Optimize HTTP headers management

Several benchmarks underlined a few hotspots for CPU and GC pressure in
the Spring Framework codebase:

1. `org.springframework.util.MimeType.<init>(String, String, Map)`
2. `org.springframework.util.LinkedCaseInsensitiveMap.convertKey(String)`

Both are linked with HTTP request headers parsing and response headers
writin during the exchange processing phase.

1) is linked to repeated calls to `HttpHeaders.getContentType`
within a single request handling. The media type parsing operation
is expensive and the result doesn't change between calls, since
the request headers are immutable at that point.

This commit improves this by caching the parsed `MediaType` for the
`"Content-Type"` request header in the `ReadOnlyHttpHeaders` class.
This change is available for both Spring MVC and Spring WebFlux.

2) is linked to insertions/lookups in the `LinkedCaseInsensitiveMap`,
which is the data structure behind `HttpHeaders`.
Those operations are creating a lot of garbage (including a lot of
`String` created by `toLowerCase`). We could choose a more efficient
data structure for storing HTTP headers data.

As a first step, this commit is focusing on Spring WebFlux and
introduces `MultiValueMap` implementations mapped by native HTTP headers
for the following servers: Tomcat, Jetty, Netty and Undertow.
Such implementations avoid unnecessary copying of the headers
and leverages as much as possible optimized operations provided by the
native implementations.

This change has a few consequences:

* `HttpHeaders` can now wrap a `MultiValueMap` directly
* The default constructor of `HttpHeaders` is still backed by a
`LinkedCaseInsensitiveMap`
* The HTTP request headers for the websocket HTTP handshake now need to
be cloned, because native headers are likely to be pooled/recycled by
the server implementation, hence gone when the initial HTTP exchange is
done

Issue: SPR-17250

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

@@ -35,8 +35,6 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.Iterator;
-import java.util.LinkedHashMap;
-import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -47,7 +45,9 @@ import java.util.stream.Collectors;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.CollectionUtils;
 import org.springframework.util.LinkedCaseInsensitiveMap;
+import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 import org.springframework.util.StringUtils;
 
@@ -78,7 +78,8 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	/**
 	 * The empty {@code HttpHeaders} instance (immutable).
 	 */
-	public static final HttpHeaders EMPTY = new HttpHeaders(new LinkedHashMap<>(), true);
+	public static final HttpHeaders EMPTY =
+			new ReadOnlyHttpHeaders(new HttpHeaders(new LinkedMultiValueMap<>(0)));
 	/**
 	 * The HTTP {@code Accept} header field name.
 	 * @see <a href="http://tools.ietf.org/html/rfc7231#section-5.3.2">Section 5.3.2 of RFC 7231</a>
@@ -397,35 +398,27 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	private static final DateTimeFormatter[] DATE_FORMATTERS = new DateTimeFormatter[] {
 			DateTimeFormatter.RFC_1123_DATE_TIME,
 			DateTimeFormatter.ofPattern("EEEE, dd-MMM-yy HH:mm:ss zz", Locale.US),
-			DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss yyyy",Locale.US).withZone(GMT)
+			DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss yyyy", Locale.US).withZone(GMT)
 	};
 
 
-	private final Map<String, List<String>> headers;
+	final MultiValueMap<String, String> headers;
-
-	private final boolean readOnly;
 
 
 	/**
-	 * Constructs a new, empty instance of the {@code HttpHeaders} object.
+	 * Construct a new, empty instance of the {@code HttpHeaders} object.
 	 */
 	public HttpHeaders() {
-		this(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH), false);
+		this(CollectionUtils.toMultiValueMap(
+				new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH)));
 	}
 
 	/**
-	 * Private constructor that can create read-only {@code HttpHeader} instances.
+	 * Construct a new {@code HttpHeaders} instance backed by an existing map.
 	 */
-	private HttpHeaders(Map<String, List<String>> headers, boolean readOnly) {
+	public HttpHeaders(MultiValueMap<String, String> headers) {
-		if (readOnly) {
+		Assert.notNull(headers, "headers must not be null");
-			Map<String, List<String>> map = new LinkedCaseInsensitiveMap<>(headers.size(), Locale.ENGLISH);
+		this.headers = headers;
-			headers.forEach((key, valueList) -> map.put(key, Collections.unmodifiableList(valueList)));
-			this.headers = Collections.unmodifiableMap(map);
-		}
-		else {
-			this.headers = headers;
-		}
-		this.readOnly = readOnly;
 	}
 
 
@@ -1474,8 +1467,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	@Override
 	@Nullable
 	public String getFirst(String headerName) {
-		List<String> headerValues = this.headers.get(headerName);
+		return this.headers.getFirst(headerName);
-		return (headerValues != null ? headerValues.get(0) : null);
 	}
 
 	/**
@@ -1488,19 +1480,17 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 */
 	@Override
 	public void add(String headerName, @Nullable String headerValue) {
-		List<String> headerValues = this.headers.computeIfAbsent(headerName, k -> new LinkedList<>());
+		this.headers.add(headerName, headerValue);
-		headerValues.add(headerValue);
 	}
 
 	@Override
 	public void addAll(String key, List<? extends String> values) {
-		List<String> currentValues = this.headers.computeIfAbsent(key, k -> new LinkedList<>());
+		this.headers.addAll(key, values);
-		currentValues.addAll(values);
 	}
 
 	@Override
 	public void addAll(MultiValueMap<String, String> values) {
-		values.forEach(this::addAll);
+		this.headers.addAll(values);
 	}
 
 	/**
@@ -1513,21 +1503,17 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 */
 	@Override
 	public void set(String headerName, @Nullable String headerValue) {
-		List<String> headerValues = new LinkedList<>();
+		this.headers.set(headerName, headerValue);
-		headerValues.add(headerValue);
-		this.headers.put(headerName, headerValues);
 	}
 
 	@Override
 	public void setAll(Map<String, String> values) {
-		values.forEach(this::set);
+		this.headers.setAll(values);
 	}
 
