocally(annotationType, clazz)) {
				return clazz;
			}
		}
		return findAnnotationDeclaringClassForTypes(annotationTypes, clazz.getSuperclass());
	}

	/**
	 * Determine whether an annotation of the specified {@code annotationType}
	 * is declared locally (i.e. <em>directly present</em>) on the supplied
	 * {@code clazz}.
	 * <p>The supplied {@link Class} may represent any type.
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * <p>Note: This method does <strong>not</strong> determine if the annotation
	 * is {@linkplain java.lang.annotation.Inherited inherited}. For greater
	 * clarity regarding inherited annotations, consider using
	 * {@link #isAnnotationInherited(Class, Class)} instead.
	 * @param annotationType the annotation type to look for
	 * @param clazz the class to check for the annotation on
	 * @return {@code true} if an annotation of the specified {@code annotationType}
	 * is <em>directly present</em>
	 * @see java.lang.Class#getDeclaredAnnotations()
	 * @see java.lang.Class#getDeclaredAnnotation(Class)
	 * @see #isAnnotationInherited(Class, Class)
	 */
	public static boolean isAnnotationDeclaredLocally(Class<? extends Annotation> annotationType, Class<?> clazz) {
		try {
			return (clazz.getDeclaredAnnotation(annotationType) != null);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(clazz, ex);
			return false;
		}
	}

	/**
	 * Determine whether an annotation of the specified {@code annotationType}
	 * is <em>present</em> on the supplied {@code clazz} and is
	 * {@linkplain java.lang.annotation.Inherited inherited}
	 * (i.e. not <em>directly present</em>).
	 * <p>Meta-annotations will <em>not</em> be searched.
	 * <p>If the supplied {@code clazz} is an interface, only the interface
	 * itself will be checked. In accordance with standard meta-annotation
	 * semantics in Java, the inheritance hierarchy for interfaces will not
	 * be traversed. See the {@linkplain java.lang.annotation.Inherited javadoc}
	 * for the {@code @Inherited} meta-annotation for further details regarding
	 * annotation inheritance.
	 * @param annotationType the annotation type to look for
	 * @param clazz the class to check for the annotation on
	 * @return {@code true} if an annotation of the specified {@code annotationType}
	 * is <em>present</em> and <em>inherited</em>
	 * @see Class#isAnnotationPresent(Class)
	 * @see #isAnnotationDeclaredLocally(Class, Class)
	 */
	public static boolean isAnnotationInherited(Class<? extends Annotation> annotationType, Class<?> clazz) {
		return (clazz.isAnnotationPresent(annotationType) && !isAnnotationDeclaredLocally(annotationType, clazz));
	}

	/**
	 * Determine if an annotation of type {@code metaAnnotationType} is
	 * <em>meta-present</em> on the supplied {@code annotationType}.
	 * @param annotationType the annotation type to search on
	 * @param metaAnnotationType the type of meta-annotation to search for
	 * @return {@code true} if such an annotation is meta-present
	 * @since 4.2.1
	 */
	public static boolean isAnnotationMetaPresent(Class<? extends Annotation> annotationType,
			@Nullable Class<? extends Annotation> metaAnnotationType) {

		Assert.notNull(annotationType, "Annotation type must not be null");
		if (metaAnnotationType == null) {
			return false;
		}

		AnnotationCacheKey cacheKey = new AnnotationCacheKey(annotationType, metaAnnotationType);
		Boolean metaPresent = metaPresentCache.get(cacheKey);
		if (metaPresent != null) {
			return metaPresent;
		}
		metaPresent = Boolean.FALSE;
		if (findAnnotation(annotationType, metaAnnotationType, false) != null) {
			metaPresent = Boolean.TRUE;
		}
		metaPresentCache.put(cacheKey, metaPresent);
		return metaPresent;
	}

	/**
	 * Determine if the supplied {@link Annotation} is defined in the core JDK
	 * {@code java.lang.annotation} package.
	 * @param annotation the annotation to check
	 * @return {@code true} if the annotation is in the {@code java.lang.annotation} package
	 */
	public static boolean isInJavaLangAnnotationPackage(@Nullable Annotation annotation) {
		return (annotation != null && isInJavaLangAnnotationPackage(annotation.annotationType()));
	}

	/**
	 * Determine if the {@link Annotation} with the supplied name is defined
	 * in the core JDK {@code java.lang.annotation} package.
	 * @param annotationType the annotation type to check
	 * @return {@code true} if the annotation is in the {@code java.lang.annotation} package
	 * @since 4.3.8
	 */
	static boolean isInJavaLangAnnotationPackage(@Nullable Class<? extends Annotation> annotationType) {
		return (annotationType != null && isInJavaLangAnnotationPackage(annotationType.getName()));
	}

	/**
	 * Determine if the {@link Annotation} with the supplied name is defined
	 * in the core JDK {@code java.lang.annotation} package.
	 * @param annotationType the name of the annotation type to check
	 * @return {@code true} if the annotation is in the {@code java.lang.annotation} package
	 * @since 4.2
	 */
	public static boolean isInJavaLangAnnotationPackage(@Nullable String annotationType) {
		return (annotationType != null && annotationType.startsWith("java.lang.annotation"));
	}

	/**
	 * Check the declared attributes of the given annotation, in particular covering
	 * Google App Engine's late arrival of {@code TypeNotPresentExceptionProxy} for
	 * {@code Class} values (instead of early {@code Class.getAnnotations() failure}.
	 * <p>This method not failing indicates that {@link #getAnnotationAttributes(Annotation)}
	 * won't failure either (when attempted later on).
	 * @param annotation the annotation to validate
	 * @throws IllegalStateException if a declared {@code Class} attribute could not be read
	 * @since 4.3.15
	 * @see Class#getAnnotations()
	 * @see #getAnnotationAttributes(Annotation)
	 */
	public static void validateAnnotation(Annotation annotation) {
		for (Method method : getAttributeMethods(annotation.annotationType())) {
			Class<?> returnType = method.getReturnType();
			if (returnType == Class.class || returnType == Class[].class) {
				try {
					method.invoke(annotation);
				}
				catch (Throwable ex) {
					throw new IllegalStateException("Could not obtain annotation attribute value for " + method, ex);
				}
			}
		}
	}

	/**
	 * Retrieve the given annotation's attributes as a {@link Map}, preserving all
	 * attribute types.
	 * <p>Equivalent to calling {@link #getAnnotationAttributes(Annotation, boolean, boolean)}
	 * with the {@code classValuesAsString} and {@code nestedAnnotationsAsMap} parameters
	 * set to {@code false}.
	 * <p>Note: This method actually returns an {@link AnnotationAttributes} instance.
	 * However, the {@code Map} signature has been preserved for binary compatibility.
	 * @param annotation the annotation to retrieve the attributes for
	 * @return the Map of annotation attributes, with attribute names as keys and
	 * corresponding attribute values as values (never {@code null})
	 * @see #getAnnotationAttributes(AnnotatedElement, Annotation)
	 * @see #getAnnotationAttributes(Annotation, boolean, boolean)
	 * @see #getAnnotationAttributes(AnnotatedElement, Annotation, boolean, boolean)
	 */
	public static Map<String, Object> getAnnotationAttributes(Annotation annotation) {
		return getAnnotationAttributes(null, annotation);
	}

