Use composite collections in attribute merging

This commit introduces composite collections (i.e. Collection, Set, Map)
and uses these composites in request predicates, where before new
collections were instantiated.

Closes gh-32245

spring-core/src/main/java/org/springframework/util/CollectionUtils.java

@@ -31,6 +31,8 @@
 import java.util.Properties;
 import java.util.Set;
 import java.util.SortedSet;
+import java.util.function.BiFunction;
+import java.util.function.Consumer;
 
 import org.springframework.lang.Nullable;
 
@@ -506,4 +508,42 @@ public static <K, V> MultiValueMap<K, V> unmodifiableMultiValueMap(
 		return new UnmodifiableMultiValueMap<>(targetMap);
 	}
 
+	/**
+	 * Return a (partially unmodifiable) map that combines the provided two
+	 * maps. Invoking {@link Map#put(Object, Object)} or {@link Map#putAll(Map)}
+	 * on the returned map results in an {@link UnsupportedOperationException}.
+	 * @param first the first map to compose
+	 * @param second the second map to compose
+	 * @return a new map that composes the given two maps
+	 * @since 6.2
+	 */
+	public static <K, V> Map<K, V> compositeMap(Map<K,V> first, Map<K,V> second) {
+		return new CompositeMap<>(first, second);
+	}
+
+	/**
+	 * Return a map that combines the provided maps. Invoking
+	 * {@link Map#put(Object, Object)} on the returned map will apply
+	 * {@code putFunction}, or will throw an
+	 * {@link UnsupportedOperationException} {@code putFunction} is
+	 * {@code null}. The same applies to {@link Map#putAll(Map)} and
+	 * {@code putAllFunction}.
+	 * @param first the first map to compose
+	 * @param second the second map to compose
+	 * @param putFunction applied when {@code Map::put} is invoked. If
+	 * {@code null}, {@code Map::put} throws an
+	 * {@code UnsupportedOperationException}.
+	 * @param putAllFunction applied when {@code Map::putAll} is invoked. If
+	 * {@code null}, {@code Map::putAll} throws an
+	 * {@code UnsupportedOperationException}.
+	 * @return a new map that composes the give maps
+	 * @since 6.2
+	 */
+	public static <K, V> Map<K, V> compositeMap(Map<K,V> first, Map<K,V> second,
+			@Nullable BiFunction<K, V, V> putFunction,
+			@Nullable Consumer<Map<K, V>> putAllFunction) {
+
+		return new CompositeMap<>(first, second, putFunction, putAllFunction);
+	}
+
 }

spring-core/src/main/java/org/springframework/util/CompositeCollection.java

@@ -0,0 +1,163 @@
+/*
+ * Copyright 2002-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.util;
+
+import java.lang.reflect.Array;
+import java.util.Collection;
+import java.util.Iterator;
+
+
+/**
+ * Composite collection that combines two other collections. This type is only
+ * exposed through {@link CompositeMap#values()}.
+ *
+ * @author Arjen Poutsma
+ * @since 6.2
+ * @param <E> the type of elements maintained by this collection
+ */
+class CompositeCollection<E> implements Collection<E> {
+
+	private final Collection<E> first;
+
+	private final Collection<E> second;
+
+
+	CompositeCollection(Collection<E> first, Collection<E> second) {
+		Assert.notNull(first, "First must not be null");
+		Assert.notNull(second, "Second must not be null");
+		this.first = first;
+		this.second = second;
+	}
+
+	@Override
+	public int size() {
+		return this.first.size() + this.second.size();
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.first.isEmpty() && this.second.isEmpty();
+	}
+
+	@Override
+	public boolean contains(Object o) {
+		if (this.first.contains(o)) {
+			return true;
+		}
+		else {
+			return this.second.contains(o);
+		}
+	}
+
+	@Override
+	public Iterator<E> iterator() {
+		CompositeIterator<E> iterator = new CompositeIterator<>();
+		iterator.add(this.first.iterator());
+		iterator.add(this.second.iterator());
+		return iterator;
+	}
+
+	@Override
+	public Object[] toArray() {
+		Object[] result = new Object[size()];
+		Object[] firstArray = this.first.toArray();
+		Object[] secondArray = this.second.toArray();
+		System.arraycopy(firstArray, 0, result, 0, firstArray.length);
+		System.arraycopy(secondArray, 0, result, firstArray.length, secondArray.length);
+		return result;
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public <T> T[] toArray(T[] a) {
+		int size = this.size();
+		T[] result;
+		if (a.length >= size) {
+			result = a;
+		}
+		else {
+			result = (T[]) Array.newInstance(a.getClass().getComponentType(), size);
+		}
+
+		int idx = 0;
+		for (E e : this) {
+			result[idx++] = (T) e;
+		}
+		if (result.length > size) {
+			result[size] = null;
+		}
+		return result;
+	}
+
+	@Override
+	public boolean add(E e) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public boolean remove(Object o) {
+		boolean firstResult = this.first.remove(o);
+		boolean secondResult = this.second.remove(o);
+		return firstResult || secondResult;
+	}
+
+	@Override
+	public boolean containsAll(Collection<?> c) {
+		for (Object o : c) {
+			if (!contains(o)) {
+				return false;
+			}
+		}
+		return true;
+	}
+
+	@Override
+	public boolean addAll(Collection<? extends E> c) {
+		boolean changed = false;
+		for (E e : c) {
+			if (add(e)) {
+				changed = true;
+			}
+		}
+		return changed;
+	}
+
+	@Override
+	public boolean removeAll(Collection<?> c) {
+		if (c.isEmpty()) {
+			return false;
+		}
+		boolean firstResult = this.first.removeAll(c);
+		boolean secondResult = this.second.removeAll(c);
+
+		return firstResult || secondResult;
+	}
+
+	@Override
+	public boolean retainAll(Collection<?> c) {
+		boolean firstResult = this.first.retainAll(c);
+		boolean secondResult = this.second.retainAll(c);
+
+		return firstResult || secondResult;
+	}
+
+	@Override
+	public void clear() {
+		this.first.clear();
+		this.second.clear();
+	}
+}