Add close() method on HTTP client response

Before this commit, there was no way to signal the HTTP client that we
were done consuming the response. Without that, the underlying client
library cannot know when it is safe to release the associated resources
(e.g. the HTTP connection).

This commit adds new `close()` methods on both `ClientHttpResponse`
and `ClientResponse`. This methods is non-blocking and its behavior
depends on the library, its configuration, HTTP version, etc.

At the `WebClient` level, `close()` is called automatically if we
consume the response body through the `ResponseSpec` or the
`ClientResponse` itself.

Note that it is *required* to call `close()` manually otherwise; not
doing so might create resource leaks or connection issues.

Issue: SPR-15920

Author: bclozel

spring-test/src/main/java/org/springframework/mock/http/client/reactive/MockClientHttpResponse.java

@@ -55,6 +55,8 @@ public class MockClientHttpResponse implements ClientHttpResponse {
 
 	private final DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
+	private boolean closed = false;
+
 
 	public MockClientHttpResponse(HttpStatus status) {
 		Assert.notNull(status, "HttpStatus is required");
@@ -94,10 +96,23 @@ private DataBuffer toDataBuffer(String body, Charset charset) {
 		return this.bufferFactory.wrap(byteBuffer);
 	}
 
+	@Override
 	public Flux<DataBuffer> getBody() {
+		if (this.closed) {
+			return Flux.error(new IllegalStateException("Connection has been closed."));
+		}
 		return this.body;
 	}
 
+	@Override
+	public void close() {
+		this.closed = true;
+	}
+
+	public boolean isClosed() {
+		return this.closed;
+	}
+
 	/**
 	 * Return the response body aggregated and converted to a String using the
 	 * charset of the Content-Type response or otherwise as "UTF-8".

spring-web/src/main/java/org/springframework/http/client/reactive/ClientHttpResponse.java

@@ -16,6 +16,8 @@
 
 package org.springframework.http.client.reactive;
 
+import java.io.Closeable;
+
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.http.ResponseCookie;
@@ -25,9 +27,10 @@
  * Represents a client-side reactive HTTP response.
  *
  * @author Arjen Poutsma
+ * @author Brian Clozel
  * @since 5.0
  */
-public interface ClientHttpResponse extends ReactiveHttpInputMessage {
+public interface ClientHttpResponse extends ReactiveHttpInputMessage, Closeable {
 
 	/**
 	 * Return the HTTP status as an {@link HttpStatus} enum value.
@@ -39,4 +42,15 @@ public interface ClientHttpResponse extends ReactiveHttpInputMessage {
 	 */
 	MultiValueMap<String, ResponseCookie> getCookies();
 
+	/**
+	 * Close this response, freeing any resources created.
+	 * <p>This non-blocking method has to be called once the response has been
+	 * processed and the resources are no longer needed; not doing so might
+	 * create resource leaks or connection issues.
+	 * <p>Depending on the client configuration and HTTP version,
+	 * this can lead to closing the connection or returning it to a connection pool.
+	 */
+	@Override
+	void close();
+
 }

spring-web/src/main/java/org/springframework/http/client/reactive/ClientHttpResponseDecorator.java

@@ -70,6 +70,10 @@ public Flux<DataBuffer> getBody() {
 		return this.delegate.getBody();
 	}
 
+	@Override
+	public void close() {
+		this.delegate.close();
+	}
 
 	@Override
 	public String toString() {

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpResponse.java

@@ -89,6 +89,10 @@ public MultiValueMap<String, ResponseCookie> getCookies() {
 		return CollectionUtils.unmodifiableMultiValueMap(result);
 	}
 
+	@Override
+	public void close() {
+		this.response.dispose();
+	}
 
 	@Override
 	public String toString() {

spring-web/src/test/java/org/springframework/mock/http/client/reactive/test/MockClientHttpResponse.java

@@ -55,13 +55,15 @@ public class MockClientHttpResponse implements ClientHttpResponse {
 
 	private final DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
+	private boolean closed = false;
 
 	public MockClientHttpResponse(HttpStatus status) {
 		Assert.notNull(status, "HttpStatus is required");
 		this.status = status;
 	}
 
 
+	@Override
 	public HttpStatus getStatusCode() {
 		return this.status;
 	}
@@ -71,6 +73,7 @@ public HttpHeaders getHeaders() {
 		return this.headers;
 	}
 
+	@Override
 	public MultiValueMap<String, ResponseCookie> getCookies() {
 		return this.cookies;
 	}
@@ -94,10 +97,23 @@ private DataBuffer toDataBuffer(String body, Charset charset) {
 		return this.bufferFactory.wrap(byteBuffer);
 	}
 
+	@Override
 	public Flux<DataBuffer> getBody() {
+		if (this.closed) {
+			return Flux.error(new IllegalStateException("Connection has been closed."));
+		}
 		return this.body;
 	}
 
+	@Override
+	public void close() {
+		this.closed = true;
+	}
+
+	public boolean isClosed() {
+		return this.closed;
+	}
+
 	/**
 	 * Return the response body aggregated and converted to a String using the
 	 * charset of the Content-Type response or otherwise as "UTF-8".