	/**
	 * Retrieve the given annotation's attributes as a {@link Map}.
	 * <p>Equivalent to calling {@link #getAnnotationAttributes(Annotation, boolean, boolean)}
	 * with the {@code nestedAnnotationsAsMap} parameter set to {@code false}.
	 * <p>Note: This method actually returns an {@link AnnotationAttributes} instance.
	 * However, the {@code Map} signature has been preserved for binary compatibility.
	 * @param annotation the annotation to retrieve the attributes for
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @return the Map of annotation attributes, with attribute names as keys and
	 * corresponding attribute values as values (never {@code null})
	 * @see #getAnnotationAttributes(Annotation, boolean, boolean)
	 */
	public static Map<String, Object> getAnnotationAttributes(
			Annotation annotation, boolean classValuesAsString) {

		return getAnnotationAttributes(annotation, classValuesAsString, false);
	}

	/**
	 * Retrieve the given annotation's attributes as an {@link AnnotationAttributes} map.
	 * <p>This method provides fully recursive annotation reading capabilities on par with
	 * the reflection-based {@link org.springframework.core.type.StandardAnnotationMetadata}.
	 * @param annotation the annotation to retrieve the attributes for
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @param nestedAnnotationsAsMap whether to convert nested annotations into
	 * {@link AnnotationAttributes} maps (for compatibility with
	 * {@link org.springframework.core.type.AnnotationMetadata}) or to preserve them as
	 * {@code Annotation} instances
	 * @return the annotation attributes (a specialized Map) with attribute names as keys
	 * and corresponding attribute values as values (never {@code null})
	 * @since 3.1.1
	 */
	public static AnnotationAttributes getAnnotationAttributes(
			Annotation annotation, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		return getAnnotationAttributes(null, annotation, classValuesAsString, nestedAnnotationsAsMap);
	}

	/**
	 * Retrieve the given annotation's attributes as an {@link AnnotationAttributes} map.
	 * <p>Equivalent to calling {@link #getAnnotationAttributes(AnnotatedElement, Annotation, boolean, boolean)}
	 * with the {@code classValuesAsString} and {@code nestedAnnotationsAsMap} parameters
	 * set to {@code false}.
	 * @param annotatedElement the element that is annotated with the supplied annotation;
	 * may be {@code null} if unknown
	 * @param annotation the annotation to retrieve the attributes for
	 * @return the annotation attributes (a specialized Map) with attribute names as keys
	 * and corresponding attribute values as values (never {@code null})
	 * @since 4.2
	 * @see #getAnnotationAttributes(AnnotatedElement, Annotation, boolean, boolean)
	 */
	public static AnnotationAttributes getAnnotationAttributes(
			@Nullable AnnotatedElement annotatedElement, Annotation annotation) {

		return getAnnotationAttributes(annotatedElement, annotation, false, false);
	}

	/**
	 * Retrieve the given annotation's attributes as an {@link AnnotationAttributes} map.
	 * <p>This method provides fully recursive annotation reading capabilities on par with
	 * the reflection-based {@link org.springframework.core.type.StandardAnnotationMetadata}.
	 * @param annotatedElement the element that is annotated with the supplied annotation;
	 * may be {@code null} if unknown
	 * @param annotation the annotation to retrieve the attributes for
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @param nestedAnnotationsAsMap whether to convert nested annotations into
	 * {@link AnnotationAttributes} maps (for compatibility with
	 * {@link org.springframework.core.type.AnnotationMetadata}) or to preserve them as
	 * {@code Annotation} instances
	 * @return the annotation attributes (a specialized Map) with attribute names as keys
	 * and corresponding attribute values as values (never {@code null})
	 * @since 4.2
	 */
	public static AnnotationAttributes getAnnotationAttributes(@Nullable AnnotatedElement annotatedElement,
			Annotation annotation, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		return getAnnotationAttributes(
				(Object) annotatedElement, annotation, classValuesAsString, nestedAnnotationsAsMap);
	}

	private static AnnotationAttributes getAnnotationAttributes(@Nullable Object annotatedElement,
			Annotation annotation, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		AnnotationAttributes attributes =
				retrieveAnnotationAttributes(annotatedElement, annotation, classValuesAsString, nestedAnnotationsAsMap);
		postProcessAnnotationAttributes(annotatedElement, attributes, classValuesAsString, nestedAnnotationsAsMap);
		return attributes;
	}

	/**
	 * Retrieve the given annotation's attributes as an {@link AnnotationAttributes} map.
	 * <p>This method provides fully recursive annotation reading capabilities on par with
	 * the reflection-based {@link org.springframework.core.type.StandardAnnotationMetadata}.
	 * <p><strong>NOTE</strong>: This variant of {@code getAnnotationAttributes()} is
	 * only intended for use within the framework. The following special rules apply:
	 * <ol>
	 * <li>Default values will be replaced with default value placeholders.</li>
	 * <li>The resulting, merged annotation attributes should eventually be
	 * {@linkplain #postProcessAnnotationAttributes post-processed} in order to
	 * ensure that placeholders have been replaced by actual default values and
	 * in order to enforce {@code @AliasFor} semantics.</li>
	 * </ol>
	 * @param annotatedElement the element that is annotated with the supplied annotation;
	 * may be {@code null} if unknown
	 * @param annotation the annotation to retrieve the attributes for
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @param nestedAnnotationsAsMap whether to convert nested annotations into
	 * {@link AnnotationAttributes} maps (for compatibility with
	 * {@link org.springframework.core.type.AnnotationMetadata}) or to preserve them as
	 * {@code Annotation} instances
	 * @return the annotation attributes (a specialized Map) with attribute names as keys
	 * and corresponding attribute values as values (never {@code null})
	 * @since 4.2
	 * @see #postProcessAnnotationAttributes
	 */
	static AnnotationAttributes retrieveAnnotationAttributes(@Nullable Object annotatedElement, Annotation annotation,
			boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		Class<? extends Annotation> annotationType = annotation.annotationType();
		AnnotationAttributes attributes = new AnnotationAttributes(annotationType);

		for (Method method : getAttributeMethods(annotationType)) {
			try {
				Object attributeValue = method.invoke(annotation);
				Object defaultValue = method.getDefaultValue();
				if (defaultValue != null && ObjectUtils.nullSafeEquals(attributeValue, defaultValue)) {
					attributeValue = new DefaultValueHolder(defaultValue);
				}
				attributes.put(method.getName(),
						adaptValue(annotatedElement, attributeValue, classValuesAsString, nestedAnnotationsAsMap));
			}
			catch (Throwable ex) {
				if (ex instanceof InvocationTargetException) {
					Throwable targetException = ((InvocationTargetException) ex).getTargetException();
					rethrowAnnotationConfigurationException(targetException);
				}
				throw new IllegalStateException("Could not obtain annotation attribute value for " + method, ex);
			}
		}

		return attributes;
	}

	/**
	 * Adapt the given value according to the given class and nested annotation settings.
	 * <p>Nested annotations will be
	 * {@linkplain #synthesizeAnnotation(Annotation, AnnotatedElement) synthesized}.
	 * @param annotatedElement the element that is annotated, used for contextual
	 * logging; may be {@code null} if unknown
	 * @param value the annotation attribute value
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @param nestedAnnotationsAsMap whether to convert nested annotations into
	 * {@link AnnotationAttributes} maps (for compatibility with
	 * {@link org.springframework.core.type.AnnotationMetadata}) or to preserve them as
	 * {@code Annotation} instances
	 * @return the adapted value, or the original value if no adaptation is needed
	 */
	@Nullable
	static Object adaptValue(@Nullable Object annotatedElement, @Nullable Object value,
			boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		if (classValuesAsString) {
			if (value instanceof Class) {
				return ((Class<?>) value).getName();
			}
			else if (value instanceof Class[]) {
				Class<?>[] clazzArray = (Class<?>[]) value;
				String[] classNames = new String[clazzArray.length];
				for (int i = 0; i < clazzArray.length; i++) {
					classNames[i] = clazzArray[i].getName();
				}
				return classNames;
			}
		}

		if (value instanceof Annotation) {
			Annotation annotation = (Annotation) value;
			if (nestedAnnotationsAsMap) {
				return getAnnotationAttributes(annotatedElement, annotation, classValuesAsString, true);
			}
			else {
				return synthesizeAnnotation(annotation, annotatedElement);
			}
		}

		if (value instanceof Annotation[]) {
			Annotation[] annotations = (Annotation[]) value;
			if (nestedAnnotationsAsMap) {
				AnnotationAttributes[] mappedAnnotations = new AnnotationAttributes[annotations.length];
				for (int i = 0; i < annotations.length; i++) {
					mappedAnnotations[i] =
							getAnnotationAttributes(annotatedElement, annotations[i], classValuesAsString, true);
				}
				return mappedAnnotations;
			}
			else {
				return synthesizeAnnotationArray(annotations, annotatedElement);
			}
		}

