Improve WebClientResponseException message in case of 1xx/2xx/3xx status

When a response fails to be completely emitted by the remote (connection
termination during the transmission of the response for example), a
WebClientResponseException can be propagated with a confusing message
which mainly reflects the status code and reason phrase, leading to
messages like "200 OK" in such an exception.

This change improves the situation by appending a hint at the underlying
cause whenever getMessage() is called on a WebClientResponseException
which was created with a non-error status code.

Closes gh-33127

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

@@ -272,6 +272,19 @@ public void setBodyDecodeFunction(Function<ResolvableType, ?> decoderFunction) {
 		this.bodyDecodeFunction = decoderFunction;
 	}
 
+	@Override
+	public String getMessage() {
+		if (shouldHintAtResponseFailure()) {
+			return super.getMessage() + ", but response failed with cause: " + getCause();
+		}
+		return super.getMessage();
+	}
+
+	private boolean shouldHintAtResponseFailure() {
+		return this.statusCode.is1xxInformational() ||
+				this.statusCode.is2xxSuccessful() ||
+				this.statusCode.is3xxRedirection();
+	}
 
 	/**
 	 * Create {@code WebClientResponseException} or an HTTP status specific subclass.

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

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2002-2024 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
+ *
+ *      https://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.web.reactive.function.client;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class WebClientResponseExceptionTests {
+
+	@Test
+	void constructWithSuccessStatusCodeAndNoCauseAdditionalMessage() {
+		assertThat(new WebClientResponseException(200, "OK", null, null, null))
+				.hasNoCause()
+				.hasMessage("200 OK, but response failed with cause: null");
+	}
+
+	@Test
+	void constructWith1xxStatusCodeAndCauseAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException(100, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("100 reasonPhrase, but response failed with cause: java.lang.RuntimeException: example cause");
+	}
+
+	@Test
+	void constructWith2xxStatusCodeAndCauseAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException(200, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("200 reasonPhrase, but response failed with cause: java.lang.RuntimeException: example cause");
+	}
+
+	@Test
+	void constructWith3xxStatusCodeAndCauseAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException(300, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("300 reasonPhrase, but response failed with cause: java.lang.RuntimeException: example cause");
+	}
+
+	@Test
+	void constructWithExplicitMessageAndNotErrorCodeAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException("explicit message", 100, "reasonPhrase", null, null, null);
+		assertThat(ex).hasMessage("explicit message, but response failed with cause: null");
+	}
+
+	@Test
+	void constructWithExplicitMessageAndNotErrorCodeAndCauseAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException("explicit message", 100, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("explicit message, but response failed with cause: java.lang.RuntimeException: example cause")
+				.hasRootCauseMessage("example cause");
+	}
+
+	@Test
+	void constructWithExplicitMessageAndErrorCodeAndCauseNoAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException("explicit message", 404, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("explicit message").hasRootCauseMessage("example cause");
+	}
+
+	@Test
+	void constructWith4xxStatusCodeAndCauseNoAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException(400, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("400 reasonPhrase").hasRootCauseMessage("example cause");
+	}
+
+	@Test
+	void constructWith5xxStatusCodeAndCauseNoAdditionalMessage() {
+		final WebClientResponseException ex = new WebClientResponseException(500, "reasonPhrase", null, null, null);
+		ex.initCause(new RuntimeException("example cause"));
+		assertThat(ex).hasMessage("500 reasonPhrase").hasRootCauseMessage("example cause");
+	}
+}