Reintroduce synthesized annotation attribute value caching

Prior to the introduction of the MergedAnnotation API in Spring Framework
5.2, our SynthesizedAnnotationInvocationHandler utilized a cache for
annotation attribute values; whereas, the new
SynthesizedMergedAnnotationInvocationHandler has no such caching in
place.

Issues such as gh-24961 indicate a regression in performance caused by
the lack of such an attribute value cache. For example, the required
attribute in @RequestParam is looked up using the internal meta-model
in the MergedAnnotation API twice per request for each @RequestParam in
a given controller handler method.

This commit reintroduces the attribute value cache to avoid the
unnecessary performance overhead associated with multiple lookups of
the same attribute in a synthesized annotation. This applies not only
to direct attribute method invocations but also to invocations of
equals() and hashCode() on a synthesized annotation.

Note, however, that this commit does NOT address multiple lookups of
annotation attribute values for invocations of toString(). That behavior
currently remains unchanged in the implementation of
org.springframework.core.annotation.TypeMappedAnnotation.toString().

Closes gh-24970

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

@@ -18,11 +18,14 @@ package org.springframework.core.annotation;
 
 import java.lang.annotation.Annotation;
 import java.lang.reflect.AnnotatedElement;
+import java.lang.reflect.Array;
 import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.lang.reflect.Proxy;
 import java.util.Arrays;
+import java.util.Map;
 import java.util.NoSuchElementException;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -50,6 +53,8 @@ final class SynthesizedMergedAnnotationInvocationHandler<A extends Annotation> i
 
 	private final AttributeMethods attributes;
 
+	private final Map<String, Object> valueCache = new ConcurrentHashMap<>(8);
+
 	@Nullable
 	private volatile Integer hashCode;
 
@@ -167,11 +172,53 @@ final class SynthesizedMergedAnnotationInvocationHandler<A extends Annotation> i
 	}
 
 	private Object getAttributeValue(Method method) {
-		String name = method.getName();
-		Class<?> type = ClassUtils.resolvePrimitiveIfNecessary(method.getReturnType());
-		return this.annotation.getValue(name, type).orElseThrow(
-				() -> new NoSuchElementException("No value found for attribute named '" + name +
-						"' in merged annotation " + this.annotation.getType().getName()));
+		Object value = this.valueCache.computeIfAbsent(method.getName(), attributeName -> {
+			Class<?> type = ClassUtils.resolvePrimitiveIfNecessary(method.getReturnType());
+			return this.annotation.getValue(attributeName, type).orElseThrow(
+					() -> new NoSuchElementException("No value found for attribute named '" + attributeName +
+							"' in merged annotation " + this.annotation.getType().getName()));
+		});
+
+		// Clone non-empty arrays so that users cannot alter the contents of values in our cache.
+		if (value.getClass().isArray() && Array.getLength(value) > 0) {
+			value = cloneArray(value);
+		}
+
+		return value;
+	}
+
+	/**
+	 * Clone the provided array, ensuring that original component type is retained.
+	 * @param array the array to clone
+	 */
+	private Object cloneArray(Object array) {
+		if (array instanceof boolean[]) {
+			return ((boolean[]) array).clone();
+		}
+		if (array instanceof byte[]) {
+			return ((byte[]) array).clone();
+		}
+		if (array instanceof char[]) {
+			return ((char[]) array).clone();
+		}
+		if (array instanceof double[]) {
+			return ((double[]) array).clone();
+		}
+		if (array instanceof float[]) {
+			return ((float[]) array).clone();
+		}
+		if (array instanceof int[]) {
+			return ((int[]) array).clone();
+		}
+		if (array instanceof long[]) {
+			return ((long[]) array).clone();
+		}
+		if (array instanceof short[]) {
+			return ((short[]) array).clone();
+		}
+
+		// else
+		return ((Object[]) array).clone();
 	}
 
 	@SuppressWarnings("unchecked")
@@ -183,6 +230,7 @@ final class SynthesizedMergedAnnotationInvocationHandler<A extends Annotation> i
 		return (A) Proxy.newProxyInstance(classLoader, interfaces, handler);
 	}
 
+
 	private static boolean isVisible(ClassLoader classLoader, Class<?> interfaceClass) {
 		if (classLoader == interfaceClass.getClassLoader()) {
 			return true;