		// Fallback
		return value;
	}

	/**
	 * Register the annotation-declared default values for the given attributes,
	 * if available.
	 * @param attributes the annotation attributes to process
	 * @since 4.3.2
	 */
	public static void registerDefaultValues(AnnotationAttributes attributes) {
		// Only do defaults scanning for public annotations; we'd run into
		// IllegalAccessExceptions otherwise, and we don't want to mess with
		// accessibility in a SecurityManager environment.
		Class<? extends Annotation> annotationType = attributes.annotationType();
		if (annotationType != null && Modifier.isPublic(annotationType.getModifiers())) {
			// Check declared default values of attributes in the annotation type.
			for (Method annotationAttribute : getAttributeMethods(annotationType)) {
				String attributeName = annotationAttribute.getName();
				Object defaultValue = annotationAttribute.getDefaultValue();
				if (defaultValue != null && !attributes.containsKey(attributeName)) {
					if (defaultValue instanceof Annotation) {
						defaultValue = getAnnotationAttributes((Annotation) defaultValue, false, true);
					}
					else if (defaultValue instanceof Annotation[]) {
						Annotation[] realAnnotations = (Annotation[]) defaultValue;
						AnnotationAttributes[] mappedAnnotations = new AnnotationAttributes[realAnnotations.length];
						for (int i = 0; i < realAnnotations.length; i++) {
							mappedAnnotations[i] = getAnnotationAttributes(realAnnotations[i], false, true);
						}
						defaultValue = mappedAnnotations;
					}
					attributes.put(attributeName, new DefaultValueHolder(defaultValue));
				}
			}
		}
	}

	/**
	 * Post-process the supplied {@link AnnotationAttributes}, preserving nested
	 * annotations as {@code Annotation} instances.
	 * <p>Specifically, this method enforces <em>attribute alias</em> semantics
	 * for annotation attributes that are annotated with {@link AliasFor @AliasFor}
	 * and replaces default value placeholders with their original default values.
	 * @param annotatedElement the element that is annotated with an annotation or
	 * annotation hierarchy from which the supplied attributes were created;
	 * may be {@code null} if unknown
	 * @param attributes the annotation attributes to post-process
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @since 4.3.2
	 * @see #postProcessAnnotationAttributes(Object, AnnotationAttributes, boolean, boolean)
	 * @see #getDefaultValue(Class, String)
	 */
	public static void postProcessAnnotationAttributes(@Nullable Object annotatedElement,
			AnnotationAttributes attributes, boolean classValuesAsString) {

		postProcessAnnotationAttributes(annotatedElement, attributes, classValuesAsString, false);
	}

	/**
	 * Post-process the supplied {@link AnnotationAttributes}.
	 * <p>Specifically, this method enforces <em>attribute alias</em> semantics
	 * for annotation attributes that are annotated with {@link AliasFor @AliasFor}
	 * and replaces default value placeholders with their original default values.
	 * @param annotatedElement the element that is annotated with an annotation or
	 * annotation hierarchy from which the supplied attributes were created;
	 * may be {@code null} if unknown
	 * @param attributes the annotation attributes to post-process
	 * @param classValuesAsString whether to convert Class references into Strings (for
	 * compatibility with {@link org.springframework.core.type.AnnotationMetadata})
	 * or to preserve them as Class references
	 * @param nestedAnnotationsAsMap whether to convert nested annotations into
	 * {@link AnnotationAttributes} maps (for compatibility with
	 * {@link org.springframework.core.type.AnnotationMetadata}) or to preserve them as
	 * {@code Annotation} instances
	 * @since 4.2
	 * @see #retrieveAnnotationAttributes(Object, Annotation, boolean, boolean)
	 * @see #getDefaultValue(Class, String)
	 */
	static void postProcessAnnotationAttributes(@Nullable Object annotatedElement,
			@Nullable AnnotationAttributes attributes, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {

		if (attributes == null) {
			return;
		}

		Class<? extends Annotation> annotationType = attributes.annotationType();

		// Track which attribute values have already been replaced so that we can short
		// circuit the search algorithms.
		Set<String> valuesAlreadyReplaced = new HashSet<>();

		if (!attributes.validated) {
			// Validate @AliasFor configuration
			Map<String, List<String>> aliasMap = getAttributeAliasMap(annotationType);
			aliasMap.forEach((attributeName, aliasedAttributeNames) -> {
				if (valuesAlreadyReplaced.contains(attributeName)) {
					return;
				}
				Object value = attributes.get(attributeName);
				boolean valuePresent = (value != null && !(value instanceof DefaultValueHolder));
				for (String aliasedAttributeName : aliasedAttributeNames) {
					if (valuesAlreadyReplaced.contains(aliasedAttributeName)) {
						continue;
					}
					Object aliasedValue = attributes.get(aliasedAttributeName);
					boolean aliasPresent = (aliasedValue != null && !(aliasedValue instanceof DefaultValueHolder));
					// Something to validate or replace with an alias?
					if (valuePresent || aliasPresent) {
						if (valuePresent && aliasPresent) {
							// Since annotation attributes can be arrays, we must use ObjectUtils.nullSafeEquals().
							if (!ObjectUtils.nullSafeEquals(value, aliasedValue)) {
								String elementAsString =
										(annotatedElement != null ? annotatedElement.toString() : "unknown element");
								throw new AnnotationConfigurationException(String.format(
										"In AnnotationAttributes for annotation [%s] declared on %s, " +
										"attribute '%s' and its alias '%s' are declared with values of [%s] and [%s], " +
										"but only one is permitted.", attributes.displayName, elementAsString,
										attributeName, aliasedAttributeName, ObjectUtils.nullSafeToString(value),
										ObjectUtils.nullSafeToString(aliasedValue)));
							}
						}
						else if (aliasPresent) {
							// Replace value with aliasedValue
							attributes.put(attributeName,
									adaptValue(annotatedElement, aliasedValue, classValuesAsString, nestedAnnotationsAsMap));
							valuesAlreadyReplaced.add(attributeName);
						}
						else {
							// Replace aliasedValue with value
							attributes.put(aliasedAttributeName,
									adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
							valuesAlreadyReplaced.add(aliasedAttributeName);
						}
					}
				}
			});
			attributes.validated = true;
		}

		// Replace any remaining placeholders with actual default values
		for (String attributeName : attributes.keySet()) {
			if (valuesAlreadyReplaced.contains(attributeName)) {
				continue;
			}
			Object value = attributes.get(attributeName);
			if (value instanceof DefaultValueHolder) {
				value = ((DefaultValueHolder) value).defaultValue;
				attributes.put(attributeName,
						adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
			}
		}
	}

	/**
	 * Retrieve the <em>value</em> of the {@code value} attribute of a
	 * single-element Annotation, given an annotation instance.
	 * @param annotation the annotation instance from which to retrieve the value
	 * @return the attribute value, or {@code null} if not found unless the attribute
	 * value cannot be retrieved due to an {@link AnnotationConfigurationException},
	 * in which case such an exception will be rethrown
	 * @see #getValue(Annotation, String)
	 */
	@Nullable
	public static Object getValue(Annotation annotation) {
		return getValue(annotation, VALUE);
	}

