Optimize MultiValueMap iteration operations

* use forEach and putIfAbsent to copy headers in DefaultClientRequestBuilder
* use forEach in ReactorClientHttpRequest and ReactorNetty2ClientHttpRequest
* circumvent ReadOnlyHttpHeaders.entrySet()
* ensure the fast path to LinkedCaseInsensitiveMap for forEach and putIfAbsent exists

Author: yuzawa-san

Diff for: spring-core/src/main/java/org/springframework/util/LinkedCaseInsensitiveMap.java

@@ -27,6 +27,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.Spliterator;
+import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -286,6 +287,11 @@ public Set<Entry<String, V>> entrySet() {
 		return entrySet;
 	}
 
+	@Override
+	public void forEach(BiConsumer<? super String, ? super V> action) {
+		this.targetMap.forEach(action);
+	}
+
 	@Override
 	public LinkedCaseInsensitiveMap<V> clone() {
 		return new LinkedCaseInsensitiveMap<>(this);

Diff for: spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java

@@ -22,6 +22,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.BiConsumer;
 
 import org.springframework.lang.Nullable;
 
@@ -69,15 +70,13 @@ public void add(K key, @Nullable V value) {
 
 	@Override
 	public void addAll(K key, List<? extends V> values) {
-		List<V> currentValues = this.targetMap.computeIfAbsent(key, k -> new ArrayList<>(1));
+		List<V> currentValues = this.targetMap.computeIfAbsent(key, k -> new ArrayList<>(values.size()));
 		currentValues.addAll(values);
 	}
 
 	@Override
 	public void addAll(MultiValueMap<K, V> values) {
-		for (Entry<K, List<V>> entry : values.entrySet()) {
-			addAll(entry.getKey(), entry.getValue());
-		}
+		values.forEach(this::addAll);
 	}
 
 	@Override
@@ -138,6 +137,12 @@ public List<V> put(K key, List<V> value) {
 		return this.targetMap.put(key, value);
 	}
 
+	@Override
+	@Nullable
+	public List<V> putIfAbsent(K key, List<V> value) {
+		return this.targetMap.putIfAbsent(key, value);
+	}
+
 	@Override
 	@Nullable
 	public List<V> remove(Object key) {
@@ -169,6 +174,11 @@ public Set<Entry<K, List<V>>> entrySet() {
 		return this.targetMap.entrySet();
 	}
 
+	@Override
+	public void forEach(BiConsumer<? super K, ? super List<V>> action) {
+		this.targetMap.forEach(action);
+	}
+
 	@Override
 	public boolean equals(@Nullable Object other) {
 		return (this == other || this.targetMap.equals(other));

Diff for: spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -40,6 +40,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.StringJoiner;
+import java.util.function.BiConsumer;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
@@ -1821,6 +1822,16 @@ public Set<Entry<String, List<String>>> entrySet() {
 		return this.headers.entrySet();
 	}
 
+	@Override
+	public void forEach(BiConsumer<? super String, ? super List<String>> action) {
+		this.headers.forEach(action);
+	}
+
+	@Override
+	public List<String> putIfAbsent(String key, List<String> value) {
+		return this.headers.putIfAbsent(key, value);
+	}
+
 
 	@Override
 	public boolean equals(@Nullable Object obj) {

Diff for: spring-web/src/main/java/org/springframework/http/ReadOnlyHttpHeaders.java

@@ -23,6 +23,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.BiConsumer;
 import java.util.stream.Collectors;
 
 import org.springframework.lang.Nullable;
@@ -155,4 +156,9 @@ public Set<Entry<String, List<String>>> entrySet() {
 						Collections::unmodifiableSet));
 	}
 
+	@Override
+	public void forEach(BiConsumer<? super String, ? super List<String>> action) {
+		this.headers.forEach((k, vs) -> action.accept(k, Collections.unmodifiableList(vs)));
+	}
+
 }

Diff for: spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java

@@ -18,7 +18,6 @@
 
 import java.net.URI;
 import java.nio.file.Path;
-import java.util.Collection;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.handler.codec.http.cookie.DefaultCookie;
@@ -129,9 +128,10 @@ protected void applyHeaders() {
 
 	@Override
 	protected void applyCookies() {
-		getCookies().values().stream().flatMap(Collection::stream)
-				.map(cookie -> new DefaultCookie(cookie.getName(), cookie.getValue()))
-				.forEach(this.request::addCookie);
+		getCookies().values().forEach(values -> values.forEach(value -> {
+			DefaultCookie cookie = new DefaultCookie(value.getName(), value.getValue());
+			this.request.addCookie(cookie);
+		}));
 	}
 
