Add local error handling in WebClient.retrieve

This commit introduces a way to customize the WebClientExceptions, as
thrown by WebClient.ResponseSpec.bodyTo[Mono|Flux]. The first
customization will override the defaults, additional customizations are
simply tried in order.

Issue: SPR-15724

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

@@ -465,6 +465,16 @@ public enum HttpStatus {
 		return Series.SERVER_ERROR.equals(series());
 	}
 
+	/**
+	 * Whether this status code is in the HTTP series
+	 * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR} or
+	 * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
+	 * This is a shortcut for checking the value of {@link #series()}.
+	 */
+	public boolean isError() {
+		return is4xxClientError() || is5xxServerError();
+	}
+
 	/**
 	 * Returns the HTTP status series of this status code.
 	 * @see HttpStatus.Series

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

@@ -21,12 +21,15 @@ import java.nio.charset.Charset;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
 import java.time.format.DateTimeFormatter;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -365,11 +368,52 @@ class DefaultWebClient implements WebClient {
 
 	private static class DefaultResponseSpec implements ResponseSpec {
 
+		private static final Function<ClientResponse, Optional<? extends Throwable>> DEFAULT_STATUS_HANDLER =
+				clientResponse -> {
+					HttpStatus statusCode = clientResponse.statusCode();
+					if (statusCode.isError()) {
+						return Optional.of(new WebClientException(
+								"ClientResponse has erroneous status code: " + statusCode.value() +
+										" " + statusCode.getReasonPhrase()));
+					} else {
+						return Optional.empty();
+					}
+				};
+
 		private final Mono<ClientResponse> responseMono;
 
+		private List<Function<ClientResponse, Optional<? extends Throwable>>> statusHandlers =
+				new ArrayList<>(1);
+
 
 		DefaultResponseSpec(Mono<ClientResponse> responseMono) {
 			this.responseMono = responseMono;
+			this.statusHandlers.add(DEFAULT_STATUS_HANDLER);
+		}
+
+		@Override
+		public ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate,
+				Function<ClientResponse, ? extends Throwable> exceptionFunction) {
+
+			Assert.notNull(statusPredicate, "'statusPredicate' must not be null");
+			Assert.notNull(exceptionFunction, "'exceptionFunction' must not be null");
+
+			if (this.statusHandlers.size() == 1 && this.statusHandlers.get(0) == DEFAULT_STATUS_HANDLER) {
+				this.statusHandlers.clear();
+			}
+
+			Function<ClientResponse, Optional<? extends Throwable>> statusHandler =
+					clientResponse -> {
+						if (statusPredicate.test(clientResponse.statusCode())) {
+							return Optional.of(exceptionFunction.apply(clientResponse));
+						}
+						else {
+							return Optional.empty();
+						}
+					};
+			this.statusHandlers.add(statusHandler);
+
+			return this;
 		}
 
 		@Override
@@ -388,18 +432,15 @@ class DefaultWebClient implements WebClient {
 
 		private <T extends Publisher<?>> T bodyToPublisher(ClientResponse response,
 				BodyExtractor<T, ? super ClientHttpResponse> extractor,
-				Function<WebClientException, T> errorFunction) {
+				Function<Throwable, T> errorFunction) {
 
-			HttpStatus status = response.statusCode();
-			if (status.is4xxClientError() || status.is5xxServerError()) {
-				WebClientException ex = new WebClientException(
-						"ClientResponse has erroneous status code: " + status.value() +
-								" " + status.getReasonPhrase());
-				return errorFunction.apply(ex);
-			}
-			else {
-				return response.body(extractor);
-			}
+			return this.statusHandlers.stream()
+					.map(statusHandler -> statusHandler.apply(response))
+					.filter(Optional::isPresent)
+					.findFirst()
+					.map(Optional::get)
+					.map(errorFunction::apply)
+					.orElse(response.body(extractor));
 		}
 
 

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

@@ -23,6 +23,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -30,6 +31,7 @@ import reactor.core.publisher.Mono;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.client.reactive.ClientHttpConnector;
@@ -537,6 +539,20 @@ public interface WebClient {
 
 	interface ResponseSpec {
 
+		/**
+		 * Register a custom error function that gets invoked when the given {@link HttpStatus}
+		 * predicate applies. The exception returned from the function will be returned from
+		 * {@link #bodyToMono(Class)} and {@link #bodyToFlux(Class)}.
+		 * <p>By default, an error handler is register that throws a {@link WebClientException}
+		 * when the response status code is 4xx or 5xx.
+		 * @param statusPredicate a predicate that indicates whether {@code exceptionFunction}
+		 * applies
+		 * @param exceptionFunction the function that returns the exception
+		 * @return this builder
+		 */
+		ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate,
+				Function<ClientResponse, ? extends Throwable> exceptionFunction);
+
 		/**
 		 * Extract the body to a {@code Mono}. If the response has status code 4xx or 5xx, the
 		 * {@code Mono} will contain a {@link WebClientException}.

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

@@ -390,6 +390,27 @@ public class WebClientIntegrationTests {
 		Assert.assertEquals("/greeting?name=Spring", recordedRequest.getPath());
 	}
 
+	@Test
+	public void retrieveBodyToCustomStatusHandler() throws Exception {
+		this.server.enqueue(new MockResponse().setResponseCode(500)
+				.setHeader("Content-Type", "text/plain").setBody("Internal Server error"));
+
+		Mono<String> result = this.webClient.get()
+				.uri("/greeting?name=Spring")
+				.retrieve()
+				.onStatus(HttpStatus::is5xxServerError, response -> new MyException("500 error!"))
+				.bodyToMono(String.class);
+
+		StepVerifier.create(result)
+				.expectError(MyException.class)
+				.verify(Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		Assert.assertEquals(1, server.getRequestCount());
+		Assert.assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		Assert.assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
 	@Test
 	public void retrieveToEntityNotFound() throws Exception {
 		this.server.enqueue(new MockResponse().setResponseCode(404)