	/**
	 * Retrieve the <em>value</em> of a named attribute, given an annotation instance.
	 * @param annotation the annotation instance from which to retrieve the value
	 * @param attributeName the name of the attribute value to retrieve
	 * @return the attribute value, or {@code null} if not found unless the attribute
	 * value cannot be retrieved due to an {@link AnnotationConfigurationException},
	 * in which case such an exception will be rethrown
	 * @see #getValue(Annotation)
	 * @see #rethrowAnnotationConfigurationException(Throwable)
	 */
	@Nullable
	public static Object getValue(@Nullable Annotation annotation, @Nullable String attributeName) {
		if (annotation == null || !StringUtils.hasText(attributeName)) {
			return null;
		}
		try {
			Method method = annotation.annotationType().getDeclaredMethod(attributeName);
			ReflectionUtils.makeAccessible(method);
			return method.invoke(annotation);
		}
		catch (NoSuchMethodException ex) {
			return null;
		}
		catch (InvocationTargetException ex) {
			rethrowAnnotationConfigurationException(ex.getTargetException());
			throw new IllegalStateException(
					"Could not obtain value for annotation attribute '" + attributeName + "' in " + annotation, ex);
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotation.getClass(), ex);
			return null;
		}
	}

	/**
	 * Retrieve the <em>default value</em> of the {@code value} attribute
	 * of a single-element Annotation, given an annotation instance.
	 * @param annotation the annotation instance from which to retrieve the default value
	 * @return the default value, or {@code null} if not found
	 * @see #getDefaultValue(Annotation, String)
	 */
	@Nullable
	public static Object getDefaultValue(Annotation annotation) {
		return getDefaultValue(annotation, VALUE);
	}

	/**
	 * Retrieve the <em>default value</em> of a named attribute, given an annotation instance.
	 * @param annotation the annotation instance from which to retrieve the default value
	 * @param attributeName the name of the attribute value to retrieve
	 * @return the default value of the named attribute, or {@code null} if not found
	 * @see #getDefaultValue(Class, String)
	 */
	@Nullable
	public static Object getDefaultValue(@Nullable Annotation annotation, @Nullable String attributeName) {
		return (annotation != null ? getDefaultValue(annotation.annotationType(), attributeName) : null);
	}

	/**
	 * Retrieve the <em>default value</em> of the {@code value} attribute
	 * of a single-element Annotation, given the {@link Class annotation type}.
	 * @param annotationType the <em>annotation type</em> for which the default value should be retrieved
	 * @return the default value, or {@code null} if not found
	 * @see #getDefaultValue(Class, String)
	 */
	@Nullable
	public static Object getDefaultValue(Class<? extends Annotation> annotationType) {
		return getDefaultValue(annotationType, VALUE);
	}

	/**
	 * Retrieve the <em>default value</em> of a named attribute, given the
	 * {@link Class annotation type}.
	 * @param annotationType the <em>annotation type</em> for which the default value should be retrieved
	 * @param attributeName the name of the attribute value to retrieve.
	 * @return the default value of the named attribute, or {@code null} if not found
	 * @see #getDefaultValue(Annotation, String)
	 */
	@Nullable
	public static Object getDefaultValue(
			@Nullable Class<? extends Annotation> annotationType, @Nullable String attributeName) {

		if (annotationType == null || !StringUtils.hasText(attributeName)) {
			return null;
		}
		try {
			return annotationType.getDeclaredMethod(attributeName).getDefaultValue();
		}
		catch (Throwable ex) {
			handleIntrospectionFailure(annotationType, ex);
			return null;
		}
	}

	/**
	 * <em>Synthesize</em> an annotation from the supplied {@code annotation}
	 * by wrapping it in a dynamic proxy that transparently enforces
	 * <em>attribute alias</em> semantics for annotation attributes that are
	 * annotated with {@link AliasFor @AliasFor}.
	 * @param annotation the annotation to synthesize
	 * @return the synthesized annotation, if the supplied annotation is
	 * <em>synthesizable</em>; {@code null} if the supplied annotation is
	 * {@code null}; otherwise, the supplied annotation unmodified
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #synthesizeAnnotation(Annotation, AnnotatedElement)
	 */
	static <A extends Annotation> A synthesizeAnnotation(A annotation) {
		return synthesizeAnnotation(annotation, null);
	}

	/**
	 * <em>Synthesize</em> an annotation from the supplied {@code annotation}
	 * by wrapping it in a dynamic proxy that transparently enforces
	 * <em>attribute alias</em> semantics for annotation attributes that are
	 * annotated with {@link AliasFor @AliasFor}.
	 * @param annotation the annotation to synthesize
	 * @param annotatedElement the element that is annotated with the supplied
	 * annotation; may be {@code null} if unknown
	 * @return the synthesized annotation if the supplied annotation is
	 * <em>synthesizable</em>; {@code null} if the supplied annotation is
	 * {@code null}; otherwise the supplied annotation unmodified
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #synthesizeAnnotation(Map, Class, AnnotatedElement)
	 * @see #synthesizeAnnotation(Class)
	 */
	public static <A extends Annotation> A synthesizeAnnotation(
			A annotation, @Nullable AnnotatedElement annotatedElement) {

		return synthesizeAnnotation(annotation, (Object) annotatedElement);
	}

	@SuppressWarnings("unchecked")
	static <A extends Annotation> A synthesizeAnnotation(A annotation, @Nullable Object annotatedElement) {
		if (annotation instanceof SynthesizedAnnotation) {
			return annotation;
		}

		Class<? extends Annotation> annotationType = annotation.annotationType();
		if (!isSynthesizable(annotationType)) {
			return annotation;
		}

		DefaultAnnotationAttributeExtractor attributeExtractor =
				new DefaultAnnotationAttributeExtractor(annotation, annotatedElement);
		InvocationHandler handler = new SynthesizedAnnotationInvocationHandler(attributeExtractor);

		// Can always expose Spring's SynthesizedAnnotation marker since we explicitly check for a
		// synthesizable annotation before (which needs to declare @AliasFor from the same package)
		Class<?>[] exposedInterfaces = new Class<?>[] {annotationType, SynthesizedAnnotation.class};
		return (A) Proxy.newProxyInstance(annotation.getClass().getClassLoader(), exposedInterfaces, handler);
	}

	/**
	 * <em>Synthesize</em> an annotation from the supplied map of annotation
	 * attributes by wrapping the map in a dynamic proxy that implements an
	 * annotation of the specified {@code annotationType} and transparently
	 * enforces <em>attribute alias</em> semantics for annotation attributes
	 * that are annotated with {@link AliasFor @AliasFor}.
	 * <p>The supplied map must contain a key-value pair for every attribute
	 * defined in the supplied {@code annotationType} that is not aliased or
	 * does not have a default value. Nested maps and nested arrays of maps
	 * will be recursively synthesized into nested annotations or nested
	 * arrays of annotations, respectively.
	 * <p>Note that {@link AnnotationAttributes} is a specialized type of
	 * {@link Map} that is an ideal candidate for this method's
	 * {@code attributes} argument.
	 * @param attributes the map of annotation attributes to synthesize
	 * @param annotationType the type of annotation to synthesize
	 * @param annotatedElement the element that is annotated with the annotation
	 * corresponding to the supplied attributes; may be {@code null} if unknown
	 * @return the synthesized annotation
	 * @throws IllegalArgumentException if a required attribute is missing or if an
	 * attribute is not of the correct type
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #synthesizeAnnotation(Annotation, AnnotatedElement)
	 * @see #synthesizeAnnotation(Class)
	 * @see #getAnnotationAttributes(AnnotatedElement, Annotation)
	 * @see #getAnnotationAttributes(AnnotatedElement, Annotation, boolean, boolean)
	 */
	@SuppressWarnings("unchecked")
	public static <A extends Annotation> A synthesizeAnnotation(Map<String, Object> attributes,
			Class<A> annotationType, @Nullable AnnotatedElement annotatedElement) {

