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-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessorMockTests.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.
@@ -31,6 +31,7 @@ import java.util.Date;
 import java.util.Set;
 
 import jakarta.servlet.FilterChain;
+import jakarta.servlet.http.HttpServletResponse;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.mockito.ArgumentCaptor;
@@ -46,6 +47,7 @@ import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.ProblemDetail;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.HttpMessageConverter;
@@ -77,6 +79,8 @@ import static org.mockito.Mockito.reset;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.springframework.http.MediaType.APPLICATION_OCTET_STREAM;
+import static org.springframework.http.MediaType.APPLICATION_PROBLEM_JSON;
+import static org.springframework.http.MediaType.APPLICATION_PROBLEM_JSON_VALUE;
 import static org.springframework.http.MediaType.TEXT_PLAIN;
 import static org.springframework.web.servlet.HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE;
 
@@ -103,6 +107,8 @@ public class HttpEntityMethodProcessorMockTests {
 
 	private HttpMessageConverter<Object> resourceRegionMessageConverter;
 
+	private HttpMessageConverter<Object> jsonMessageConverter;
+
 	private MethodParameter paramHttpEntity;
 
 	private MethodParameter paramRequestEntity;
@@ -123,6 +129,8 @@ public class HttpEntityMethodProcessorMockTests {
 
 	private MethodParameter returnTypeInt;
 
+	private MethodParameter returnTypeProblemDetail;
+
 	private ModelAndViewContainer mavContainer;
 
 	private MockHttpServletRequest servletRequest;
@@ -147,8 +155,12 @@ public class HttpEntityMethodProcessorMockTests {
 		given(resourceRegionMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(MediaType.ALL));
 		given(resourceRegionMessageConverter.getSupportedMediaTypes(any())).willReturn(Collections.singletonList(MediaType.ALL));
 
+		jsonMessageConverter = mock(HttpMessageConverter.class);
+		given(jsonMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(MediaType.APPLICATION_PROBLEM_JSON));
+		given(jsonMessageConverter.getSupportedMediaTypes(any())).willReturn(Collections.singletonList(MediaType.APPLICATION_PROBLEM_JSON));
+
 		processor = new HttpEntityMethodProcessor(Arrays.asList(
-				stringHttpMessageConverter, resourceMessageConverter, resourceRegionMessageConverter));
+				stringHttpMessageConverter, resourceMessageConverter, resourceRegionMessageConverter, jsonMessageConverter));
 
 		Method handle1 = getClass().getMethod("handle1", HttpEntity.class, ResponseEntity.class,
 				Integer.TYPE, RequestEntity.class);
@@ -163,6 +175,7 @@ public class HttpEntityMethodProcessorMockTests {
 		returnTypeHttpEntitySubclass = new MethodParameter(getClass().getMethod("handle2x", HttpEntity.class), -1);
 		returnTypeInt = new MethodParameter(getClass().getMethod("handle3"), -1);
 		returnTypeResponseEntityResource = new MethodParameter(getClass().getMethod("handle5"), -1);
+		returnTypeProblemDetail = new MethodParameter(getClass().getMethod("handle6"), -1);
 
 		mavContainer = new ModelAndViewContainer();
 		servletRequest = new MockHttpServletRequest("GET", "/foo");
@@ -184,6 +197,7 @@ public class HttpEntityMethodProcessorMockTests {
 		assertThat(processor.supportsReturnType(returnTypeResponseEntity)).as("ResponseEntity return type not supported").isTrue();
 		assertThat(processor.supportsReturnType(returnTypeHttpEntity)).as("HttpEntity return type not supported").isTrue();
 		assertThat(processor.supportsReturnType(returnTypeHttpEntitySubclass)).as("Custom HttpEntity subclass not supported").isTrue();
+		assertThat(processor.supportsReturnType(returnTypeProblemDetail)).isTrue();
 		assertThat(processor.supportsReturnType(paramRequestEntity)).as("RequestEntity parameter supported").isFalse();
 		assertThat(processor.supportsReturnType(returnTypeInt)).as("non-ResponseBody return type supported").isFalse();
 	}
@@ -268,6 +282,36 @@ public class HttpEntityMethodProcessorMockTests {
 		verify(stringHttpMessageConverter).write(eq(body), eq(accepted), isA(HttpOutputMessage.class));
 	}
 
+	@Test
+	public void shouldHandleProblemDetail() throws Exception {
+		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
+		servletRequest.addHeader("Accept", APPLICATION_PROBLEM_JSON_VALUE);
+		given(jsonMessageConverter.canWrite(ProblemDetail.class, APPLICATION_PROBLEM_JSON)).willReturn(true);
+
+		processor.handleReturnValue(problemDetail, returnTypeProblemDetail, mavContainer, webRequest);
+
+		assertThat(mavContainer.isRequestHandled()).isTrue();
+		assertThat(webRequest.getNativeResponse(HttpServletResponse.class).getStatus()).isEqualTo(400);
+		verify(jsonMessageConverter).write(eq(problemDetail), eq(APPLICATION_PROBLEM_JSON), isA(HttpOutputMessage.class));
+
+		assertThat(problemDetail).isNotNull()
+				.extracting(ProblemDetail::getInstance).isNotNull()
+				.extracting(URI::toString)
+				.as("Instance was not set to the request path")
+				.isEqualTo(servletRequest.getRequestURI());
+
+
+		// But if instance is set, it should be respected
+		problemDetail.setInstance(URI.create("/something/else"));
+		processor.handleReturnValue(problemDetail, returnTypeProblemDetail, mavContainer, webRequest);
+
+		assertThat(problemDetail).isNotNull()
+				.extracting(ProblemDetail::getInstance).isNotNull()
+				.extracting(URI::toString)
+				.as("Instance was not set to the request path")
+				.isEqualTo("/something/else");
+	}
+
 	@Test
 	public void shouldHandleReturnValueWithProducibleMediaType() throws Exception {
 		String body = "Foo";
@@ -797,6 +841,11 @@ public class HttpEntityMethodProcessorMockTests {
 		return null;
 	}
 
+	@SuppressWarnings("unused")
+	public ProblemDetail handle6() {
+		return null;
+	}
+
 	@SuppressWarnings("unused")
 	public static class CustomHttpEntity extends HttpEntity<Object> {
 	}