 	@Override
 	public Map<String, String> toSingleValueMap() {
-		LinkedHashMap<String, String> singleValueMap = new LinkedHashMap<>(this.headers.size());
+		return this.headers.toSingleValueMap();
-		this.headers.forEach((key, valueList) -> singleValueMap.put(key, valueList.get(0)));
-		return singleValueMap;
 	}
 
 
@@ -1623,7 +1609,12 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 */
 	public static HttpHeaders readOnlyHttpHeaders(HttpHeaders headers) {
 		Assert.notNull(headers, "HttpHeaders must not be null");
-		return (headers.readOnly ? headers : new HttpHeaders(headers, true));
+		if (headers instanceof ReadOnlyHttpHeaders) {
+			return headers;
+		}
+		else {
+			return new ReadOnlyHttpHeaders(headers);
+		}
 	}
 
 }

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

@@ -0,0 +1,135 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.http;
+
+import java.util.AbstractMap;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * {@code HttpHeaders} object that can only be read, not written to.
+ *
+ * @author Brian Clozel
+ * @since 5.1
+ */
+class ReadOnlyHttpHeaders extends HttpHeaders {
+
+	private static final long serialVersionUID = -8578554704772377436L;
+
+	@Nullable
+	private MediaType cachedContentType;
+
+	ReadOnlyHttpHeaders(HttpHeaders headers) {
+		super(headers.headers);
+	}
+
+	@Override
+	public MediaType getContentType() {
+		if (this.cachedContentType != null) {
+			return this.cachedContentType;
+		}
+		else {
+			MediaType contentType = super.getContentType();
+			this.cachedContentType = contentType;
+			return contentType;
+		}
+	}
+
+	@Override
+	public List<String> get(Object key) {
+		List<String> values = this.headers.get(key);
+		if (values != null) {
+			return Collections.unmodifiableList(values);
+		}
+		return values;
+	}
+
+	@Override
+	public void add(String headerName, @Nullable String headerValue) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void addAll(String key, List<? extends String> values) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void addAll(MultiValueMap<String, String> values) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void set(String headerName, @Nullable String headerValue) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void setAll(Map<String, String> values) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public Map<String, String> toSingleValueMap() {
+		return Collections.unmodifiableMap(this.headers.toSingleValueMap());
+	}
+
+	@Override
+	public Set<String> keySet() {
+		return Collections.unmodifiableSet(this.headers.keySet());
+	}
+
+	@Override
+	public List<String> put(String key, List<String> value) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public List<String> remove(Object key) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void putAll(Map<? extends String, ? extends List<String>> map) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void clear() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public Collection<List<String>> values() {
+		return Collections.unmodifiableCollection(this.headers.values());
+	}
+
+	@Override
+	public Set<Entry<String, List<String>>> entrySet() {
+		return Collections.unmodifiableSet(this.headers.entrySet().stream()
+				.map(AbstractMap.SimpleImmutableEntry::new)
+				.collect(Collectors.toSet()));
+	}
+
+}

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -153,6 +153,9 @@ public class ServletServerHttpResponse implements ServerHttpResponse {
 			Assert.isInstanceOf(String.class, key, "Key must be a String-based header name");
 
 			Collection<String> values1 = servletResponse.getHeaders((String) key);
+			if (headersWritten) {
+				return new ArrayList<>(values1);
+			}
 			boolean isEmpty1 = CollectionUtils.isEmpty(values1);
 
 			List<String> values2 = super.get(key);

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerServerHttpResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -24,6 +24,7 @@ import reactor.core.publisher.Mono;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
+import org.springframework.http.HttpHeaders;
 
 /**
  * Abstract base class for listener-based server responses, e.g. Servlet 3.1
@@ -41,6 +42,10 @@ public abstract class AbstractListenerServerHttpResponse extends AbstractServerH
 		super(dataBufferFactory);
 	}
 
+	public AbstractListenerServerHttpResponse(DataBufferFactory dataBufferFactory, HttpHeaders headers) {
+		super(dataBufferFactory, headers);
+	}
+
 
 	@Override
 	protected final Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body) {

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java

@@ -75,9 +75,14 @@ public abstract class AbstractServerHttpResponse implements ServerHttpResponse {
 
 
 	public AbstractServerHttpResponse(DataBufferFactory dataBufferFactory) {
+		this(dataBufferFactory, new HttpHeaders());
+	}
+
+	public AbstractServerHttpResponse(DataBufferFactory dataBufferFactory, HttpHeaders headers) {
 		Assert.notNull(dataBufferFactory, "DataBufferFactory must not be null");
+		Assert.notNull(headers, "HttpHeaders must not be null");
 		this.dataBufferFactory = dataBufferFactory;
-		this.headers = new HttpHeaders();
+		this.headers = headers;
 		this.cookies = new LinkedMultiValueMap<>();
 	}
 

spring-web/src/main/java/org/springframework/http/server/reactive/JettyHeadersAdapter.java