		MapAnnotationAttributeExtractor attributeExtractor =
				new MapAnnotationAttributeExtractor(attributes, annotationType, annotatedElement);
		InvocationHandler handler = new SynthesizedAnnotationInvocationHandler(attributeExtractor);
		Class<?>[] exposedInterfaces = (canExposeSynthesizedMarker(annotationType) ?
				new Class<?>[] {annotationType, SynthesizedAnnotation.class} : new Class<?>[] {annotationType});
		return (A) Proxy.newProxyInstance(annotationType.getClassLoader(), exposedInterfaces, handler);
	}

	/**
	 * <em>Synthesize</em> an annotation from its default attributes values.
	 * <p>This method simply delegates to
	 * {@link #synthesizeAnnotation(Map, Class, AnnotatedElement)},
	 * supplying an empty map for the source attribute values and {@code null}
	 * for the {@link AnnotatedElement}.
	 * @param annotationType the type of annotation to synthesize
	 * @return the synthesized annotation
	 * @throws IllegalArgumentException if a required attribute is missing
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #synthesizeAnnotation(Map, Class, AnnotatedElement)
	 * @see #synthesizeAnnotation(Annotation, AnnotatedElement)
	 */
	public static <A extends Annotation> A synthesizeAnnotation(Class<A> annotationType) {
		return synthesizeAnnotation(Collections.emptyMap(), annotationType, null);
	}

	/**
	 * <em>Synthesize</em> an array of annotations from the supplied array
	 * of {@code annotations} by creating a new array of the same size and
	 * type and populating it with {@linkplain #synthesizeAnnotation(Annotation)
	 * synthesized} versions of the annotations from the input array.
	 * @param annotations the array of annotations to synthesize
	 * @param annotatedElement the element that is annotated with the supplied
	 * array of annotations; may be {@code null} if unknown
	 * @return a new array of synthesized annotations, or {@code null} if
	 * the supplied array is {@code null}
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #synthesizeAnnotation(Annotation, AnnotatedElement)
	 * @see #synthesizeAnnotation(Map, Class, AnnotatedElement)
	 */
	static Annotation[] synthesizeAnnotationArray(Annotation[] annotations, @Nullable Object annotatedElement) {
		Annotation[] synthesized = (Annotation[]) Array.newInstance(
				annotations.getClass().getComponentType(), annotations.length);
		for (int i = 0; i < annotations.length; i++) {
			synthesized[i] = synthesizeAnnotation(annotations[i], annotatedElement);
		}
		return synthesized;
	}

	/**
	 * <em>Synthesize</em> an array of annotations from the supplied array
	 * of {@code maps} of annotation attributes by creating a new array of
	 * {@code annotationType} with the same size and populating it with
	 * {@linkplain #synthesizeAnnotation(Map, Class, AnnotatedElement)
	 * synthesized} versions of the maps from the input array.
	 * @param maps the array of maps of annotation attributes to synthesize
	 * @param annotationType the type of annotations to synthesize
	 * (never {@code null})
	 * @return a new array of synthesized annotations, or {@code null} if
	 * the supplied array is {@code null}
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2.1
	 * @see #synthesizeAnnotation(Map, Class, AnnotatedElement)
	 * @see #synthesizeAnnotationArray(Annotation[], Object)
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	static <A extends Annotation> A[] synthesizeAnnotationArray(
			@Nullable Map<String, Object>[] maps, Class<A> annotationType) {

		if (maps == null) {
			return null;
		}

		A[] synthesized = (A[]) Array.newInstance(annotationType, maps.length);
		for (int i = 0; i < maps.length; i++) {
			synthesized[i] = synthesizeAnnotation(maps[i], annotationType, null);
		}
		return synthesized;
	}

	/**
	 * Get a map of all attribute aliases declared via {@code @AliasFor}
	 * in the supplied annotation type.
	 * <p>The map is keyed by attribute name with each value representing
	 * a list of names of aliased attributes.
	 * <p>For <em>explicit</em> alias pairs such as x and y (i.e. where x
	 * is an {@code @AliasFor("y")} and y is an {@code @AliasFor("x")}, there
	 * will be two entries in the map: {@code x -> (y)} and {@code y -> (x)}.
	 * <p>For <em>implicit</em> aliases (i.e. attributes that are declared
	 * as attribute overrides for the same attribute in the same meta-annotation),
	 * there will be n entries in the map. For example, if x, y, and z are
	 * implicit aliases, the map will contain the following entries:
	 * {@code x -> (y, z)}, {@code y -> (x, z)}, {@code z -> (x, y)}.
	 * <p>An empty return value implies that the annotation does not declare
	 * any attribute aliases.
	 * @param annotationType the annotation type to find attribute aliases in
	 * @return a map containing attribute aliases (never {@code null})
	 * @since 4.2
	 */
	static Map<String, List<String>> getAttributeAliasMap(@Nullable Class<? extends Annotation> annotationType) {
		if (annotationType == null) {
			return Collections.emptyMap();
		}

		Map<String, List<String>> map = attributeAliasesCache.get(annotationType);
		if (map != null) {
			return map;
		}

		map = new LinkedHashMap<>();
		for (Method attribute : getAttributeMethods(annotationType)) {
			List<String> aliasNames = getAttributeAliasNames(attribute);
			if (!aliasNames.isEmpty()) {
				map.put(attribute.getName(), aliasNames);
			}
		}

		attributeAliasesCache.put(annotationType, map);
		return map;
	}

	/**
	 * Check whether we can expose our {@link SynthesizedAnnotation} marker for the given annotation type.
	 * @param annotationType the annotation type that we are about to create a synthesized proxy for
	 */
	private static boolean canExposeSynthesizedMarker(Class<? extends Annotation> annotationType) {
		try {
			return (Class.forName(SynthesizedAnnotation.class.getName(), false, annotationType.getClassLoader()) ==
					SynthesizedAnnotation.class);
		}
		catch (ClassNotFoundException ex) {
			return false;
		}
	}

	/**
	 * Determine if annotations of the supplied {@code annotationType} are
	 * <em>synthesizable</em> (i.e. in need of being wrapped in a dynamic
	 * proxy that provides functionality above that of a standard JDK
	 * annotation).
	 * <p>Specifically, an annotation is <em>synthesizable</em> if it declares
	 * any attributes that are configured as <em>aliased pairs</em> via
	 * {@link AliasFor @AliasFor} or if any nested annotations used by the
	 * annotation declare such <em>aliased pairs</em>.
	 * @since 4.2
	 * @see SynthesizedAnnotation
	 * @see SynthesizedAnnotationInvocationHandler
	 */
	@SuppressWarnings("unchecked")
	private static boolean isSynthesizable(Class<? extends Annotation> annotationType) {
		if (isInJavaLangAnnotationPackage(annotationType)) {
			return false;
		}

		Boolean synthesizable = synthesizableCache.get(annotationType);
		if (synthesizable != null) {
			return synthesizable;
		}

		synthesizable = Boolean.FALSE;
		for (Method attribute : getAttributeMethods(annotationType)) {
			if (!getAttributeAliasNames(attribute).isEmpty()) {
				synthesizable = Boolean.TRUE;
				break;
			}
			Class<?> returnType = attribute.getReturnType();
			if (Annotation[].class.isAssignableFrom(returnType)) {
				Class<? extends Annotation> nestedAnnotationType =
						(Class<? extends Annotation>) returnType.getComponentType();
				if (isSynthesizable(nestedAnnotationType)) {
					synthesizable = Boolean.TRUE;
					break;
				}
			}
			else if (Annotation.class.isAssignableFrom(returnType)) {
				Class<? extends Annotation> nestedAnnotationType = (Class<? extends Annotation>) returnType;
				if (isSynthesizable(nestedAnnotationType)) {
					synthesizable = Boolean.TRUE;
					break;
				}
			}
		}

		synthesizableCache.put(annotationType, synthesizable);
		return synthesizable;
	}

