Improve WebClient error handling

This commit introduces two new `WebClient` methods: `retrieveMono` and
`retrieveFlux`, both of which offer direct access to the response body.
More importantly, these methods publish a WebClientException if the
response status code is in the 4xx or 5xx series.

Issue: SPR-14852

spring-web/src/test/java/org/springframework/web/client/reactive/ExchangeFilterFunctionsTests.java

@@ -16,15 +16,20 @@
 
 package org.springframework.web.client.reactive;
 
+import java.util.Optional;
+
 import org.junit.Test;
 import reactor.core.publisher.Mono;
+import reactor.test.subscriber.ScriptedSubscriber;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 /**
  * @author Arjen Poutsma
@@ -80,6 +85,75 @@ public class ExchangeFilterFunctionsTests {
 		assertTrue(filterInvoked[0]);
 	}
 
+
+	@Test
+	public void clientNoError() throws Exception {
+		ClientRequest<Void> request = ClientRequest.GET("http://example.com").build();
+		ClientResponse response = mock(ClientResponse.class);
+		when(response.statusCode()).thenReturn(HttpStatus.OK);
+		ExchangeFunction exchange = r -> Mono.just(response);
+
+		ExchangeFilterFunction standardErrors = ExchangeFilterFunctions.clientError();
+
+		Mono<ClientResponse> result = standardErrors.filter(request, exchange);
+
+		ScriptedSubscriber.<ClientResponse>create()
+				.expectNext(response)
+				.expectComplete()
+				.verify(result);
+	}
+
+	@Test
+	public void serverError() throws Exception {
+		ClientRequest<Void> request = ClientRequest.GET("http://example.com").build();
+		ClientResponse response = mock(ClientResponse.class);
+		when(response.statusCode()).thenReturn(HttpStatus.INTERNAL_SERVER_ERROR);
+		ExchangeFunction exchange = r -> Mono.just(response);
+
+		ExchangeFilterFunction standardErrors = ExchangeFilterFunctions.serverError();
+
+		Mono<ClientResponse> result = standardErrors.filter(request, exchange);
+
+		ScriptedSubscriber.<ClientResponse>create()
+				.expectError(WebClientException.class)
+				.verify(result);
+	}
+
+	@Test
+	public void errorPredicate() throws Exception {
+		ClientRequest<Void> request = ClientRequest.GET("http://example.com").build();
+		ClientResponse response = mock(ClientResponse.class);
+		when(response.statusCode()).thenReturn(HttpStatus.NOT_FOUND);
+		ExchangeFunction exchange = r -> Mono.just(response);
+
+		ExchangeFilterFunction errorPredicate = ExchangeFilterFunctions
+				.errorPredicate(clientResponse -> clientResponse.statusCode().is4xxClientError());
+
+		Mono<ClientResponse> result = errorPredicate.filter(request, exchange);
+
+		ScriptedSubscriber.<ClientResponse>create()
+				.expectError(WebClientException.class)
+				.verify(result);
+	}
+
+
+	@Test
+	public void errorMapperFunction() throws Exception {
+		ClientRequest<Void> request = ClientRequest.GET("http://example.com").build();
+		ClientResponse response = mock(ClientResponse.class);
+		ExchangeFunction exchange = r -> Mono.just(response);
+
+		ExchangeFilterFunction errorMapper = ExchangeFilterFunctions
+				.errorMapper(clientResponse -> Optional.of(new IllegalStateException()));
+
+		Mono<ClientResponse> result = errorMapper.filter(request, exchange);
+
+		ScriptedSubscriber.<ClientResponse>create()
+				.expectError(IllegalStateException.class)
+				.verify(result);
+	}
+
+
 	@Test
 	public void basicAuthentication() throws Exception {
 		ClientRequest<Void> request = ClientRequest.GET("http://example.com").build();

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

@@ -111,6 +111,50 @@ public class WebClientIntegrationTests {
 		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
 	}
 
+	@Test
+	public void retrieveMono() throws Exception {
+		HttpUrl baseUrl = server.url("/greeting?name=Spring");
+		this.server.enqueue(new MockResponse().setBody("Hello Spring!"));
+
+		ClientRequest<Void> request = ClientRequest.GET(baseUrl.toString()).build();
+
+		Mono<String> result = this.webClient
+				.retrieveMono(request, String.class);
+
+		ScriptedSubscriber
+				.<String>create()
+				.expectNext("Hello Spring!")
+				.expectComplete()
+				.verify(result);
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		assertEquals(1, server.getRequestCount());
+		assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
+	@Test
+	public void retrieveFlux() throws Exception {
+		HttpUrl baseUrl = server.url("/greeting?name=Spring");
+		this.server.enqueue(new MockResponse().setBody("Hello Spring!"));
+
+		ClientRequest<Void> request = ClientRequest.GET(baseUrl.toString()).build();
+
+		Flux<String> result = this.webClient
+				.retrieveFlux(request, String.class);
+
+		ScriptedSubscriber
+				.<String>create()
+				.expectNext("Hello Spring!")
+				.expectComplete()
+				.verify(result);
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		assertEquals(1, server.getRequestCount());
+		assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
 	@Test
 	public void jsonString() throws Exception {
 		HttpUrl baseUrl = server.url("/json");
@@ -274,6 +318,50 @@ public class WebClientIntegrationTests {
 		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
 	}
 
+	@Test
+	public void retrieveNotFound() throws Exception {
+		HttpUrl baseUrl = server.url("/greeting?name=Spring");
+		this.server.enqueue(new MockResponse().setResponseCode(404)
+				.setHeader("Content-Type", "text/plain").setBody("Not Found"));
+
+		ClientRequest<Void> request = ClientRequest.GET(baseUrl.toString()).build();
+
+		Mono<String> result = this.webClient
+				.retrieveMono(request, String.class);
+
+		ScriptedSubscriber
+				.<String>create()
+				.expectError(WebClientException.class)
+				.verify(result, Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		assertEquals(1, server.getRequestCount());
+		assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
+	@Test
+	public void retrieveServerError() throws Exception {
+		HttpUrl baseUrl = server.url("/greeting?name=Spring");
+		this.server.enqueue(new MockResponse().setResponseCode(500)
+				.setHeader("Content-Type", "text/plain").setBody("Not Found"));
+
+		ClientRequest<Void> request = ClientRequest.GET(baseUrl.toString()).build();
+
+		Mono<String> result = this.webClient
+				.retrieveMono(request, String.class);
+
+		ScriptedSubscriber
+				.<String>create()
+				.expectError(WebClientException.class)
+				.verify(result, Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		assertEquals(1, server.getRequestCount());
+		assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
 	@Test
 	public void filter() throws Exception {
 		HttpUrl baseUrl = server.url("/greeting?name=Spring");