@@ -0,0 +1,222 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.http.server.reactive;
+
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Enumeration;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import org.eclipse.jetty.http.HttpField;
+import org.eclipse.jetty.http.HttpFields;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * {@code MultiValueMap} implementation for wrapping Jetty HTTP headers.
+ *
+ * @author Brian Clozel
+ * @since 5.1
+ */
+class JettyHeadersAdapter implements MultiValueMap<String, String> {
+
+	private final HttpFields headers;
+
+	JettyHeadersAdapter(HttpFields headers) {
+		this.headers = headers;
+	}
+
+	@Override
+	public String getFirst(String key) {
+		return this.headers.get(key);
+	}
+
+	@Override
+	public void add(String key, @Nullable String value) {
+		this.headers.add(key, value);
+	}
+
+	@Override
+	public void addAll(String key, List<? extends String> values) {
+		values.forEach(value -> add(key, value));
+	}
+
+	@Override
+	public void addAll(MultiValueMap<String, String> values) {
+		values.forEach(this::addAll);
+	}
+
+	@Override
+	public void set(String key, @Nullable String value) {
+		this.headers.put(key, value);
+	}
+
+	@Override
+	public void setAll(Map<String, String> values) {
+		values.forEach(this::set);
+	}
+
+	@Override
+	public Map<String, String> toSingleValueMap() {
+		Map<String, String> singleValueMap = new LinkedHashMap<>(this.headers.size());
+		Iterator<HttpField> iterator = this.headers.iterator();
+		iterator.forEachRemaining(field -> {
+			if (!singleValueMap.containsKey(field.getName())) {
+				singleValueMap.put(field.getName(), field.getValue());
+			}
+		});
+		return singleValueMap;
+	}
+
+	@Override
+	public int size() {
+		return this.headers.getFieldNamesCollection().size();
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.headers.size() == 0;
+	}
+
+	@Override
+	public boolean containsKey(Object key) {
+		if (key instanceof String) {
+			return this.headers.containsKey((String) key);
+		}
+		return false;
+	}
+
+	@Override
+	public boolean containsValue(Object value) {
+		if (value instanceof String) {
+			return this.headers.stream()
+					.anyMatch(field -> field.contains((String) value));
+		}
+		return false;
+	}
+
+	@Nullable
+	@Override
+	public List<String> get(Object key) {
+		if (key instanceof String) {
+			return this.headers.getValuesList((String) key);
+		}
+		return null;
+	}
+
+	@Nullable
+	@Override
+	public List<String> put(String key, List<String> value) {
+		List<String> oldValues = get(key);
+		this.headers.put(key, value);
+		return oldValues;
+	}
+
+	@Nullable
+	@Override
+	public List<String> remove(Object key) {
+		if (key instanceof String) {
+			List<String> oldValues = get(key);
+			this.headers.remove((String) key);
+			return oldValues;
+		}
+		return null;
+	}
+
+	@Override
+	public void putAll(Map<? extends String, ? extends List<String>> m) {
+		m.forEach(this::put);
+	}
+
+	@Override
+	public void clear() {
+		this.headers.clear();
+	}
+
+	@Override
+	public Set<String> keySet() {
+		return this.headers.getFieldNamesCollection();
+	}
+
+	@Override
+	public Collection<List<String>> values() {
+		return this.headers.getFieldNamesCollection().stream()
+				.map(this.headers::getValuesList).collect(Collectors.toList());
+	}
+
+	@Override
+	public Set<Entry<String, List<String>>> entrySet() {
+		return new AbstractSet<Entry<String, List<String>>>() {
+			@Override
+			public Iterator<Entry<String, List<String>>> iterator() {
+				return new EntryIterator();
+			}
+
+			@Override
+			public int size() {
+				return headers.size();
+			}
+		};
+	}
+
+	private class EntryIterator implements Iterator<Entry<String, List<String>>> {
+
+		private Enumeration<String> names = headers.getFieldNames();
+
+		@Override
+		public boolean hasNext() {
+			return this.names.hasMoreElements();
+		}
+
+		@Override
+		public Entry<String, List<String>> next() {
+			return new HeaderEntry(this.names.nextElement());
+		}
+	}
+
+	private class HeaderEntry implements Entry<String, List<String>> {
+
+		private final String key;
+
+		HeaderEntry(String key) {
+			this.key = key;
+		}
+
+		@Override
+		public String getKey() {
+			return this.key.toString();
+		}
+
+		@Override
+		public List<String> getValue() {
+			return headers.getValuesList(this.key);
+		}
+
+		@Override
+		public List<String> setValue(List<String> value) {
+			List<String> previousValues = headers.getValuesList(this.key);
+			headers.put(this.key, value);
+			return previousValues;
+		}
+	}
+}

spring-web/src/main/java/org/springframework/http/server/reactive/JettyHttpHandlerAdapter.java

@@ -17,15 +17,21 @@
 package org.springframework.http.server.reactive;
 
 import java.io.IOException;
+import java.net.URISyntaxException;
 import java.nio.ByteBuffer;
 import javax.servlet.AsyncContext;
 import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.eclipse.jetty.http.HttpFields;
 import org.eclipse.jetty.server.HttpOutput;
+import org.eclipse.jetty.server.Request;
+import org.eclipse.jetty.server.Response;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
+import org.springframework.http.HttpHeaders;
 
 /**
  * {@link ServletHttpHandlerAdapter} extension that uses Jetty APIs for writing
@@ -42,6 +48,12 @@ public class JettyHttpHandlerAdapter extends ServletHttpHandlerAdapter {
 	}
 
 
+	@Override
+	protected ServletServerHttpRequest createRequest(HttpServletRequest request, AsyncContext context)
+			throws IOException, URISyntaxException {
+		return new JettyServerHttpRequest(request, context, getServletPath(), getDataBufferFactory(), getBufferSize());
+	}
+
 	@Override
 	protected ServletServerHttpResponse createResponse(HttpServletResponse response,
 			AsyncContext context, ServletServerHttpRequest request) throws IOException {
@@ -50,14 +62,38 @@ public class JettyHttpHandlerAdapter extends ServletHttpHandlerAdapter {
 				response, context, getDataBufferFactory(), getBufferSize(), request);
 	}
 
+	private static final class JettyServerHttpRequest extends ServletServerHttpRequest {
+
+		JettyServerHttpRequest(HttpServletRequest request, AsyncContext asyncContext,
+				String servletPath, DataBufferFactory bufferFactory, int bufferSize)
+				throws IOException, URISyntaxException {
+
+			super(createHeaders(request), request, asyncContext, servletPath, bufferFactory, bufferSize);
+		}
+
+		private static HttpHeaders createHeaders(HttpServletRequest request) {
+			HttpFields fields = ((Request) request).getMetaData().getFields();
+			return new HttpHeaders(new JettyHeadersAdapter(fields));
+		}
+	}
+
 
 	private static final class JettyServerHttpResponse extends ServletServerHttpResponse {
 
-		public JettyServerHttpResponse(HttpServletResponse response, AsyncContext asyncContext,
+		JettyServerHttpResponse(HttpServletResponse response, AsyncContext asyncContext,
 				DataBufferFactory bufferFactory, int bufferSize, ServletServerHttpRequest request)
 				throws IOException {
 
-			super(response, asyncContext, bufferFactory, bufferSize, request);
+			super(createHeaders(response), response, asyncContext, bufferFactory, bufferSize, request);
+		}
+
+		private static HttpHeaders createHeaders(HttpServletResponse response) {
+			HttpFields fields = ((Response) response).getHttpFields();
+			return new HttpHeaders(new JettyHeadersAdapter(fields));
+		}
+
+		@Override
+		protected void applyHeaders() {
 		}
 