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/ClientResponse.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.client;
 
+import java.io.Closeable;
 import java.util.List;
 import java.util.Optional;
 import java.util.OptionalLong;
@@ -43,7 +44,7 @@
  * @author Arjen Poutsma
  * @since 5.0
  */
-public interface ClientResponse {
+public interface ClientResponse extends Closeable {
 
 	/**
 	 * Return the status code of this response.
@@ -132,6 +133,18 @@ public interface ClientResponse {
 	 */
 	<T> Mono<ResponseEntity<List<T>>> toEntityList(ParameterizedTypeReference<T> typeReference);
 
+	/**
+	 * Close this response, freeing any resources created.
+	 * <p>This non-blocking method has to be called once the response has been processed
+	 * and the resources are no longer needed.
+	 * <p>{@code ClientResponse.bodyTo*}, {@code ClientResponse.toEntity*}
+	 * and all methods under {@code WebClient.retrieve()} will close the response
+	 * automatically.
+	 * <p>It is required to call close() manually otherwise; not doing so might
+	 * create resource leaks or connection issues.
+	 */
+	@Override
+	void close();
 
 	/**
 	 * Represents the headers of the HTTP response.

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponse.java

@@ -42,6 +42,7 @@
  * Default implementation of {@link ClientResponse}.
  *
  * @author Arjen Poutsma
+ * @author Brian Clozel
  * @since 5.0
  */
 class DefaultClientResponse implements ClientResponse {
@@ -97,22 +98,24 @@ public Map<String, Object> hints() {
 
 	@Override
 	public <T> Mono<T> bodyToMono(Class<? extends T> elementClass) {
-		return body(BodyExtractors.toMono(elementClass));
+		Mono<T> body = body(BodyExtractors.toMono(elementClass));
+		return body.doOnTerminate(this.response::close);
 	}
 
 	@Override
 	public <T> Mono<T> bodyToMono(ParameterizedTypeReference<T> typeReference) {
-		return body(BodyExtractors.toMono(typeReference));
+		return body(BodyExtractors.toMono(typeReference)).doOnTerminate(this.response::close);
 	}
 
 	@Override
 	public <T> Flux<T> bodyToFlux(Class<? extends T> elementClass) {
-		return body(BodyExtractors.toFlux(elementClass));
+		Flux<T> body = body(BodyExtractors.toFlux(elementClass));
+		return body.doOnTerminate(this.response::close);
 	}
 
 	@Override
 	public <T> Flux<T> bodyToFlux(ParameterizedTypeReference<T> typeReference) {
-		return body(BodyExtractors.toFlux(typeReference));
+		return body(BodyExtractors.toFlux(typeReference)).doOnTerminate(this.response::close);
 	}
 
 	@Override
@@ -131,7 +134,8 @@ private <T> Mono<ResponseEntity<T>> toEntityInternal(Mono<T> bodyMono) {
 		return bodyMono
 				.map(body -> new ResponseEntity<>(body, headers, statusCode))
 				.switchIfEmpty(Mono.defer(
-						() -> Mono.just(new ResponseEntity<>(headers, statusCode))));
+						() -> Mono.just(new ResponseEntity<>(headers, statusCode))))
+				.doOnTerminate(this.response::close);
 	}
 
 	@Override
@@ -150,9 +154,14 @@ private <T> Mono<ResponseEntity<List<T>>> toEntityListInternal(Flux<T> bodyFlux)
 		HttpStatus statusCode = statusCode();
 		return bodyFlux
 				.collectList()
-				.map(body -> new ResponseEntity<>(body, headers, statusCode));
+				.map(body -> new ResponseEntity<>(body, headers, statusCode))
+				.doOnTerminate(this.response::close);
 	}
 
+	@Override
+	public void close() {
+		this.response.close();
+	}
 
 	private class DefaultHeaders implements Headers {
 

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -413,50 +413,64 @@ public ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate,
 		@SuppressWarnings("unchecked")
 		public <T> Mono<T> bodyToMono(Class<T> bodyType) {
 			return this.responseMono.flatMap(
-					response -> bodyToPublisher(response, BodyExtractors.toMono(bodyType),
+					response -> bodyToMono(response, BodyExtractors.toMono(bodyType),
 							this::monoThrowableToMono));
 		}
 
