Fix handling of ResponseStatusException

This commit updates DefaultErrorAttributes to handle
ResponseStatusException explicitly. This exception is used in a
WebFlux application to signal that the processing of the query has
failed with an HTTP status code and a reason phrase. The latter is now
properly mapped to the `message` attribute of the response body.

Closes gh-11614

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorAttributes.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -25,6 +25,7 @@ import java.util.Map;
 import org.springframework.http.HttpStatus;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.ObjectError;
+import org.springframework.web.bind.support.WebExchangeBindException;
 import org.springframework.web.reactive.function.server.ServerRequest;
 import org.springframework.web.server.ResponseStatusException;
 import org.springframework.web.server.ServerWebExchange;
@@ -44,6 +45,7 @@ import org.springframework.web.server.ServerWebExchange;
  * </ul>
  *
  * @author Brian Clozel
+ * @author Stephane Nicoll
  * @since 2.0.0
  * @see ErrorAttributes
  */
@@ -77,24 +79,36 @@ public class DefaultErrorAttributes implements ErrorAttributes {
 		errorAttributes.put("timestamp", new Date());
 		errorAttributes.put("path", request.path());
 		Throwable error = getError(request);
-		if (this.includeException) {
-			errorAttributes.put("exception", error.getClass().getName());
+		HttpStatus errorStatus = determineHttpStatus(error);
+		errorAttributes.put("status", errorStatus.value());
+		errorAttributes.put("error", errorStatus.getReasonPhrase());
+		errorAttributes.put("message", determineMessage(error));
+		handleException(errorAttributes, determineException(error), includeStackTrace);
+		return errorAttributes;
+	}
+
+	private HttpStatus determineHttpStatus(Throwable error) {
+		if (error instanceof ResponseStatusException) {
+			return ((ResponseStatusException) error).getStatus();
 		}
-		if (includeStackTrace) {
-			addStackTrace(errorAttributes, error);
+		return HttpStatus.INTERNAL_SERVER_ERROR;
+	}
+
+	private String determineMessage(Throwable error) {
+		if (error instanceof WebExchangeBindException) {
+			return error.getMessage();
 		}
-		addErrorMessage(errorAttributes, error);
 		if (error instanceof ResponseStatusException) {
-			HttpStatus errorStatus = ((ResponseStatusException) error).getStatus();
-			errorAttributes.put("status", errorStatus.value());
-			errorAttributes.put("error", errorStatus.getReasonPhrase());
+			return ((ResponseStatusException) error).getReason();
 		}
-		else {
-			errorAttributes.put("status", HttpStatus.INTERNAL_SERVER_ERROR.value());
-			errorAttributes.put("error",
-					HttpStatus.INTERNAL_SERVER_ERROR.getReasonPhrase());
+		return error.getMessage();
+	}
+
+	private Throwable determineException(Throwable error) {
+		if (error instanceof ResponseStatusException) {
+			return error.getCause() != null ? error.getCause() : error;
 		}
-		return errorAttributes;
+		return error;
 	}
 
 	private void addStackTrace(Map<String, Object> errorAttributes, Throwable error) {
@@ -104,8 +118,14 @@ public class DefaultErrorAttributes implements ErrorAttributes {
 		errorAttributes.put("trace", stackTrace.toString());
 	}
 
-	private void addErrorMessage(Map<String, Object> errorAttributes, Throwable error) {
-		errorAttributes.put("message", error.getMessage());
+	private void handleException(Map<String, Object> errorAttributes,
+			Throwable error, boolean includeStackTrace) {
+		if (this.includeException) {
+			errorAttributes.put("exception", error.getClass().getName());
+		}
+		if (includeStackTrace) {
+			addStackTrace(errorAttributes, error);
+		}
 		if (error instanceof BindingResult) {
 			BindingResult result = (BindingResult) error;
 			if (result.getErrorCount() > 0) {

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorAttributesTests.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -46,6 +46,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link DefaultErrorAttributes}.
  *
  * @author Brian Clozel
+ * @author Stephane Nicoll
  */
 public class DefaultErrorAttributesTests {
 
@@ -125,6 +126,39 @@ public class DefaultErrorAttributesTests {
 		assertThat(attributes.get("message")).isEqualTo("Test");
 	}
 
+	@Test
+	public void processResponseStatusException() {
+		RuntimeException nested = new RuntimeException("Test");
+		ResponseStatusException error = new ResponseStatusException(
+				HttpStatus.BAD_REQUEST, "invalid request", nested);
+		this.errorAttributes = new DefaultErrorAttributes(true);
+		MockServerHttpRequest request = MockServerHttpRequest.get("/test").build();
+		ServerRequest serverRequest = buildServerRequest(request, error);
+		Map<String, Object> attributes = this.errorAttributes
+				.getErrorAttributes(serverRequest, false);
+		assertThat(attributes.get("status")).isEqualTo(400);
+		assertThat(attributes.get("message")).isEqualTo("invalid request");
+		assertThat(attributes.get("exception"))
+				.isEqualTo(RuntimeException.class.getName());
+		assertThat(this.errorAttributes.getError(serverRequest)).isSameAs(error);
+	}
+
+	@Test
+	public void processResponseStatusExceptionWithNoNestedCause() {
+		ResponseStatusException error = new ResponseStatusException(
+				HttpStatus.NOT_ACCEPTABLE, "could not process request");
+		this.errorAttributes = new DefaultErrorAttributes(true);
+		MockServerHttpRequest request = MockServerHttpRequest.get("/test").build();
+		ServerRequest serverRequest = buildServerRequest(request, error);
+		Map<String, Object> attributes = this.errorAttributes
+				.getErrorAttributes(serverRequest, false);
+		assertThat(attributes.get("status")).isEqualTo(406);
+		assertThat(attributes.get("message")).isEqualTo("could not process request");
+		assertThat(attributes.get("exception"))
+				.isEqualTo(ResponseStatusException.class.getName());
+		assertThat(this.errorAttributes.getError(serverRequest)).isSameAs(error);
+	}
+
 	@Test
 	public void notIncludeTrace() {
 		RuntimeException ex = new RuntimeException("Test");