 		@Override

spring-web/src/main/java/org/springframework/http/server/reactive/NettyHeadersAdapter.java

@@ -0,0 +1,217 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.http.server.reactive;
+
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import io.netty.handler.codec.http.HttpHeaders;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * {@code MultiValueMap} implementation for wrapping Netty HTTP headers.
+ *
+ * @author Brian Clozel
+ * @since 5.1
+ */
+class NettyHeadersAdapter implements MultiValueMap<String, String> {
+
+	private final HttpHeaders headers;
+
+	NettyHeadersAdapter(HttpHeaders headers) {
+		this.headers = headers;
+	}
+
+	@Override
+	@Nullable
+	public String getFirst(String key) {
+		return this.headers.get(key);
+	}
+
+	@Override
+	public void add(String key, @Nullable String value) {
+		this.headers.add(key, value);
+	}
+
+	@Override
+	public void addAll(String key, List<? extends String> values) {
+		this.headers.add(key, values);
+	}
+
+	@Override
+	public void addAll(MultiValueMap<String, String> values) {
+		values.forEach(this.headers::add);
+	}
+
+	@Override
+	public void set(String key, @Nullable String value) {
+		this.headers.set(key, value);
+	}
+
+	@Override
+	public void setAll(Map<String, String> values) {
+		values.forEach(this.headers::set);
+	}
+
+	@Override
+	public Map<String, String> toSingleValueMap() {
+		Map<String, String> singleValueMap = new LinkedHashMap<>(this.headers.size());
+		this.headers.entries()
+				.forEach(entry -> {
+					if (!singleValueMap.containsKey(entry.getKey())) {
+						singleValueMap.put(entry.getKey(), entry.getValue());
+					}
+				});
+		return singleValueMap;
+	}
+
+	@Override
+	public int size() {
+		return this.headers.size();
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.headers.isEmpty();
+	}
+
+	@Override
+	public boolean containsKey(Object key) {
+		return (key instanceof String) && this.headers.contains((String) key);
+	}
+
+	@Override
+	public boolean containsValue(Object value) {
+		return (value instanceof String) &&
+				this.headers.entries().stream()
+						.anyMatch(entry -> value != null && value.equals(entry.getValue()));
+	}
+
+	@Override
+	@Nullable
+	public List<String> get(Object key) {
+		if (key instanceof String) {
+			return this.headers.getAll((String) key);
+		}
+		return null;
+	}
+
+	@Nullable
+	@Override
+	public List<String> put(String key, @Nullable List<String> value) {
+		List<String> previousValues = this.headers.getAll(key);
+		this.headers.add(key, value);
+		return previousValues;
+	}
+
+	@Nullable
+	@Override
+	public List<String> remove(Object key) {
+		if (key instanceof String) {
+			List<String> previousValues = this.headers.getAll((String) key);
+			this.headers.remove((String) key);
+			return previousValues;
+		}
+		return null;
+	}
+
+	@Override
+	public void putAll(Map<? extends String, ? extends List<String>> m) {
+		m.forEach(this.headers::add);
+	}
+
+	@Override
+	public void clear() {
+		this.headers.clear();
+	}
+
+	@Override
+	public Set<String> keySet() {
+		return this.headers.names();
+	}
+
+	@Override
+	public Collection<List<String>> values() {
+		return this.headers.names().stream()
+				.map(this.headers::getAll).collect(Collectors.toList());
+	}
+
+	@Override
+	public Set<Entry<String, List<String>>> entrySet() {
+		return new AbstractSet<Entry<String, List<String>>>() {
+			@Override
+			public Iterator<Entry<String, List<String>>> iterator() {
+				return new EntryIterator();
+			}
+
+			@Override
+			public int size() {
+				return headers.size();
+			}
+		};
+	}
+
+	private class EntryIterator implements Iterator<Entry<String, List<String>>> {
+
+		private Iterator<String> names = headers.names().iterator();
+
+		@Override
+		public boolean hasNext() {
+			return this.names.hasNext();
+		}
+
+		@Override
+		public Entry<String, List<String>> next() {
+			return new HeaderEntry(this.names.next());
+		}
+	}
+
+	private class HeaderEntry implements Entry<String, List<String>> {
+
+		private final String key;
+
+		HeaderEntry(String key) {
+			this.key = key;
+		}
+
+		@Override
+		public String getKey() {
+			return this.key;
+		}
+
+		@Override
+		public List<String> getValue() {
+			return headers.getAll(this.key);
+		}
+
+		@Override
+		public List<String> setValue(List<String> value) {
+			List<String> previousValues = headers.getAll(this.key);
+			headers.set(this.key, value);
+			return previousValues;
+		}
+	}
+
+}

spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpRequest.java

@@ -125,11 +125,8 @@ class ReactorServerHttpRequest extends AbstractServerHttpRequest {
 	}
 
 	private static HttpHeaders initHeaders(HttpServerRequest channel) {
-		HttpHeaders headers = new HttpHeaders();
+		NettyHeadersAdapter headersMap = new NettyHeadersAdapter(channel.requestHeaders());
-		for (String name : channel.requestHeaders().names()) {
+		return new HttpHeaders(headersMap);
-			headers.put(name, channel.requestHeaders().getAll(name));
-		}
-		return headers;
 	}
 
 

spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java

@@ -19,6 +19,7 @@ package org.springframework.http.server.reactive;
 import java.nio.file.Path;
 
 import io.netty.buffer.ByteBuf;
+import io.netty.handler.codec.http.HttpHeaderNames;
 import io.netty.handler.codec.http.HttpResponseStatus;
 import io.netty.handler.codec.http.cookie.Cookie;
 import io.netty.handler.codec.http.cookie.DefaultCookie;
@@ -30,6 +31,7 @@ import reactor.netty.http.server.HttpServerResponse;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.ResponseCookie;
 import org.springframework.http.ZeroCopyHttpOutputMessage;
 import org.springframework.util.Assert;
@@ -47,11 +49,16 @@ class ReactorServerHttpResponse extends AbstractServerHttpResponse implements Ze
 
 
 	public ReactorServerHttpResponse(HttpServerResponse response, DataBufferFactory bufferFactory) {
-		super(bufferFactory);
+		super(bufferFactory, initHeaders(response));
 		Assert.notNull(response, "HttpServerResponse must not be null");
 		this.response = response;
 	}
 