	/**
	 * Get the names of the aliased attributes configured via
	 * {@link AliasFor @AliasFor} for the supplied annotation {@code attribute}.
	 * @param attribute the attribute to find aliases for
	 * @return the names of the aliased attributes (never {@code null}, though
	 * potentially <em>empty</em>)
	 * @throws IllegalArgumentException if the supplied attribute method is
	 * {@code null} or not from an annotation
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 * @see #getAttributeOverrideName(Method, Class)
	 */
	static List<String> getAttributeAliasNames(Method attribute) {
		AliasDescriptor descriptor = AliasDescriptor.from(attribute);
		return (descriptor != null ? descriptor.getAttributeAliasNames() : Collections.emptyList());
	}

	/**
	 * Get the name of the overridden attribute configured via
	 * {@link AliasFor @AliasFor} for the supplied annotation {@code attribute}.
	 * @param attribute the attribute from which to retrieve the override
	 * (never {@code null})
	 * @param metaAnnotationType the type of meta-annotation in which the
	 * overridden attribute is allowed to be declared
	 * @return the name of the overridden attribute, or {@code null} if not
	 * found or not applicable for the specified meta-annotation type
	 * @throws IllegalArgumentException if the supplied attribute method is
	 * {@code null} or not from an annotation, or if the supplied meta-annotation
	 * type is {@code null} or {@link Annotation}
	 * @throws AnnotationConfigurationException if invalid configuration of
	 * {@code @AliasFor} is detected
	 * @since 4.2
	 */
	@Nullable
	static String getAttributeOverrideName(Method attribute, @Nullable Class<? extends Annotation> metaAnnotationType) {
		AliasDescriptor descriptor = AliasDescriptor.from(attribute);
		return (descriptor != null && metaAnnotationType != null ?
				descriptor.getAttributeOverrideName(metaAnnotationType) : null);
	}

	/**
	 * Get all methods declared in the supplied {@code annotationType} that
	 * match Java's requirements for annotation <em>attributes</em>.
	 * <p>All methods in the returned list will be
	 * {@linkplain ReflectionUtils#makeAccessible(Method) made accessible}.
	 * @param annotationType the type in which to search for attribute methods
	 * (never {@code null})
	 * @return all annotation attribute methods in the specified annotation
	 * type (never {@code null}, though potentially <em>empty</em>)
	 * @since 4.2
	 */
	static List<Method> getAttributeMethods(Class<? extends Annotation> annotationType) {
		List<Method> methods = attributeMethodsCache.get(annotationType);
		if (methods != null) {
			return methods;
		}

		methods = new ArrayList<>();
		for (Method method : annotationType.getDeclaredMethods()) {
			if (isAttributeMethod(method)) {
				ReflectionUtils.makeAccessible(method);
				methods.add(method);
			}
		}

		attributeMethodsCache.put(annotationType, methods);
		return methods;
	}

	/**
	 * Get the annotation with the supplied {@code annotationName} on the
	 * supplied {@code element}.
	 * @param element the element to search on
	 * @param annotationName the fully qualified class name of the annotation
	 * type to find
	 * @return the annotation if found; {@code null} otherwise
	 * @since 4.2
	 */
	@Nullable
	static Annotation getAnnotation(AnnotatedElement element, String annotationName) {
		for (Annotation annotation : element.getAnnotations()) {
			if (annotation.annotationType().getName().equals(annotationName)) {
				return annotation;
			}
		}
		return null;
	}

	/**
	 * Determine if the supplied {@code method} is an annotation attribute method.
	 * @param method the method to check
	 * @return {@code true} if the method is an attribute method
	 * @since 4.2
	 */
	static boolean isAttributeMethod(@Nullable Method method) {
		return (method != null && method.getParameterCount() == 0 && method.getReturnType() != void.class);
	}

	/**
	 * Determine if the supplied method is an "annotationType" method.
	 * @return {@code true} if the method is an "annotationType" method
	 * @since 4.2
	 * @see Annotation#annotationType()
	 */
	static boolean isAnnotationTypeMethod(@Nullable Method method) {
		return (method != null && method.getName().equals("annotationType") && method.getParameterCount() == 0);
	}

	/**
	 * Resolve the container type for the supplied repeatable {@code annotationType}.
	 * <p>Automatically detects a <em>container annotation</em> declared via
	 * {@link java.lang.annotation.Repeatable}. If the supplied annotation type
	 * is not annotated with {@code @Repeatable}, this method simply returns
	 * {@code null}.
	 * @since 4.2
	 */
	@Nullable
	static Class<? extends Annotation> resolveContainerAnnotationType(Class<? extends Annotation> annotationType) {
		Repeatable repeatable = getAnnotation(annotationType, Repeatable.class);
		return (repeatable != null ? repeatable.value() : null);
	}

	/**
	 * If the supplied throwable is an {@link AnnotationConfigurationException},
	 * it will be cast to an {@code AnnotationConfigurationException} and thrown,
	 * allowing it to propagate to the caller.
	 * <p>Otherwise, this method does nothing.
	 * @param ex the throwable to inspect
	 * @since 4.2
	 */
	static void rethrowAnnotationConfigurationException(Throwable ex) {
		if (ex instanceof AnnotationConfigurationException) {
			throw (AnnotationConfigurationException) ex;
		}
	}

	/**
	 * Handle the supplied annotation introspection exception.
	 * <p>If the supplied exception is an {@link AnnotationConfigurationException},
	 * it will simply be thrown, allowing it to propagate to the caller, and
	 * nothing will be logged.
	 * <p>Otherwise, this method logs an introspection failure (in particular
	 * {@code TypeNotPresentExceptions}) before moving on, assuming nested
	 * Class values were not resolvable within annotation attributes and
	 * thereby effectively pretending there were no annotations on the specified
	 * element.
	 * @param element the element that we tried to introspect annotations on
	 * @param ex the exception that we encountered
	 * @see #rethrowAnnotationConfigurationException
	 */
	static void handleIntrospectionFailure(@Nullable AnnotatedElement element, Throwable ex) {
		rethrowAnnotationConfigurationException(ex);

		Log loggerToUse = logger;
		if (loggerToUse == null) {
			loggerToUse = LogFactory.getLog(AnnotationUtils.class);
			logger = loggerToUse;
		}
		if (element instanceof Class && Annotation.class.isAssignableFrom((Class<?>) element)) {
			// Meta-annotation or (default) value lookup on an annotation type
			if (loggerToUse.isDebugEnabled()) {
				loggerToUse.debug("Failed to meta-introspect annotation " + element + ": " + ex);
			}
		}
		else {
			// Direct annotation lookup on regular Class, Method, Field
			if (loggerToUse.isInfoEnabled()) {
				loggerToUse.info("Failed to introspect annotations on " + element + ": " + ex);
			}
		}
	}

	/**
	 * Clear the internal annotation metadata cache.
	 * @since 4.3.15
	 */
	public static void clearCache() {
		findAnnotationCache.clear();
		metaPresentCache.clear();
		annotatedBaseTypeCache.clear();
		synthesizableCache.clear();
		attributeAliasesCache.clear();
		attributeMethodsCache.clear();
		aliasDescriptorCache.clear();
	}


	/**
	 * Cache key for the AnnotatedElement cache.
	 */
	private static final class AnnotationCacheKey implements Comparable<AnnotationCacheKey> {

		private final AnnotatedElement element;

		private final Class<? extends Annotation> annotationType;

		public AnnotationCacheKey(AnnotatedElement element, Class<? extends Annotation> annotationType) {
			this.element = element;
			this.annotationType = annotationType;
		}

