Avoid cloning empty Annotation array in TypeDescriptor

Rework AnnotatedElementAdapter to avoid cloning the underlying
Annotation array if it is empty when getAnnotations() is called.

Additionally, make the class static and add a factory method that
returns a singleton instance for null or empty Annotation arrays.

Closes gh-32405

spring-core/src/main/java/org/springframework/core/convert/TypeDescriptor.java

@@ -30,6 +30,7 @@ import java.util.stream.Stream;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -52,8 +53,6 @@ import org.springframework.util.ObjectUtils;
 @SuppressWarnings("serial")
 public class TypeDescriptor implements Serializable {
 
-	private static final Annotation[] EMPTY_ANNOTATION_ARRAY = new Annotation[0];
-
 	private static final Map<Class<?>, TypeDescriptor> commonTypesCache = new HashMap<>(32);
 
 	private static final Class<?>[] CACHED_COMMON_TYPES = {
@@ -84,7 +83,7 @@ public class TypeDescriptor implements Serializable {
 	public TypeDescriptor(MethodParameter methodParameter) {
 		this.resolvableType = ResolvableType.forMethodParameter(methodParameter);
 		this.type = this.resolvableType.resolve(methodParameter.getNestedParameterType());
-		this.annotatedElement = new AnnotatedElementAdapter(methodParameter.getParameterIndex() == -1 ?
+		this.annotatedElement = AnnotatedElementAdapter.from(methodParameter.getParameterIndex() == -1 ?
 				methodParameter.getMethodAnnotations() : methodParameter.getParameterAnnotations());
 	}
 
@@ -96,7 +95,7 @@ public class TypeDescriptor implements Serializable {
 	public TypeDescriptor(Field field) {
 		this.resolvableType = ResolvableType.forField(field);
 		this.type = this.resolvableType.resolve(field.getType());
-		this.annotatedElement = new AnnotatedElementAdapter(field.getAnnotations());
+		this.annotatedElement = AnnotatedElementAdapter.from(field.getAnnotations());
 	}
 
 	/**
@@ -109,7 +108,7 @@ public class TypeDescriptor implements Serializable {
 		Assert.notNull(property, "Property must not be null");
 		this.resolvableType = ResolvableType.forMethodParameter(property.getMethodParameter());
 		this.type = this.resolvableType.resolve(property.getType());
-		this.annotatedElement = new AnnotatedElementAdapter(property.getAnnotations());
+		this.annotatedElement = AnnotatedElementAdapter.from(property.getAnnotations());
 	}
 
 	/**
@@ -125,7 +124,7 @@ public class TypeDescriptor implements Serializable {
 	public TypeDescriptor(ResolvableType resolvableType, @Nullable Class<?> type, @Nullable Annotation[] annotations) {
 		this.resolvableType = resolvableType;
 		this.type = (type != null ? type : resolvableType.toClass());
-		this.annotatedElement = new AnnotatedElementAdapter(annotations);
+		this.annotatedElement = AnnotatedElementAdapter.from(annotations);
 	}
 
 
@@ -742,15 +741,23 @@ public class TypeDescriptor implements Serializable {
 	 * @see AnnotatedElementUtils#isAnnotated(AnnotatedElement, Class)
 	 * @see AnnotatedElementUtils#getMergedAnnotation(AnnotatedElement, Class)
 	 */
-	private class AnnotatedElementAdapter implements AnnotatedElement, Serializable {
+	private static final class AnnotatedElementAdapter implements AnnotatedElement, Serializable {
+		private static final AnnotatedElementAdapter EMPTY = new AnnotatedElementAdapter(new Annotation[0]);
 
-		@Nullable
+		@NonNull
 		private final Annotation[] annotations;
 
-		public AnnotatedElementAdapter(@Nullable Annotation[] annotations) {
+		private AnnotatedElementAdapter(@NonNull Annotation[] annotations) {
 			this.annotations = annotations;
 		}
 
+		private static AnnotatedElementAdapter from(@Nullable Annotation[] annotations) {
+			if (annotations == null || annotations.length == 0) {
+				return EMPTY;
+			}
+			return new AnnotatedElementAdapter(annotations);
+		}
+
 		@Override
 		public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass) {
 			for (Annotation annotation : getAnnotations()) {
@@ -775,7 +782,7 @@ public class TypeDescriptor implements Serializable {
 
 		@Override
 		public Annotation[] getAnnotations() {
-			return (this.annotations != null ? this.annotations.clone() : EMPTY_ANNOTATION_ARRAY);
+			return isEmpty() ? this.annotations : this.annotations.clone();
 		}
 
 		@Override
@@ -784,7 +791,7 @@ public class TypeDescriptor implements Serializable {
 		}
 
 		public boolean isEmpty() {
-			return ObjectUtils.isEmpty(this.annotations);
+			return this.annotations.length == 0;
 		}
 
 		@Override
@@ -800,7 +807,7 @@ public class TypeDescriptor implements Serializable {
 
 		@Override
 		public String toString() {
-			return TypeDescriptor.this.toString();
+			return "{AnnotatedElementAdapter annotations=" + Arrays.toString(this.annotations) + "}";
 		}
 	}