+	private static HttpHeaders initHeaders(HttpServerResponse channel) {
+		channel.responseHeaders().remove(HttpHeaderNames.TRANSFER_ENCODING);
+		NettyHeadersAdapter headersMap = new NettyHeadersAdapter(channel.responseHeaders());
+		return new HttpHeaders(headersMap);
+	}
 
 	@SuppressWarnings("unchecked")
 	@Override
@@ -80,11 +87,9 @@ class ReactorServerHttpResponse extends AbstractServerHttpResponse implements Ze
 
 	@Override
 	protected void applyHeaders() {
-		getHeaders().forEach((headerName, headerValues) -> {
+		if (getHeaders().getContentLength() == -1) {
-			for (String value : headerValues) {
+			this.response.chunkedTransfer(true);
-				this.response.responseHeaders().add(headerName, value);
+		}
-			}
-		});
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpRequest.java

@@ -69,12 +69,18 @@ class ServletServerHttpRequest extends AbstractServerHttpRequest {
 
 	private final byte[] buffer;
 
-
 	public ServletServerHttpRequest(HttpServletRequest request, AsyncContext asyncContext,
 			String servletPath, DataBufferFactory bufferFactory, int bufferSize)
 			throws IOException, URISyntaxException {
 
-		super(initUri(request), request.getContextPath() + servletPath, initHeaders(request));
+		this(createDefaultHttpHeaders(request), request, asyncContext, servletPath, bufferFactory, bufferSize);
+	}
+
+	public ServletServerHttpRequest(HttpHeaders headers, HttpServletRequest request, AsyncContext asyncContext,
+			String servletPath, DataBufferFactory bufferFactory, int bufferSize)
+			throws IOException, URISyntaxException {
+
+		super(initUri(request), request.getContextPath() + servletPath, initHeaders(headers, request));
 
 		Assert.notNull(bufferFactory, "'bufferFactory' must not be null");
 		Assert.isTrue(bufferSize > 0, "'bufferSize' must be higher than 0");
@@ -91,6 +97,18 @@ class ServletServerHttpRequest extends AbstractServerHttpRequest {
 		this.bodyPublisher.registerReadListener();
 	}
 
+
+	private static HttpHeaders createDefaultHttpHeaders(HttpServletRequest request) {
+		HttpHeaders headers = new HttpHeaders();
+		for (Enumeration<?> names = request.getHeaderNames(); names.hasMoreElements(); ) {
+			String name = (String) names.nextElement();
+			for (Enumeration<?> values = request.getHeaders(name); values.hasMoreElements(); ) {
+				headers.add(name, (String) values.nextElement());
+			}
+		}
+		return headers;
+	}
+
 	private static URI initUri(HttpServletRequest request) throws URISyntaxException {
 		Assert.notNull(request, "'request' must not be null");
 		StringBuffer url = request.getRequestURL();
@@ -101,16 +119,7 @@ class ServletServerHttpRequest extends AbstractServerHttpRequest {
 		return new URI(url.toString());
 	}
 
-	private static HttpHeaders initHeaders(HttpServletRequest request) {
+	private static HttpHeaders initHeaders(HttpHeaders headers, HttpServletRequest request) {
-		HttpHeaders headers = new HttpHeaders();
-		for (Enumeration<?> names = request.getHeaderNames();
-			names.hasMoreElements(); ) {
-			String name = (String) names.nextElement();
-			for (Enumeration<?> values = request.getHeaders(name);
-				values.hasMoreElements(); ) {
-				headers.add(name, (String) values.nextElement());
-			}
-		}
 		MediaType contentType = headers.getContentType();
 		if (contentType == null) {
 			String requestContentType = request.getContentType();
@@ -231,7 +240,8 @@ class ServletServerHttpRequest extends AbstractServerHttpRequest {
 	private final class RequestAsyncListener implements AsyncListener {
 
 		@Override
-		public void onStartAsync(AsyncEvent event) {}
+		public void onStartAsync(AsyncEvent event) {
+		}
 
 		@Override
 		public void onTimeout(AsyncEvent event) {

spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java

@@ -33,6 +33,7 @@ import org.reactivestreams.Publisher;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseCookie;
 import org.springframework.lang.Nullable;
@@ -62,11 +63,16 @@ class ServletServerHttpResponse extends AbstractListenerServerHttpResponse {
 
 	private final ServletServerHttpRequest request;
 
-
 	public ServletServerHttpResponse(HttpServletResponse response, AsyncContext asyncContext,
 			DataBufferFactory bufferFactory, int bufferSize, ServletServerHttpRequest request) throws IOException {
 
-		super(bufferFactory);
+		this(new HttpHeaders(), response, asyncContext, bufferFactory, bufferSize, request);
+	}
+
+	public ServletServerHttpResponse(HttpHeaders headers, HttpServletResponse response, AsyncContext asyncContext,
+			DataBufferFactory bufferFactory, int bufferSize, ServletServerHttpRequest request) throws IOException {
+
+		super(bufferFactory, headers);
 
 		Assert.notNull(response, "HttpServletResponse must not be null");
 		Assert.notNull(bufferFactory, "DataBufferFactory must not be null");

spring-web/src/main/java/org/springframework/http/server/reactive/TomcatHeadersAdapter.java

