Add ErrorResponse and ErrorResponseException

ErrorResponse represents a complete error response with status, headers,
and an  RFC 7807 ProblemDetail body.

ErrorResponseException implements ErrorResponse and is usable on its
own or as a base class. ResponseStatusException extends
ErrorResponseException and now also supports RFC 7807 and so does its
sub-hierarchy.

ErrorResponse can be returned from `@ExceptionHandler` methods and is
mapped to ResponseEntity.

See gh-27052

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandler.java

@@ -37,6 +37,7 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.web.ErrorResponse;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.HandlerResultHandler;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
@@ -44,7 +45,7 @@ import org.springframework.web.server.ServerWebExchange;
 
 /**
  * Handles return values of type {@link HttpEntity}, {@link ResponseEntity},
- * {@link HttpHeaders}, and {@link ProblemDetail}.
+ * {@link HttpHeaders}, {@link ErrorResponse}, and {@link ProblemDetail}.
  *
  * <p>By default the order for this result handler is set to 0. It is generally
  * safe to place it early in the order as it looks for a concrete return type.
@@ -108,7 +109,8 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 			return false;
 		}
 		return ((HttpEntity.class.isAssignableFrom(type) && !RequestEntity.class.isAssignableFrom(type)) ||
-				HttpHeaders.class.isAssignableFrom(type) || ProblemDetail.class.isAssignableFrom(type));
+				ErrorResponse.class.isAssignableFrom(type) || ProblemDetail.class.isAssignableFrom(type) ||
+				HttpHeaders.class.isAssignableFrom(type));
 	}
 
 
@@ -138,12 +140,15 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 			if (returnValue instanceof HttpEntity) {
 				httpEntity = (HttpEntity<?>) returnValue;
 			}
-			else if (returnValue instanceof HttpHeaders) {
-				httpEntity = new ResponseEntity<>((HttpHeaders) returnValue, HttpStatus.OK);
+			else if (returnValue instanceof ErrorResponse response) {
+				httpEntity = new ResponseEntity<>(response.getBody(), response.getHeaders(), response.getRawStatusCode());
 			}
 			else if (returnValue instanceof ProblemDetail detail) {
 				httpEntity = new ResponseEntity<>(returnValue, HttpHeaders.EMPTY, detail.getStatus());
 			}
+			else if (returnValue instanceof HttpHeaders) {
+				httpEntity = new ResponseEntity<>((HttpHeaders) returnValue, HttpStatus.OK);
+			}
 			else {
 				throw new IllegalArgumentException(
 						"HttpEntity or HttpHeaders expected but got: " + returnValue.getClass());

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/RequestMappingInfoHandlerMappingTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -330,7 +330,7 @@ public class RequestMappingInfoHandlerMappingTests {
 					UnsupportedMediaTypeStatusException umtse = (UnsupportedMediaTypeStatusException) ex;
 					MediaType mediaType = new MediaType("foo", "bar");
 					assertThat(umtse.getSupportedMediaTypes()).containsExactly(mediaType);
-					assertThat(umtse.getResponseHeaders().getAcceptPatch()).containsExactly(mediaType);
+					assertThat(umtse.getHeaders().getAcceptPatch()).containsExactly(mediaType);
 				})
 				.verify();
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -55,6 +55,8 @@ import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.http.codec.xml.Jaxb2XmlEncoder;
 import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.util.ObjectUtils;
+import org.springframework.web.ErrorResponse;
+import org.springframework.web.ErrorResponseException;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolverBuilder;
@@ -130,6 +132,9 @@ public class ResponseEntityResultHandlerTests {
 		returnType = on(TestController.class).resolveReturnType(HttpHeaders.class);
 		assertThat(this.resultHandler.supports(handlerResult(value, returnType))).isTrue();
 
+		returnType = on(TestController.class).resolveReturnType(ErrorResponse.class);
+		assertThat(this.resultHandler.supports(handlerResult(value, returnType))).isTrue();
+
 		returnType = on(TestController.class).resolveReturnType(ProblemDetail.class);
 		assertThat(this.resultHandler.supports(handlerResult(value, returnType))).isTrue();
 
@@ -236,6 +241,28 @@ public class ResponseEntityResultHandlerTests {
 		testHandle(returnValue, returnType);
 	}
 
+	@Test
+	public void handleErrorResponse() {
+		ErrorResponseException ex = new ErrorResponseException(HttpStatus.BAD_REQUEST);
+		ex.getHeaders().add("foo", "bar");
+		MethodParameter returnType = on(TestController.class).resolveReturnType(ErrorResponse.class);
+		HandlerResult result = handlerResult(ex, returnType);
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));
+		exchange.getResponse().getHeaders().setContentType(MediaType.APPLICATION_PROBLEM_JSON);
+		this.resultHandler.handleResult(exchange, result).block(Duration.ofSeconds(5));
+
+		assertThat(exchange.getResponse().getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
+		assertThat(exchange.getResponse().getHeaders()).hasSize(3);
+		assertThat(exchange.getResponse().getHeaders().get("foo")).containsExactly("bar");
+		assertThat(exchange.getResponse().getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_PROBLEM_JSON);
+		assertResponseBody(exchange,
+				"{\"type\":\"about:blank\"," +
+						"\"title\":\"Bad Request\"," +
+						"\"status\":400," +
+						"\"detail\":null," +
+						"\"instance\":\"/path\"}");
+	}
+
 	@Test
 	public void handleProblemDetail() {
 		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
@@ -529,6 +556,8 @@ public class ResponseEntityResultHandlerTests {
 
 		ResponseEntity<Person> responseEntityPerson() { return null; }
 
+		ErrorResponse errorResponse() { return null; }
+
 		ProblemDetail problemDetail() { return null; }
 
 		HttpHeaders httpHeaders() { return null; }