Extract BindingResult if necessary

Previously, no `errors` attribute is made available in the standard JSON
error document if a request body object is invalid. This is due to the
fact that the framework throws a `MethodArgumentNotValidException holding
a `BindingResult` object that was not detected.

We now make sure to extract the `BindingResult` from such exception.

Closes gh-4166

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

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -32,6 +32,7 @@ import org.springframework.http.HttpStatus;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.ObjectError;
+import org.springframework.web.bind.MethodArgumentNotValidException;
 import org.springframework.web.context.request.RequestAttributes;
 import org.springframework.web.servlet.HandlerExceptionResolver;
 import org.springframework.web.servlet.ModelAndView;
@@ -52,6 +53,7 @@ import org.springframework.web.servlet.ModelAndView;
  *
  * @author Phillip Webb
  * @author Dave Syer
+ * @author Stephane Nicoll
  * @since 1.1.0
  * @see ErrorAttributes
  */
@@ -130,11 +132,11 @@ public class DefaultErrorAttributes
 	}
 
 	private void addErrorMessage(Map<String, Object> errorAttributes, Throwable error) {
-		if (!(error instanceof BindingResult)) {
+		BindingResult result = extractBindingResult(error);
+		if (result == null) {
 			errorAttributes.put("message", error.getMessage());
 			return;
 		}
-		BindingResult result = (BindingResult) error;
 		if (result.getErrorCount() > 0) {
 			errorAttributes.put("errors", result.getAllErrors());
 			errorAttributes.put("message",
@@ -146,6 +148,16 @@ public class DefaultErrorAttributes
 		}
 	}
 
+	private BindingResult extractBindingResult(Throwable error) {
+		if (error instanceof BindingResult) {
+			return (BindingResult) error;
+		}
+		if (error instanceof MethodArgumentNotValidException) {
+			return ((MethodArgumentNotValidException) error).getBindingResult();
+		}
+		return null;
+	}
+
 	private void addStackTrace(Map<String, Object> errorAttributes, Throwable error) {
 		StringWriter stackTrace = new StringWriter();
 		error.printStackTrace(new PrintWriter(stackTrace));

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

@@ -24,6 +24,8 @@ import java.util.Map;
 
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
+import javax.validation.Valid;
+import javax.validation.constraints.NotNull;
 
 import org.junit.After;
 import org.junit.Test;
@@ -45,7 +47,9 @@ import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 import org.springframework.test.context.web.WebAppConfiguration;
 import org.springframework.validation.BindException;
+import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.bind.annotation.ResponseStatus;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.servlet.View;
@@ -164,6 +168,20 @@ public class BasicErrorControllerIntegrationTests {
 		assertThat(resp, containsString("org.springframework.validation.BindException"));
 	}
 
+	@Test
+	@SuppressWarnings("rawtypes")
+	public void testRequestBodyValidationForMachineClient() throws Exception {
+		load();
+		RequestEntity request = RequestEntity.post(URI.create(createUrl("/bodyValidation")))
+				.contentType(MediaType.APPLICATION_JSON).body("{}");
+		ResponseEntity<Map> entity = new TestRestTemplate().exchange(request, Map.class);
+		String resp = entity.getBody().toString();
+		assertThat(resp, containsString("Error count: 1"));
+		assertThat(resp, containsString("errors=[{"));
+		assertThat(resp, containsString("codes=["));
+		assertThat(resp, containsString("org.springframework.web.bind.MethodArgumentNotValidException"));
+	}
+
 	private void assertErrorAttributes(Map<?, ?> content, String status, String error,
 			Class<?> exception, String message, String path) {
 		assertEquals("Wrong status", status, content.get("status"));
@@ -239,6 +257,11 @@ public class BasicErrorControllerIntegrationTests {
 				throw error;
 			}
 
+			@RequestMapping(path = "/bodyValidation", method = RequestMethod.POST, produces = "application/json")
+			public String bodyValidation(@Valid @RequestBody DummyBody body) {
+				return body.content;
+			}
+
 			@ResponseStatus(value = HttpStatus.BAD_REQUEST, reason = "Expected!")
 			@SuppressWarnings("serial")
 			private static class ExpectedException extends RuntimeException {
@@ -255,6 +278,20 @@ public class BasicErrorControllerIntegrationTests {
 
 			}
 
+			private static class DummyBody {
+
+				@NotNull
+				private String content;
+
+				public String getContent() {
+					return this.content;
+				}
+
+				public void setContent(String content) {
+					this.content = content;
+				}
+			}
+
 		}
 
 	}

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

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -29,6 +29,7 @@ import org.springframework.validation.BindException;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.MapBindingResult;
 import org.springframework.validation.ObjectError;
+import org.springframework.web.bind.MethodArgumentNotValidException;
 import org.springframework.web.context.request.RequestAttributes;
 import org.springframework.web.context.request.ServletRequestAttributes;
 import org.springframework.web.servlet.ModelAndView;
@@ -162,7 +163,20 @@ public class DefaultErrorAttributesTests {
 		BindingResult bindingResult = new MapBindingResult(
 				Collections.singletonMap("a", "b"), "objectName");
 		bindingResult.addError(new ObjectError("c", "d"));
-		BindException ex = new BindException(bindingResult);
+		Exception ex = new BindException(bindingResult);
+		testBindingResult(bindingResult, ex);
+	}
+
+	@Test
+	public void extractMethodArgumentNotValidExceptionBindingResultErrors() throws Exception {
+		BindingResult bindingResult = new MapBindingResult(
+				Collections.singletonMap("a", "b"), "objectName");
+		bindingResult.addError(new ObjectError("c", "d"));
+		Exception ex = new MethodArgumentNotValidException(null, bindingResult);
+		testBindingResult(bindingResult, ex);
+	}
+
+	private void testBindingResult(BindingResult bindingResult, Exception ex) {
 		this.request.setAttribute("javax.servlet.error.exception", ex);
 		Map<String, Object> attributes = this.errorAttributes
 				.getErrorAttributes(this.requestAttributes, false);