@@ -0,0 +1,237 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.http.server.reactive;
+
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Enumeration;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import org.apache.tomcat.util.buf.MessageBytes;
+import org.apache.tomcat.util.http.MimeHeaders;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * {@code MultiValueMap} implementation for wrapping Tomcat HTTP headers.
+ *
+ * @author Brian Clozel
+ * @since 5.1
+ */
+class TomcatHeadersAdapter implements MultiValueMap<String, String> {
+
+	private final MimeHeaders headers;
+
+	TomcatHeadersAdapter(MimeHeaders headers) {
+		this.headers = headers;
+	}
+
+	@Override
+	public String getFirst(String key) {
+		return this.headers.getHeader(key);
+	}
+
+	@Override
+	public void add(String key, String value) {
+		this.headers.addValue(key).setString(value);
+	}
+
+	@Override
+	public void addAll(String key, List<? extends String> values) {
+		values.forEach(value -> add(key, value));
+	}
+
+	@Override
+	public void addAll(MultiValueMap<String, String> values) {
+		values.forEach(this::addAll);
+	}
+
+	@Override
+	public void set(String key, String value) {
+		this.headers.setValue(key).setString(value);
+	}
+
+	@Override
+	public void setAll(Map<String, String> values) {
+		values.forEach(this::set);
+	}
+
+	@Override
+	public Map<String, String> toSingleValueMap() {
+		Map<String, String> singleValueMap = new LinkedHashMap<>(this.headers.size());
+		this.keySet().forEach(key -> singleValueMap.put(key, getFirst(key)));
+		return singleValueMap;
+	}
+
+	@Override
+	public int size() {
+		Enumeration<String> names = this.headers.names();
+		int size = 0;
+		while (names.hasMoreElements()) {
+			size++;
+			names.nextElement();
+		}
+		return size;
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.headers.size() == 0;
+	}
+
+	@Override
+	public boolean containsKey(Object key) {
+		if (key instanceof String) {
+			return this.headers.findHeader((String) key, 0) != -1;
+		}
+		return false;
+	}
+
+	@Override
+	public boolean containsValue(Object value) {
+		if (value instanceof String) {
+			MessageBytes needle = MessageBytes.newInstance();
+			needle.setString((String) value);
+			for (int i = 0; i < this.headers.size(); i++) {
+				if (this.headers.getValue(i).equals(needle)) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
+	@Override
+	@Nullable
+	public List<String> get(Object key) {
+		if (key instanceof String) {
+			return Collections.list(this.headers.values((String) key));
+		}
+		return null;
+	}
+
+	@Override
+	@Nullable
+	public List<String> put(String key, List<String> value) {
+		List<String> previousValues = get(key);
+		value.forEach(v -> this.headers.addValue(key).setString(v));
+		return previousValues;
+	}
+
+	@Override
+	@Nullable
+	public List<String> remove(Object key) {
+		if (key instanceof String) {
+			List<String> previousValues = get(key);
+			this.headers.removeHeader((String) key);
+			return previousValues;
+		}
+		return null;
+	}
+
+	@Override
+	public void putAll(Map<? extends String, ? extends List<String>> m) {
+		m.forEach(this::put);
+	}
+
+	@Override
+	public void clear() {
+		this.headers.clear();
+	}
+
+	@Override
+	public Set<String> keySet() {
+		Set<String> result = new HashSet<>(8);
+		Enumeration<String> names = this.headers.names();
+		while (names.hasMoreElements()) {
+			result.add(names.nextElement());
+		}
+		return result;
+	}
+
+	@Override
+	public Collection<List<String>> values() {
+		return keySet().stream().map(this::get).collect(Collectors.toList());
+	}
+
+	@Override
+	public Set<Entry<String, List<String>>> entrySet() {
+		return new AbstractSet<Entry<String, List<String>>>() {
+			@Override
+			public Iterator<Entry<String, List<String>>> iterator() {
+				return new EntryIterator();
+			}
+
+			@Override
+			public int size() {
+				return headers.size();
+			}
+		};
+	}
+
+	private class EntryIterator implements Iterator<Entry<String, List<String>>> {
+
+		private Enumeration<String> names = headers.names();
+
+		@Override
+		public boolean hasNext() {
+			return this.names.hasMoreElements();
+		}
+
+		@Override
+		public Entry<String, List<String>> next() {
+			return new HeaderEntry(this.names.nextElement());
+		}
+	}
+
+	private final class HeaderEntry implements Entry<String, List<String>> {
+
+		private final String key;
+
+		private HeaderEntry(String key) {
+			this.key = key;
+		}
+
+		@Override
+		public String getKey() {
+			return this.key;
+		}
+
+		@Nullable
+		@Override
+		public List<String> getValue() {
+			return get(this.key);
+		}
+
+		@Nullable
+		@Override
+		public List<String> setValue(List<String> value) {
+			List<String> previous = getValue();
+			headers.removeHeader(this.key);
+			addAll(this.key, value);
+			return previous;
+		}
+	}
+}

spring-web/src/main/java/org/springframework/http/server/reactive/TomcatHttpHandlerAdapter.java

@@ -17,6 +17,7 @@
 package org.springframework.http.server.reactive;
 
 import java.io.IOException;
+import java.lang.reflect.Field;
 import java.net.URISyntaxException;
 import java.nio.ByteBuffer;
 import javax.servlet.AsyncContext;
@@ -27,17 +28,25 @@ import javax.servlet.http.HttpServletResponse;
 
 import org.apache.catalina.connector.CoyoteInputStream;
 import org.apache.catalina.connector.CoyoteOutputStream;
+import org.apache.catalina.connector.RequestFacade;
+import org.apache.catalina.connector.ResponseFacade;
+import org.apache.coyote.Request;
+import org.apache.coyote.Response;
 
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.http.HttpHeaders;
 import org.springframework.util.Assert;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * {@link ServletHttpHandlerAdapter} extension that uses Tomcat APIs for reading
  * from the request and writing to the response with {@link ByteBuffer}.
  *
  * @author Violeta Georgieva
+ * @author Brian Clozel
+ * @author Brian Clozel
  * @since 5.0
  * @see org.springframework.web.server.adapter.AbstractReactiveWebInitializer
  */
@@ -66,21 +75,39 @@ public class TomcatHttpHandlerAdapter extends ServletHttpHandlerAdapter {
 				response, asyncContext, getDataBufferFactory(), getBufferSize(), request);
 	}
 
+	private static final class TomcatServerHttpRequest extends ServletServerHttpRequest {
 
-	private final class TomcatServerHttpRequest extends ServletServerHttpRequest {
+		private static final Field COYOTE_REQUEST_FIELD = ReflectionUtils.findField(RequestFacade.class, "request");
 