 		@Override
 		@SuppressWarnings("unchecked")
 		public <T> Mono<T> bodyToMono(ParameterizedTypeReference<T> typeReference) {
 			return this.responseMono.flatMap(
-					response -> bodyToPublisher(response, BodyExtractors.toMono(typeReference),
+					response -> bodyToMono(response, BodyExtractors.toMono(typeReference),
 							mono -> (Mono<T>)mono));
 		}
 
 		private <T> Mono<T> monoThrowableToMono(Mono<? extends Throwable> mono) {
 			return mono.flatMap(Mono::error);
 		}
 
+		private <T> Mono<T> bodyToMono(ClientResponse response,
+				BodyExtractor<Mono<T>, ? super ClientHttpResponse> extractor,
+				Function<Mono<? extends Throwable>, Mono<T>> errorFunction) {
+
+			return this.statusHandlers.stream()
+					.filter(statusHandler -> statusHandler.test(response.statusCode()))
+					.findFirst()
+					.map(statusHandler -> statusHandler.apply(response))
+					.map(errorFunction::apply)
+					.orElse(response.body(extractor))
+					.doAfterTerminate(response::close);
+		}
+
  		@Override
 		public <T> Flux<T> bodyToFlux(Class<T> elementType) {
 			return this.responseMono.flatMapMany(
-					response -> bodyToPublisher(response, BodyExtractors.toFlux(elementType),
+					response -> bodyToFlux(response, BodyExtractors.toFlux(elementType),
 							this::monoThrowableToFlux));
 		}
 
 		@Override
 		public <T> Flux<T> bodyToFlux(ParameterizedTypeReference<T> typeReference) {
 			return this.responseMono.flatMapMany(
-					response -> bodyToPublisher(response, BodyExtractors.toFlux(typeReference),
+					response -> bodyToFlux(response, BodyExtractors.toFlux(typeReference),
 							this::monoThrowableToFlux));
 		}
 
 		private <T> Flux<T> monoThrowableToFlux(Mono<? extends Throwable> mono) {
 			return mono.flatMapMany(Flux::error);
 		}
 
-		private <T extends Publisher<?>> T bodyToPublisher(ClientResponse response,
-				BodyExtractor<T, ? super ClientHttpResponse> extractor,
-				Function<Mono<? extends Throwable>, T> errorFunction) {
+		private <T> Flux<T> bodyToFlux(ClientResponse response,
+				BodyExtractor<Flux<T>, ? super ClientHttpResponse> extractor,
+				Function<Mono<? extends Throwable>, Flux<T>> errorFunction) {
 
 			return this.statusHandlers.stream()
 					.filter(statusHandler -> statusHandler.test(response.statusCode()))
 					.findFirst()
 					.map(statusHandler -> statusHandler.apply(response))
 					.map(errorFunction::apply)
-					.orElse(response.body(extractor));
+					.orElse(response.body(extractor))
+					.doAfterTerminate(response::close);
 		}
 
 		private static Mono<WebClientResponseException> createResponseException(ClientResponse response) {

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/WebClient.java

@@ -461,6 +461,17 @@ interface RequestHeadersSpec<S extends RequestHeadersSpec<S>> {
 		 *     .exchange()
 		 *     .flatMapMany(response -> response.bodyToFlux(Pojo.class));
 		 * </pre>
+		 * <p>If the response body is not consumed with {@code bodyTo*}
+		 * or {@code toEntity*} methods, it is your responsibility
+		 * to release the HTTP resources with {@link ClientResponse#close()}.
+		 * <pre>
+		 * Mono&lt;HttpStatus&gt; mono = client.get().uri("/")
+		 *     .exchange()
+		 *     .map(response -> {
+		 *         response.close();
+		 *         return response.statusCode();
+		 *     });
+		 * </pre>
 		 * @return a {@code Mono} with the response
 		 * @see #retrieve()
 		 */

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -70,6 +70,12 @@ public void shutdown() throws Exception {
 	public void headers() throws Exception {
 		this.server.enqueue(new MockResponse().setHeader("Content-Type", "text/plain").setBody("Hello Spring!"));
 
+		this.webClient.get().uri("/test")
+				.exchange()
+				.map(response -> {
+					response.close();
+					return response.statusCode();
+				});
 		Mono<HttpHeaders> result = this.webClient.get()
 				.uri("/greeting?name=Spring")
 				.exchange()