 	@Override

Diff for: spring-web/src/main/java/org/springframework/http/client/reactive/ReactorNetty2ClientHttpRequest.java

@@ -18,7 +18,6 @@
 
 import java.net.URI;
 import java.nio.file.Path;
-import java.util.Collection;
 
 import io.netty5.buffer.Buffer;
 import io.netty5.handler.codec.http.headers.DefaultHttpCookiePair;
@@ -130,9 +129,10 @@ protected void applyHeaders() {
 
 	@Override
 	protected void applyCookies() {
-		getCookies().values().stream().flatMap(Collection::stream)
-				.map(cookie -> new DefaultHttpCookiePair(cookie.getName(), cookie.getValue()))
-				.forEach(this.request::addCookie);
+		getCookies().values().forEach(values -> values.forEach(value -> {
+			DefaultHttpCookiePair cookie = new DefaultHttpCookiePair(value.getName(), value.getValue());
+			this.request.addCookie(cookie);
+		}));
 	}
 
 	@Override

Diff for: spring-web/src/test/java/org/springframework/http/HttpHeadersTests.java

@@ -704,6 +704,31 @@ void readOnlyHttpHeadersRetainEntrySetOrder() {
 		assertThat(readOnlyHttpHeaders.entrySet()).extracting(Entry::getKey).containsExactly(expectedKeys);
 	}
 
+	@Test
+	void readOnlyHttpHeadersCopyOrderTest() {
+		headers.add("aardvark", "enigma");
+		headers.add("beaver", "enigma");
+		headers.add("cat", "enigma");
+		headers.add("dog", "enigma");
+		headers.add("elephant", "enigma");
+
+		String[] expectedKeys = new String[] { "aardvark", "beaver", "cat", "dog", "elephant" };
+
+		HttpHeaders readOnlyHttpHeaders = HttpHeaders.readOnlyHttpHeaders(headers);
+
+		HttpHeaders forEachHeaders = new HttpHeaders();
+		readOnlyHttpHeaders.forEach(forEachHeaders::putIfAbsent);
+		assertThat(forEachHeaders.entrySet()).extracting(Entry::getKey).containsExactly(expectedKeys);
+
+		HttpHeaders putAllHeaders = new HttpHeaders();
+		putAllHeaders.putAll(readOnlyHttpHeaders);
+		assertThat(putAllHeaders.entrySet()).extracting(Entry::getKey).containsExactly(expectedKeys);
+
+		HttpHeaders addAllHeaders = new HttpHeaders();
+		addAllHeaders.addAll(readOnlyHttpHeaders);
+		assertThat(addAllHeaders.entrySet()).extracting(Entry::getKey).containsExactly(expectedKeys);
+	}
+
 	@Test // gh-25034
 	void equalsUnwrapsHttpHeaders() {
 		HttpHeaders headers1 = new HttpHeaders();

Diff for: spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientRequestBuilder.java

@@ -255,10 +255,7 @@ public String logPrefix() {
 		public Mono<Void> writeTo(ClientHttpRequest request, ExchangeStrategies strategies) {
 			HttpHeaders requestHeaders = request.getHeaders();
 			if (!this.headers.isEmpty()) {
-				this.headers.entrySet().stream()
-						.filter(entry -> !requestHeaders.containsKey(entry.getKey()))
-						.forEach(entry -> requestHeaders
-								.put(entry.getKey(), entry.getValue()));
+				this.headers.forEach(requestHeaders::putIfAbsent);
 			}
 
 			MultiValueMap<String, HttpCookie> requestCookies = request.getCookies();

Diff for: spring-websocket/src/main/java/org/springframework/web/socket/WebSocketHttpHeaders.java

@@ -22,6 +22,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.BiConsumer;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.lang.Nullable;
@@ -295,6 +296,16 @@ public Set<Entry<String, List<String>>> entrySet() {
 		return this.headers.entrySet();
 	}
 
+	@Override
+	public void forEach(BiConsumer<? super String, ? super List<String>> action) {
+		this.headers.forEach(action);
+	}
+
+	@Override
+	public List<String> putIfAbsent(String key, List<String> value) {
+		return this.headers.putIfAbsent(key, value);
+	}
+
 
 	@Override
 	public boolean equals(@Nullable Object other) {