-		public TomcatServerHttpRequest(HttpServletRequest request, AsyncContext context,
+		private final int bufferSize;
+
+		private final DataBufferFactory factory;
+
+		static {
+			ReflectionUtils.makeAccessible(COYOTE_REQUEST_FIELD);
+		}
+
+		TomcatServerHttpRequest(HttpServletRequest request, AsyncContext context,
 				String servletPath, DataBufferFactory factory, int bufferSize)
 				throws IOException, URISyntaxException {
 
-			super(request, context, servletPath, factory, bufferSize);
+			super(createTomcatHttpHeaders(request), request, context, servletPath, factory, bufferSize);
+			this.factory = factory;
+			this.bufferSize = bufferSize;
+		}
+
+		private static HttpHeaders createTomcatHttpHeaders(HttpServletRequest request) {
+			Request tomcatRequest = ((org.apache.catalina.connector.Request) ReflectionUtils
+					.getField(COYOTE_REQUEST_FIELD, request)).getCoyoteRequest();
+			TomcatHeadersAdapter headers = new TomcatHeadersAdapter(tomcatRequest.getMimeHeaders());
+			return new HttpHeaders(headers);
 		}
 
 		@Override
 		protected DataBuffer readFromInputStream() throws IOException {
 			boolean release = true;
-			int capacity = getBufferSize();
+			int capacity = this.bufferSize;
-			DataBuffer dataBuffer = getDataBufferFactory().allocateBuffer(capacity);
+			DataBuffer dataBuffer = this.factory.allocateBuffer(capacity);
 			try {
 				ByteBuffer byteBuffer = dataBuffer.asByteBuffer(0, capacity);
 
@@ -111,10 +138,27 @@ public class TomcatHttpHandlerAdapter extends ServletHttpHandlerAdapter {
 
 	private static final class TomcatServerHttpResponse extends ServletServerHttpResponse {
 
-		public TomcatServerHttpResponse(HttpServletResponse response, AsyncContext context,
+		private static final Field COYOTE_RESPONSE_FIELD = ReflectionUtils.findField(ResponseFacade.class, "response");
+
+		static {
+			ReflectionUtils.makeAccessible(COYOTE_RESPONSE_FIELD);
+		}
+
+		TomcatServerHttpResponse(HttpServletResponse response, AsyncContext context,
 				DataBufferFactory factory, int bufferSize, ServletServerHttpRequest request) throws IOException {
 
-			super(response, context, factory, bufferSize, request);
+			super(createTomcatHttpHeaders(response), response, context, factory, bufferSize, request);
+		}
+
+		private static HttpHeaders createTomcatHttpHeaders(HttpServletResponse response) {
+			Response tomcatResponse = ((org.apache.catalina.connector.Response) ReflectionUtils
+					.getField(COYOTE_RESPONSE_FIELD, response)).getCoyoteResponse();
+			TomcatHeadersAdapter headers = new TomcatHeadersAdapter(tomcatResponse.getMimeHeaders());
+			return new HttpHeaders(headers);
+		}
+
+		@Override
+		protected void applyHeaders() {
 		}
 
 		@Override

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java

@@ -0,0 +1,222 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.http.server.reactive;
+
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import io.undertow.util.HeaderMap;
+import io.undertow.util.HeaderValues;
+import io.undertow.util.HttpString;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.MultiValueMap;
+
+/**
+ * {@code MultiValueMap} implementation for wrapping Undertow HTTP headers.
+ *
+ * @author Brian Clozel
+ * @since 5.1
+ */
+class UndertowHeadersAdapter implements MultiValueMap<String, String> {
+
+	private final HeaderMap headers;
+
+	UndertowHeadersAdapter(HeaderMap headers) {
+		this.headers = headers;
+	}
+
+	@Override
+	public String getFirst(String key) {
+		return this.headers.getFirst(key);
+	}
+
+	@Override
+	public void add(String key, @Nullable String value) {
+		this.headers.add(HttpString.tryFromString(key), value);
+	}
+
+	@Override
+	@SuppressWarnings("unchecked")
+	public void addAll(String key, List<? extends String> values) {
+		this.headers.addAll(HttpString.tryFromString(key), (List<String>) values);
+	}
+
+	@Override
+	public void addAll(MultiValueMap<String, String> values) {
+		values.forEach((key, list) -> this.headers.addAll(HttpString.tryFromString(key), list));
+	}
+
+	@Override
+	public void set(String key, @Nullable String value) {
+		this.headers.put(HttpString.tryFromString(key), value);
+	}
+
+	@Override
+	public void setAll(Map<String, String> values) {
+		values.forEach((key, list) -> this.headers.put(HttpString.tryFromString(key), list));
+	}
+
+	@Override
+	public Map<String, String> toSingleValueMap() {
+		Map<String, String> singleValueMap = new LinkedHashMap<>(this.headers.size());
+		this.headers.forEach(values ->
+				singleValueMap.put(values.getHeaderName().toString(), values.getFirst()));
+		return singleValueMap;
+	}
+
+	@Override
+	public int size() {
+		return this.headers.size();
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.headers.size() == 0;
+	}
+
+	@Override
+	public boolean containsKey(Object key) {
+		if (key instanceof String) {
+			return this.headers.contains((String) key);
+		}
+		return false;
+	}
+
+	@Override
+	public boolean containsValue(Object value) {
+		if (value instanceof String) {
+			return this.headers.getHeaderNames().stream()
+					.map(this.headers::get)
+					.anyMatch(values -> values.contains(value));
+		}
+		return false;
+	}
+
+	@Override
+	@Nullable
+	public List<String> get(Object key) {
+		if (key instanceof String) {
+			return this.headers.get((String) key);
+		}
+		return null;
+	}
+
+	@Override
+	@Nullable
+	public List<String> put(String key, List<String> value) {
+		HeaderValues previousValues = this.headers.get(key);
+		this.headers.putAll(HttpString.tryFromString(key), value);
+		return previousValues;
+	}
+
+	@Override
+	@Nullable
+	public List<String> remove(Object key) {
+		if (key instanceof String) {
+			this.headers.remove((String) key);
+		}
+		return null;
+	}
+
+	@Override
+	public void putAll(Map<? extends String, ? extends List<String>> m) {
+		m.forEach((key, values) ->
+				this.headers.putAll(HttpString.tryFromString(key), values));
+	}
+
+	@Override
+	public void clear() {
+		this.headers.clear();
+	}
+
+	@Override
+	public Set<String> keySet() {
+		return this.headers.getHeaderNames().stream()
+				.map(HttpString::toString)
+				.collect(Collectors.toSet());
+	}
+
+	@Override
+	public Collection<List<String>> values() {
+		return this.headers.getHeaderNames().stream()
+				.map(this.headers::get)
+				.collect(Collectors.toList());
+	}
+
+	@Override
+	public Set<Entry<String, List<String>>> entrySet() {
+		return new AbstractSet<Entry<String, List<String>>>() {
+			@Override
+			public Iterator<Entry<String, List<String>>> iterator() {
+				return new EntryIterator();
+			}
+
+			@Override
+			public int size() {
+				return headers.size();
+			}
+		};
+	}
+
+	private class EntryIterator implements Iterator<Entry<String, List<String>>> {
+
+		private Iterator<HttpString> names = headers.getHeaderNames().iterator();
+
+		@Override
+		public boolean hasNext() {
+			return this.names.hasNext();
+		}
+
+		@Override
+		public Entry<String, List<String>> next() {
+			return new HeaderEntry(this.names.next());
+		}
+	}
+
+	private class HeaderEntry implements Entry<String, List<String>> {
+
+		private final HttpString key;
+
+		HeaderEntry(HttpString key) {
+			this.key = key;
+		}
+
+		@Override
+		public String getKey() {
+			return this.key.toString();
+		}
+
+		@Override
+		public List<String> getValue() {
+			return headers.get(this.key);
+		}
+
+		@Override
+		public List<String> setValue(List<String> value) {
+			List<String> previousValues = headers.get(this.key);
+			headers.putAll(this.key, value);
+			return previousValues;
+		}
+	}
+}

