Support content negotiation for RFC 7807

Closes gh-28189

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

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation;
 
 import java.lang.reflect.Method;
+import java.time.Duration;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -25,14 +26,19 @@ import io.reactivex.rxjava3.core.Single;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import org.springframework.core.codec.ByteBufferEncoder;
 import org.springframework.core.codec.CharSequenceEncoder;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.http.codec.ResourceHttpMessageWriter;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.http.codec.xml.Jaxb2XmlEncoder;
+import org.springframework.lang.Nullable;
 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.ResponseBody;
 import org.springframework.web.bind.annotation.RestController;
@@ -40,8 +46,11 @@ import org.springframework.web.method.HandlerMethod;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolverBuilder;
+import org.springframework.web.testfixture.server.MockServerWebExchange;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest.get;
 import static org.springframework.web.testfixture.method.ResolvableMethod.on;
 
 /**
@@ -82,7 +91,7 @@ public class ResponseBodyResultHandlerTests {
 		testSupports(controller, method);
 
 		method = on(TestController.class).annotNotPresent(ResponseBody.class).resolveMethod("doWork");
-		HandlerResult handlerResult = getHandlerResult(controller, method);
+		HandlerResult handlerResult = getHandlerResult(controller, null, method);
 		assertThat(this.resultHandler.supports(handlerResult)).isFalse();
 	}
 
@@ -105,13 +114,42 @@ public class ResponseBodyResultHandlerTests {
 	}
 
 	private void testSupports(Object controller, Method method) {
-		HandlerResult handlerResult = getHandlerResult(controller, method);
+		HandlerResult handlerResult = getHandlerResult(controller, null, method);
 		assertThat(this.resultHandler.supports(handlerResult)).isTrue();
 	}
 
-	private HandlerResult getHandlerResult(Object controller, Method method) {
-		HandlerMethod handlerMethod = new HandlerMethod(controller, method);
-		return new HandlerResult(handlerMethod, null, handlerMethod.getReturnType());
+	@Test
+	void problemDetailContentNegotiation() {
+
+		// Default
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));
+		testProblemDetailMediaType(exchange, MediaType.APPLICATION_PROBLEM_JSON);
+
+		// JSON requested
+		exchange = MockServerWebExchange.from(get("/path").accept(MediaType.APPLICATION_JSON));
+		testProblemDetailMediaType(exchange, MediaType.APPLICATION_JSON);
+
+		// No match fallback
+		exchange = MockServerWebExchange.from(get("/path").accept(MediaType.APPLICATION_PDF));
+		testProblemDetailMediaType(exchange, MediaType.APPLICATION_PROBLEM_JSON);
+	}
+
+	private void testProblemDetailMediaType(MockServerWebExchange exchange, MediaType expectedMediaType) {
+		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
+
+		Method method = on(TestRestController.class).returning(ProblemDetail.class).resolveMethod();
+		HandlerResult result = getHandlerResult(new TestRestController(), problemDetail, method);
+
+		this.resultHandler.handleResult(exchange, result).block(Duration.ofSeconds(5));
+
+		assertThat(exchange.getResponse().getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
+		assertThat(exchange.getResponse().getHeaders().getContentType()).isEqualTo(expectedMediaType);
+		assertResponseBody(exchange,
+				"{\"type\":\"about:blank\"," +
+						"\"title\":\"Bad Request\"," +
+						"\"status\":400," +
+						"\"detail\":null," +
+						"\"instance\":\"/path\"}");
 	}
 
 	@Test
@@ -119,6 +157,17 @@ public class ResponseBodyResultHandlerTests {
 		assertThat(this.resultHandler.getOrder()).isEqualTo(100);
 	}
 
+	private HandlerResult getHandlerResult(Object controller, @Nullable Object returnValue, Method method) {
+		HandlerMethod handlerMethod = new HandlerMethod(controller, method);
+		return new HandlerResult(handlerMethod, returnValue, handlerMethod.getReturnType());
+	}
+
+	private void assertResponseBody(MockServerWebExchange exchange, @Nullable String responseBody) {
+		StepVerifier.create(exchange.getResponse().getBody())
+				.consumeNextWith(buf -> assertThat(buf.toString(UTF_8)).isEqualTo(responseBody))
+				.expectComplete()
+				.verify();
+	}
 
 
 	@RestController
@@ -142,6 +191,11 @@ public class ResponseBodyResultHandlerTests {
 		public Completable handleToCompletable() {
 			return null;
 		}
+
+		public ProblemDetail handleToProblemDetail() {
+			return null;
+		}
+
 	}