Avoid wasted memory on empty maps and sets

This commit avoids wasted memory on empty hash maps in
MergedAnnotationReadingVisitor and empty sets in InjectionMetadata.

Closes gh-29742

Author: Vasiliy Kudriavtsev

spring-beans/src/main/java/org/springframework/beans/factory/annotation/InjectionMetadata.java

@@ -107,15 +107,20 @@ protected boolean needsRefresh(Class<?> clazz) {
 	}
 
 	public void checkConfigMembers(RootBeanDefinition beanDefinition) {
-		Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size());
-		for (InjectedElement element : this.injectedElements) {
-			Member member = element.getMember();
-			if (!beanDefinition.isExternallyManagedConfigMember(member)) {
-				beanDefinition.registerExternallyManagedConfigMember(member);
-				checkedElements.add(element);
+		if (this.injectedElements.isEmpty()) {
+			this.checkedElements = Collections.emptySet();
+		}
+		else {
+			Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size()*4/3 + 1);
+			for (InjectedElement element : this.injectedElements) {
+				Member member = element.getMember();
+				if (!beanDefinition.isExternallyManagedConfigMember(member)) {
+					beanDefinition.registerExternallyManagedConfigMember(member);
+					checkedElements.add(element);
+				}
 			}
+			this.checkedElements = checkedElements;
 		}
-		this.checkedElements = checkedElements;
 	}
 
 	public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {

spring-core/src/main/java/org/springframework/core/type/classreading/MergedAnnotationReadingVisitor.java

@@ -19,6 +19,7 @@
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Array;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -92,8 +93,10 @@ public AnnotationVisitor visitArray(String name) {
 
 	@Override
 	public void visitEnd() {
+		Map<String, Object> compactedAttributes
+				= this.attributes.size() == 0 ? Collections.emptyMap() : this.attributes;
 		MergedAnnotation<A> annotation = MergedAnnotation.of(
-				this.classLoader, this.source, this.annotationType, this.attributes);
+				this.classLoader, this.source, this.annotationType, compactedAttributes);
 		this.consumer.accept(annotation);
 	}