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpRequest.java

@@ -30,7 +30,6 @@ import io.undertow.connector.ByteBufferPool;
 import io.undertow.connector.PooledByteBuffer;
 import io.undertow.server.HttpServerExchange;
 import io.undertow.server.handlers.Cookie;
-import io.undertow.util.HeaderValues;
 import org.xnio.channels.StreamSourceChannel;
 import reactor.core.publisher.Flux;
 
@@ -79,11 +78,9 @@ class UndertowServerHttpRequest extends AbstractServerHttpRequest {
 	}
 
 	private static HttpHeaders initHeaders(HttpServerExchange exchange) {
-		HttpHeaders headers = new HttpHeaders();
+		UndertowHeadersAdapter headersMap =
-		for (HeaderValues values : exchange.getRequestHeaders()) {
+				new UndertowHeadersAdapter(exchange.getRequestHeaders());
-			headers.put(values.getHeaderName().toString(), values);
+		return new HttpHeaders(headersMap);
-		}
-		return headers;
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java

@@ -25,7 +25,6 @@ import java.nio.file.StandardOpenOption;
 import io.undertow.server.HttpServerExchange;
 import io.undertow.server.handlers.Cookie;
 import io.undertow.server.handlers.CookieImpl;
-import io.undertow.util.HttpString;
 import org.reactivestreams.Processor;
 import org.reactivestreams.Publisher;
 import org.xnio.channels.Channels;
@@ -35,6 +34,7 @@ import reactor.core.publisher.Mono;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.ResponseCookie;
 import org.springframework.http.ZeroCopyHttpOutputMessage;
 import org.springframework.lang.Nullable;
@@ -58,15 +58,21 @@ class UndertowServerHttpResponse extends AbstractListenerServerHttpResponse impl
 	private StreamSinkChannel responseChannel;
 
 
-	public UndertowServerHttpResponse(
+	UndertowServerHttpResponse(
 			HttpServerExchange exchange, DataBufferFactory bufferFactory, UndertowServerHttpRequest request) {
 
-		super(bufferFactory);
+		super(bufferFactory, createHeaders(exchange));
 		Assert.notNull(exchange, "HttpServerExchange must not be null");
 		this.exchange = exchange;
 		this.request = request;
 	}
 
+	private static HttpHeaders createHeaders(HttpServerExchange exchange) {
+		UndertowHeadersAdapter headersMap =
+				new UndertowHeadersAdapter(exchange.getResponseHeaders());
+		return new HttpHeaders(headersMap);
+	}
+
 
 	@SuppressWarnings("unchecked")
 	@Override
@@ -85,8 +91,6 @@ class UndertowServerHttpResponse extends AbstractListenerServerHttpResponse impl
 
 	@Override
 	protected void applyHeaders() {
-		getHeaders().forEach((headerName, headerValues) ->
-				this.exchange.getResponseHeaders().addAll(HttpString.tryFromString(headerName), headerValues));
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/web/cors/reactive/DefaultCorsProcessor.java

@@ -87,7 +87,7 @@ public class DefaultCorsProcessor implements CorsProcessor {
 	}
 
 	private boolean responseHasCors(ServerHttpResponse response) {
-		return (response.getHeaders().getAccessControlAllowOrigin() != null);
+		return response.getHeaders().getFirst(HttpHeaders.ACCESS_CONTROL_ALLOW_ORIGIN) != null;
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/socket/server/support/HandshakeWebSocketService.java

@@ -272,7 +272,10 @@ public class HandshakeWebSocketService implements WebSocketService, Lifecycle {
 			@Nullable String protocol, Map<String, Object> attributes) {
 
 		URI uri = request.getURI();
-		HttpHeaders headers = request.getHeaders();
+		// Copy request headers, as they might be pooled and recycled by
+		// the server implementation once the handshake HTTP exchange is done.
+		HttpHeaders headers = new HttpHeaders();
+		headers.addAll(request.getHeaders());
 		Mono<Principal> principal = exchange.getPrincipal();
 		String logPrefix = exchange.getLogPrefix();
 		InetSocketAddress remoteAddress = request.getRemoteAddress();