		@Override
		public boolean equals(Object other) {
			if (this == other) {
				return true;
			}
			if (!(other instanceof AnnotationCacheKey)) {
				return false;
			}
			AnnotationCacheKey otherKey = (AnnotationCacheKey) other;
			return (this.element.equals(otherKey.element) && this.annotationType.equals(otherKey.annotationType));
		}

		@Override
		public int hashCode() {
			return (this.element.hashCode() * 29 + this.annotationType.hashCode());
		}

		@Override
		public String toString() {
			return "@" + this.annotationType + " on " + this.element;
		}

		@Override
		public int compareTo(AnnotationCacheKey other) {
			int result = this.element.toString().compareTo(other.element.toString());
			if (result == 0) {
				result = this.annotationType.getName().compareTo(other.annotationType.getName());
			}
			return result;
		}
	}


	private static class AnnotationCollector<A extends Annotation> {

		private final Class<A> annotationType;

		@Nullable
		private final Class<? extends Annotation> containerAnnotationType;

		private final boolean declaredMode;

		private final Set<AnnotatedElement> visited = new HashSet<>();

		private final Set<A> result = new LinkedHashSet<>();

		AnnotationCollector(Class<A> annotationType,
				@Nullable Class<? extends Annotation> containerAnnotationType, boolean declaredMode) {

			this.annotationType = annotationType;
			this.containerAnnotationType = (containerAnnotationType != null ? containerAnnotationType :
					resolveContainerAnnotationType(annotationType));
			this.declaredMode = declaredMode;
		}

		Set<A> getResult(AnnotatedElement element) {
			process(element);
			return Collections.unmodifiableSet(this.result);
		}

		@SuppressWarnings("unchecked")
		private void process(AnnotatedElement element) {
			if (this.visited.add(element)) {
				try {
					Annotation[] annotations = (this.declaredMode ? element.getDeclaredAnnotations() : element.getAnnotations());
					for (Annotation ann : annotations) {
						Class<? extends Annotation> currentAnnotationType = ann.annotationType();
						if (ObjectUtils.nullSafeEquals(this.annotationType, currentAnnotationType)) {
							this.result.add(synthesizeAnnotation((A) ann, element));
						}
						else if (ObjectUtils.nullSafeEquals(this.containerAnnotationType, currentAnnotationType)) {
							this.result.addAll(getValue(element, ann));
						}
						else if (!isInJavaLangAnnotationPackage(currentAnnotationType)) {
							process(currentAnnotationType);
						}
					}
				}
				catch (Throwable ex) {
					handleIntrospectionFailure(element, ex);
				}
			}
		}

		@SuppressWarnings("unchecked")
		private List<A> getValue(AnnotatedElement element, Annotation annotation) {
			try {
				List<A> synthesizedAnnotations = new ArrayList<>();
				A[] value = (A[]) AnnotationUtils.getValue(annotation);
				if (value != null) {
					for (A anno : value) {
						synthesizedAnnotations.add(synthesizeAnnotation(anno, element));
					}
				}
				return synthesizedAnnotations;
			}
			catch (Throwable ex) {
				handleIntrospectionFailure(element, ex);
			}
			// Unable to read value from repeating annotation container -> ignore it.
			return Collections.emptyList();
		}
	}


	/**
	 * {@code AliasDescriptor} encapsulates the declaration of {@code @AliasFor}
	 * on a given annotation attribute and includes support for validating
	 * the configuration of aliases (both explicit and implicit).
	 * @since 4.2.1
	 * @see #from
	 * @see #getAttributeAliasNames
	 * @see #getAttributeOverrideName
	 */
	private static final class AliasDescriptor {

		private final Method sourceAttribute;

		private final Class<? extends Annotation> sourceAnnotationType;

		private final String sourceAttributeName;

		private final Method aliasedAttribute;

		private final Class<? extends Annotation> aliasedAnnotationType;

		private final String aliasedAttributeName;

		private final boolean isAliasPair;

		/**
		 * Create an {@code AliasDescriptor} <em>from</em> the declaration
		 * of {@code @AliasFor} on the supplied annotation attribute and
		 * validate the configuration of {@code @AliasFor}.
		 * @param attribute the annotation attribute that is annotated with
		 * {@code @AliasFor}
		 * @return an alias descriptor, or {@code null} if the attribute
		 * is not annotated with {@code @AliasFor}
		 * @see #validateAgainst
		 */
		@Nullable
		public static AliasDescriptor from(Method attribute) {
			AliasDescriptor descriptor = aliasDescriptorCache.get(attribute);
			if (descriptor != null) {
				return descriptor;
			}

			AliasFor aliasFor = attribute.getAnnotation(AliasFor.class);
			if (aliasFor == null) {
				return null;
			}

			descriptor = new AliasDescriptor(attribute, aliasFor);
			descriptor.validate();
			aliasDescriptorCache.put(attribute, descriptor);
			return descriptor;
		}

		@SuppressWarnings("unchecked")
		private AliasDescriptor(Method sourceAttribute, AliasFor aliasFor) {
			Class<?> declaringClass = sourceAttribute.getDeclaringClass();

			this.sourceAttribute = sourceAttribute;
			this.sourceAnnotationType = (Class<? extends Annotation>) declaringClass;
			this.sourceAttributeName = sourceAttribute.getName();

			this.aliasedAnnotationType = (Annotation.class == aliasFor.annotation() ?
					this.sourceAnnotationType : aliasFor.annotation());
			this.aliasedAttributeName = getAliasedAttributeName(aliasFor, sourceAttribute);
			if (this.aliasedAnnotationType == this.sourceAnnotationType &&
					this.aliasedAttributeName.equals(this.sourceAttributeName)) {
				String msg = String.format("@AliasFor declaration on attribute '%s' in annotation [%s] points to " +
						"itself. Specify 'annotation' to point to a same-named attribute on a meta-annotation.",
						sourceAttribute.getName(), declaringClass.getName());
				throw new AnnotationConfigurationException(msg);
			}
			try {
				this.aliasedAttribute = this.aliasedAnnotationType.getDeclaredMethod(this.aliasedAttributeName);
			}
			catch (NoSuchMethodException ex) {
				String msg = String.format(
						"Attribute '%s' in annotation [%s] is declared as an @AliasFor nonexistent attribute '%s' in annotation [%s].",
						this.sourceAttributeName, this.sourceAnnotationType.getName(), this.aliasedAttributeName,
						this.aliasedAnnotationType.getName());
				throw new AnnotationConfigurationException(msg, ex);
			}

			this.isAliasPair = (this.sourceAnnotationType == this.aliasedAnnotationType);
		}

		private void validate() {
			// Target annotation is not meta-present?
			if (!this.isAliasPair && !isAnnotationMetaPresent(this.sourceAnnotationType, this.aliasedAnnotationType)) {
				String msg = String.format("@AliasFor declaration on attribute '%s' in annotation [%s] declares " +
						"an alias for attribute '%s' in meta-annotation [%s] which is not meta-present.",
						this.sourceAttributeName, this.sourceAnnotationType.getName(), this.aliasedAttributeName,
						this.aliasedAnnotationType.getName());
				throw new AnnotationConfigurationException(msg);
			}

			if (this.isAliasPair) {
				AliasFor mirrorAliasFor = this.aliasedAttribute.getAnnotation(AliasFor.class);
				if (mirrorAliasFor == null) {
					String msg = String.format("Attribute '%s' in annotation [%s] must be declared as an @AliasFor [%s].",
							this.aliasedAttributeName, this.sourceAnnotationType.getName(), this.sourceAttributeName);
					throw new AnnotationConfigurationException(msg);
				}

				String mirrorAliasedAttributeName = getAliasedAttributeName(mirrorAliasFor, this.aliasedAttribute);
				if (!this.sourceAttributeName.equals(mirrorAliasedAttributeName)) {
					String msg = String.format("Attribute '%s' in annotation [%s] must be declared as an @AliasFor [%s], not [%s].",
							this.aliasedAttributeName, this.sourceAnnotationType.getName(), this.sourceAttributeName,
							mirrorAliasedAttributeName);
					throw new AnnotationConfigurationException(msg);
				}
			}

