Do not create intermediate list in MergedAnnotationCollectors.toAnnotationSet()

Prior to this commit, MergedAnnotationCollectors.toAnnotationSet()
created an intermediate ArrayList for storing the results prior to
creating a LinkedHashSet in the finishing step.

Since the creation of the intermediate list is unnecessary, this commit
simplifies the implementation of toAnnotationSet() by using the
Collector.of() factory method that does not accept a `finisher` argument.
The resulting Collector internally uses a `castingIdentity()` function
as the `finisher`.

Closes gh-26031

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;
 	}