Add text/plain error response support

Refine `BasicErrorController` mappings so that only JSON and XML get
structured responses. A simple string response is returned for all
other media types.

Fixes gh-12513

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/BasicErrorController.java

@@ -31,7 +31,6 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.stereotype.Controller;
 import org.springframework.util.Assert;
 import org.springframework.web.bind.annotation.RequestMapping;
-import org.springframework.web.bind.annotation.ResponseBody;
 import org.springframework.web.servlet.ModelAndView;
 
 /**
@@ -82,6 +81,16 @@ public class BasicErrorController extends AbstractErrorController {
 		return this.errorProperties.getPath();
 	}
 
+	@RequestMapping(produces = { "application/xml", "text/xml", "application/json",
+			"application/*+xml", "application/*+json" })
+	public ResponseEntity<Map<String, Object>> errorStructured(
+			HttpServletRequest request) {
+		Map<String, Object> body = getErrorAttributes(request,
+				isIncludeStackTrace(request, MediaType.ALL));
+		HttpStatus status = getStatus(request);
+		return new ResponseEntity<Map<String, Object>>(body, status);
+	}
+
 	@RequestMapping(produces = "text/html")
 	public ModelAndView errorHtml(HttpServletRequest request,
 			HttpServletResponse response) {
@@ -94,12 +103,20 @@ public class BasicErrorController extends AbstractErrorController {
 	}
 
 	@RequestMapping
-	@ResponseBody
-	public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
-		Map<String, Object> body = getErrorAttributes(request,
-				isIncludeStackTrace(request, MediaType.ALL));
+	public ResponseEntity<String> errorText(HttpServletRequest request) {
+		Map<String, Object> attributes = getErrorAttributes(request,
+				isIncludeStackTrace(request, MediaType.TEXT_PLAIN));
+		int padding = 0;
+		for (Map.Entry<String, Object> entry : attributes.entrySet()) {
+			padding = Math.max(padding, entry.getKey().length());
+		}
+		StringBuffer body = new StringBuffer();
+		for (Map.Entry<String, Object> entry : attributes.entrySet()) {
+			body.append(String.format("%-" + padding + "s : %s%n", entry.getKey(),
+					entry.getValue()));
+		}
 		HttpStatus status = getStatus(request);
-		return new ResponseEntity<Map<String, Object>>(body, status);
+		return new ResponseEntity<String>(body.toString(), status);
 	}
 
 	/**

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/BasicErrorControllerIntegrationTests.java

@@ -158,6 +158,7 @@ public class BasicErrorControllerIntegrationTests {
 		load();
 		RequestEntity request = RequestEntity
 				.post(URI.create(createUrl("/bodyValidation")))
+				.accept(MediaType.APPLICATION_JSON)
 				.contentType(MediaType.APPLICATION_JSON).body("{}");
 		ResponseEntity<Map> entity = new TestRestTemplate().exchange(request, Map.class);
 		String resp = entity.getBody().toString();
@@ -167,6 +168,19 @@ public class BasicErrorControllerIntegrationTests {
 		assertThat(resp).contains(MethodArgumentNotValidException.class.getName());
 	}
 
+	@Test
+	public void testRequestBodyValidationForText() throws Exception {
+		load();
+		RequestEntity<Void> request = RequestEntity.post(URI.create(createUrl("/")))
+				.accept(MediaType.TEXT_PLAIN).build();
+		ResponseEntity<String> entity = new TestRestTemplate().exchange(request,
+				String.class);
+		String resp = entity.getBody().toString();
+		assertThat(resp).contains("status");
+		assertThat(resp).contains("error");
+		assertThat(resp).contains(IllegalStateException.class.getName());
+	}
+
 	@Test
 	public void testConventionTemplateMapping() throws Exception {
 		load();