			Class<?> returnType = this.sourceAttribute.getReturnType();
			Class<?> aliasedReturnType = this.aliasedAttribute.getReturnType();
			if (returnType != aliasedReturnType &&
					(!aliasedReturnType.isArray() || returnType != aliasedReturnType.getComponentType())) {
				String msg = String.format("Misconfigured aliases: attribute '%s' in annotation [%s] " +
						"and attribute '%s' in annotation [%s] must declare the same return type.",
						this.sourceAttributeName, this.sourceAnnotationType.getName(), this.aliasedAttributeName,
						this.aliasedAnnotationType.getName());
				throw new AnnotationConfigurationException(msg);
			}

			if (this.isAliasPair) {
				validateDefaultValueConfiguration(this.aliasedAttribute);
			}
		}

		private void validateDefaultValueConfiguration(Method aliasedAttribute) {
			Object defaultValue = this.sourceAttribute.getDefaultValue();
			Object aliasedDefaultValue = aliasedAttribute.getDefaultValue();

			if (defaultValue == null || aliasedDefaultValue == null) {
				String msg = String.format("Misconfigured aliases: attribute '%s' in annotation [%s] " +
						"and attribute '%s' in annotation [%s] must declare default values.",
						this.sourceAttributeName, this.sourceAnnotationType.getName(), aliasedAttribute.getName(),
						aliasedAttribute.getDeclaringClass().getName());
				throw new AnnotationConfigurationException(msg);
			}

			if (!ObjectUtils.nullSafeEquals(defaultValue, aliasedDefaultValue)) {
				String msg = String.format("Misconfigured aliases: attribute '%s' in annotation [%s] " +
						"and attribute '%s' in annotation [%s] must declare the same default value.",
						this.sourceAttributeName, this.sourceAnnotationType.getName(), aliasedAttribute.getName(),
						aliasedAttribute.getDeclaringClass().getName());
				throw new AnnotationConfigurationException(msg);
			}
		}

		/**
		 * Validate this descriptor against the supplied descriptor.
		 * <p>This method only validates the configuration of default values
		 * for the two descriptors, since other aspects of the descriptors
		 * are validated when they are created.
		 */
		private void validateAgainst(AliasDescriptor otherDescriptor) {
			validateDefaultValueConfiguration(otherDescriptor.sourceAttribute);
		}

		/**
		 * Determine if this descriptor represents an explicit override for
		 * an attribute in the supplied {@code metaAnnotationType}.
		 * @see #isAliasFor
		 */
		private boolean isOverrideFor(Class<? extends Annotation> metaAnnotationType) {
			return (this.aliasedAnnotationType == metaAnnotationType);
		}

		/**
		 * Determine if this descriptor and the supplied descriptor both
		 * effectively represent aliases for the same attribute in the same
		 * target annotation, either explicitly or implicitly.
		 * <p>This method searches the attribute override hierarchy, beginning
		 * with this descriptor, in order to detect implicit and transitively
		 * implicit aliases.
		 * @return {@code true} if this descriptor and the supplied descriptor
		 * effectively alias the same annotation attribute
		 * @see #isOverrideFor
		 */
		private boolean isAliasFor(AliasDescriptor otherDescriptor) {
			for (AliasDescriptor lhs = this; lhs != null; lhs = lhs.getAttributeOverrideDescriptor()) {
				for (AliasDescriptor rhs = otherDescriptor; rhs != null; rhs = rhs.getAttributeOverrideDescriptor()) {
					if (lhs.aliasedAttribute.equals(rhs.aliasedAttribute)) {
						return true;
					}
				}
			}
			return false;
		}

		public List<String> getAttributeAliasNames() {
			// Explicit alias pair?
			if (this.isAliasPair) {
				return Collections.singletonList(this.aliasedAttributeName);
			}

			// Else: search for implicit aliases
			List<String> aliases = new ArrayList<>();
			for (AliasDescriptor otherDescriptor : getOtherDescriptors()) {
				if (this.isAliasFor(otherDescriptor)) {
					this.validateAgainst(otherDescriptor);
					aliases.add(otherDescriptor.sourceAttributeName);
				}
			}
			return aliases;
		}

		private List<AliasDescriptor> getOtherDescriptors() {
			List<AliasDescriptor> otherDescriptors = new ArrayList<>();
			for (Method currentAttribute : getAttributeMethods(this.sourceAnnotationType)) {
				if (!this.sourceAttribute.equals(currentAttribute)) {
					AliasDescriptor otherDescriptor = AliasDescriptor.from(currentAttribute);
					if (otherDescriptor != null) {
						otherDescriptors.add(otherDescriptor);
					}
				}
			}
			return otherDescriptors;
		}

		@Nullable
		public String getAttributeOverrideName(Class<? extends Annotation> metaAnnotationType) {
			// Search the attribute override hierarchy, starting with the current attribute
			for (AliasDescriptor desc = this; desc != null; desc = desc.getAttributeOverrideDescriptor()) {
				if (desc.isOverrideFor(metaAnnotationType)) {
					return desc.aliasedAttributeName;
				}
			}

			// Else: explicit attribute override for a different meta-annotation
			return null;
		}

		@Nullable
		private AliasDescriptor getAttributeOverrideDescriptor() {
			if (this.isAliasPair) {
				return null;
			}
			return AliasDescriptor.from(this.aliasedAttribute);
		}

		/**
		 * Get the name of the aliased attribute configured via the supplied
		 * {@link AliasFor @AliasFor} annotation on the supplied {@code attribute},
		 * or the original attribute if no aliased one specified (indicating that
		 * the reference goes to a same-named attribute on a meta-annotation).
		 * <p>This method returns the value of either the {@code attribute}
		 * or {@code value} attribute of {@code @AliasFor}, ensuring that only
		 * one of the attributes has been declared while simultaneously ensuring
		 * that at least one of the attributes has been declared.
		 * @param aliasFor the {@code @AliasFor} annotation from which to retrieve
		 * the aliased attribute name
		 * @param attribute the attribute that is annotated with {@code @AliasFor}
		 * @return the name of the aliased attribute (never {@code null} or empty)
		 * @throws AnnotationConfigurationException if invalid configuration of
		 * {@code @AliasFor} is detected
		 */
		private String getAliasedAttributeName(AliasFor aliasFor, Method attribute) {
			String attributeName = aliasFor.attribute();
			String value = aliasFor.value();
			boolean attributeDeclared = StringUtils.hasText(attributeName);
			boolean valueDeclared = StringUtils.hasText(value);

			// Ensure user did not declare both 'value' and 'attribute' in @AliasFor
			if (attributeDeclared && valueDeclared) {
				String msg = String.format("In @AliasFor declared on attribute '%s' in annotation [%s], attribute 'attribute' " +
						"and its alias 'value' are present with values of [%s] and [%s], but only one is permitted.",
						attribute.getName(), attribute.getDeclaringClass().getName(), attributeName, value);
				throw new AnnotationConfigurationException(msg);
			}

			// Either explicit attribute name or pointing to same-named attribute by default
			attributeName = (attributeDeclared ? attributeName : value);
			return (StringUtils.hasText(attributeName) ? attributeName.trim() : attribute.getName());
		}

		@Override
		public String toString() {
			return String.format("%s: @%s(%s) is an alias for @%s(%s)", getClass().getSimpleName(),
					this.sourceAnnotationType.getSimpleName(), this.sourceAttributeName,
					this.aliasedAnnotationType.getSimpleName(), this.aliasedAttributeName);
		}
	}


	private static class DefaultValueHolder {

		final Object defaultValue;

		public DefaultValueHolder(Object defaultValue) {
			this.defaultValue = defaultValue;
		}
	}

}
