Add ProblemDetail and @ExceptionHandler support

ProblemDetail is a representation of an RFC 7807 "problem", and this
commits adds support for it in Spring MVC and WebFlux as a return value
from `@ExceptionHandler` methods, optionally wrapped with
ResponseEntity for headers.

See gh-27052

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandler.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.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.net.URI;
 import java.time.Instant;
 import java.util.List;
 import java.util.Set;
@@ -30,6 +31,7 @@ import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.HttpMessageWriter;
@@ -41,7 +43,8 @@ import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.server.ServerWebExchange;
 
 /**
- * Handles {@link HttpEntity} and {@link ResponseEntity} return values.
+ * Handles return values of type {@link HttpEntity}, {@link ResponseEntity},
+ * {@link HttpHeaders}, 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.
@@ -100,10 +103,12 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 		return valueType;
 	}
 
-	private boolean isSupportedType(@Nullable Class<?> clazz) {
-		return (clazz != null && ((HttpEntity.class.isAssignableFrom(clazz) &&
-				!RequestEntity.class.isAssignableFrom(clazz)) ||
-				HttpHeaders.class.isAssignableFrom(clazz)));
+	private boolean isSupportedType(@Nullable Class<?> type) {
+		if (type == null) {
+			return false;
+		}
+		return ((HttpEntity.class.isAssignableFrom(type) && !RequestEntity.class.isAssignableFrom(type)) ||
+				HttpHeaders.class.isAssignableFrom(type) || ProblemDetail.class.isAssignableFrom(type));
 	}
 
 
@@ -136,11 +141,21 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 			else if (returnValue instanceof HttpHeaders) {
 				httpEntity = new ResponseEntity<>((HttpHeaders) returnValue, HttpStatus.OK);
 			}
+			else if (returnValue instanceof ProblemDetail detail) {
+				httpEntity = new ResponseEntity<>(returnValue, HttpHeaders.EMPTY, detail.getStatus());
+			}
 			else {
 				throw new IllegalArgumentException(
 						"HttpEntity or HttpHeaders expected but got: " + returnValue.getClass());
 			}
 
+			if (httpEntity.getBody() instanceof ProblemDetail detail) {
+				if (detail.getInstance() == null) {
+					URI path = URI.create(exchange.getRequest().getPath().value());
+					detail.setInstance(path);
+				}
+			}
+
 			if (httpEntity instanceof ResponseEntity) {
 				exchange.getResponse().setRawStatusCode(
 						((ResponseEntity<?>) httpEntity).getStatusCodeValue());

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

@@ -46,6 +46,7 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.HttpMessageWriter;
@@ -129,6 +130,9 @@ public class ResponseEntityResultHandlerTests {
 		returnType = on(TestController.class).resolveReturnType(HttpHeaders.class);
 		assertThat(this.resultHandler.supports(handlerResult(value, returnType))).isTrue();
 
+		returnType = on(TestController.class).resolveReturnType(ProblemDetail.class);
+		assertThat(this.resultHandler.supports(handlerResult(value, returnType))).isTrue();
+
 		// SPR-15785
 		value = ResponseEntity.ok("testing");
 		returnType = on(TestController.class).resolveReturnType(Object.class);
@@ -232,6 +236,26 @@ public class ResponseEntityResultHandlerTests {
 		testHandle(returnValue, returnType);
 	}
 
+	@Test
+	public void handleProblemDetail() {
+		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
+		MethodParameter returnType = on(TestController.class).resolveReturnType(ProblemDetail.class);
+		HandlerResult result = handlerResult(problemDetail, 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().size()).isEqualTo(2);
+		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 handleReturnValueLastModified() throws Exception {
 		Instant currentTime = Instant.now().truncatedTo(ChronoUnit.SECONDS);
@@ -505,6 +529,8 @@ public class ResponseEntityResultHandlerTests {
 
 		ResponseEntity<Person> responseEntityPerson() { return null; }
 
+		ProblemDetail problemDetail() { return null; }
+
 		HttpHeaders httpHeaders() { return null; }
 
 		Mono<ResponseEntity<String>> mono() { return null; }