diff --git a/spring-core/src/main/java/org/springframework/core/annotation/MergedAnnotationCollectors.java b/spring-core/src/main/java/org/springframework/core/annotation/MergedAnnotationCollectors.java
index 4efb5cb6064..d09369f9165 100644
--- a/spring-core/src/main/java/org/springframework/core/annotation/MergedAnnotationCollectors.java
+++ b/spring-core/src/main/java/org/springframework/core/annotation/MergedAnnotationCollectors.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -18,8 +18,8 @@ package org.springframework.core.annotation;
 
 import java.lang.annotation.Annotation;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.LinkedHashSet;
-import java.util.List;
 import java.util.Set;
 import java.util.function.Function;
 import java.util.function.IntFunction;
@@ -31,10 +31,11 @@ import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
 /**
- * Collector implementations that provide various reduction operations for
+ * {@link Collector} implementations that provide various reduction operations for
  * {@link MergedAnnotation} instances.
  *
  * @author Phillip Webb
+ * @author Sam Brannen
  * @since 5.2
  */
 public abstract class MergedAnnotationCollectors {
@@ -52,13 +53,16 @@ public abstract class MergedAnnotationCollectors {
 	 * Create a new {@link Collector} that accumulates merged annotations to a
 	 * {@link LinkedHashSet} containing {@linkplain MergedAnnotation#synthesize()
 	 * synthesized} versions.
+	 * <p>The collector returned by this method is effectively equivalent to
+	 * {@code Collectors.mapping(MergedAnnotation::synthesize, Collectors.toCollection(LinkedHashSet::new))}
+	 * but avoids the creation of a composite collector.
 	 * @param <A> the annotation type
 	 * @return a {@link Collector} which collects and synthesizes the
 	 * annotations into a {@link Set}
 	 */
 	public static <A extends Annotation> Collector<MergedAnnotation<A>, ?, Set<A>> toAnnotationSet() {
-		return Collector.of(ArrayList<A>::new, (list, annotation) -> list.add(annotation.synthesize()),
-				MergedAnnotationCollectors::addAll, LinkedHashSet::new);
+		return Collector.of(LinkedHashSet::new, (set, annotation) -> set.add(annotation.synthesize()),
+				MergedAnnotationCollectors::combiner);
 	}
 
 	/**
@@ -90,14 +94,14 @@ public abstract class MergedAnnotationCollectors {
 			IntFunction<R[]> generator) {
 
 		return Collector.of(ArrayList::new, (list, annotation) -> list.add(annotation.synthesize()),
-				MergedAnnotationCollectors::addAll, list -> list.toArray(generator.apply(list.size())));
+				MergedAnnotationCollectors::combiner, list -> list.toArray(generator.apply(list.size())));
 	}
 
 	/**
-	 * Create a new {@link Collector} that accumulates merged annotations to an
+	 * Create a new {@link Collector} that accumulates merged annotations to a
 	 * {@link MultiValueMap} with items {@linkplain MultiValueMap#add(Object, Object)
 	 * added} from each merged annotation
-	 * {@link MergedAnnotation#asMap(Adapt...) as a map}.
+	 * {@linkplain MergedAnnotation#asMap(Adapt...) as a map}.
 	 * @param <A> the annotation type
 	 * @param adaptations the adaptations that should be applied to the annotation values
 	 * @return a {@link Collector} which collects and synthesizes the
@@ -111,13 +115,13 @@ public abstract class MergedAnnotationCollectors {
 	}
 
 	/**
-	 * Create a new {@link Collector} that accumulates merged annotations to an
+	 * Create a new {@link Collector} that accumulates merged annotations to a
 	 * {@link MultiValueMap} with items {@linkplain MultiValueMap#add(Object, Object)
 	 * added} from each merged annotation
-	 * {@link MergedAnnotation#asMap(Adapt...) as a map}.
+	 * {@linkplain MergedAnnotation#asMap(Adapt...) as a map}.
 	 * @param <A> the annotation type
-	 * @param adaptations the adaptations that should be applied to the annotation values
 	 * @param finisher the finisher function for the new {@link MultiValueMap}
+	 * @param adaptations the adaptations that should be applied to the annotation values
 	 * @return a {@link Collector} which collects and synthesizes the
 	 * annotations into a {@link LinkedMultiValueMap}
 	 * @see #toMultiValueMap(MergedAnnotation.Adapt...)
@@ -130,7 +134,7 @@ public abstract class MergedAnnotationCollectors {
 				IDENTITY_FINISH_CHARACTERISTICS : NO_CHARACTERISTICS);
 		return Collector.of(LinkedMultiValueMap::new,
 				(map, annotation) -> annotation.asMap(adaptations).forEach(map::add),
-				MergedAnnotationCollectors::merge, finisher, characteristics);
+				MergedAnnotationCollectors::combiner, finisher, characteristics);
 	}
 
 
@@ -138,13 +142,22 @@ public abstract class MergedAnnotationCollectors {
 		return instance == candidate;
 	}
 
-	private static <E, L extends List<E>> L addAll(L list, L additions) {
-		list.addAll(additions);
-		return list;
+	/**
+	 * {@link Collector#combiner() Combiner} for collections.
+	 * <p>This method is only invoked if the {@link java.util.stream.Stream} is
+	 * processed in {@linkplain java.util.stream.Stream#parallel() parallel}.
+	 */
+	private static <E, C extends Collection<E>> C combiner(C collection, C additions) {
+		collection.addAll(additions);
+		return collection;
 	}
 
-	private static <K, V> MultiValueMap<K, V> merge(MultiValueMap<K, V> map,
-			MultiValueMap<K, V> additions) {
+	/**
+	 * {@link Collector#combiner() Combiner} for multi-value maps.
+	 * <p>This method is only invoked if the {@link java.util.stream.Stream} is
+	 * processed in {@linkplain java.util.stream.Stream#parallel() parallel}.
+	 */
+	private static <K, V> MultiValueMap<K, V> combiner(MultiValueMap<K, V> map, MultiValueMap<K, V> additions) {
 		map.addAll(additions);
 		return map;
 	}