SPR-8642 IMPROVE ERROR REPORTING WHEN RESOLVING MULTIPART REQUEST METHOD ARGUMENTS

Separate client from server errors as much as possible in this order:
- raise MultipartException when resolving a multipart arg and the 
  request is not a multipart request (400)
- raise IllegalArgumentException when the arg type is MultipartFile 
  but the request is not of type MultipartHttpServletRequest (500)
- raise MissingServletRequestPartException when a MultipartResolver
  is in use but the part is not found (400)
- detect presence of Servlet 3.0 before using standard multipart 
  parsing to find a request part or raise 
  IllegalArgumentException (500)

Unfortunately it is not always possible to separate client from 
server errors mainly because the Servlet 3.0 API does not 
distinguish between the case of 0 request parts vs multipart 
processing not being configured.

org.springframework.web.servlet/src/main/java/org/springframework/web/servlet/mvc/method/annotation/support/RequestPartMethodArgumentResolver.java

@@ -28,7 +28,6 @@ import org.springframework.http.HttpInputMessage;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.util.Assert;
 import org.springframework.validation.BindingResult;
-import org.springframework.web.bind.ServletRequestBindingException;
 import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestParam;
@@ -37,10 +36,11 @@ import org.springframework.web.bind.support.WebDataBinderFactory;
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.method.annotation.support.MethodArgumentNotValidException;
 import org.springframework.web.method.support.ModelAndViewContainer;
+import org.springframework.web.multipart.MultipartException;
 import org.springframework.web.multipart.MultipartFile;
 import org.springframework.web.multipart.MultipartHttpServletRequest;
-import org.springframework.web.multipart.MultipartRequest;
 import org.springframework.web.multipart.MultipartResolver;
+import org.springframework.web.multipart.support.MissingServletRequestPartException;
 import org.springframework.web.multipart.support.RequestPartServletServerHttpRequest;
 import org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver;
 import org.springframework.web.util.WebUtils;
@@ -66,8 +66,8 @@ import org.springframework.web.util.WebUtils;
  * 
  * <p>Automatic validation can be applied to a @{@link RequestPart} method argument 
  * through the use of {@code @Valid}. In case of validation failure, a 
- * {@link RequestPartNotValidException} is thrown and handled automatically through
- * the {@link DefaultHandlerExceptionResolver}. 
+ * {@link MethodArgumentNotValidException} is thrown and handled automatically by
+ * the {@link DefaultHandlerExceptionResolver}.
  * 
  * @author Rossen Stoyanchev
  * @since 3.1
@@ -111,44 +111,65 @@ public class RequestPartMethodArgumentResolver extends AbstractMessageConverterM
 								  NativeWebRequest request, 
 								  WebDataBinderFactory binderFactory) throws Exception {
 
-		String partName = getPartName(parameter);
-		Object arg;
-
 		HttpServletRequest servletRequest = request.getNativeRequest(HttpServletRequest.class);
+		if (!isMultipartRequest(servletRequest)) {
+			throw new MultipartException("The current request is not a multipart request.");
+		}
+		
 		MultipartHttpServletRequest multipartRequest = 
 			WebUtils.getNativeRequest(servletRequest, MultipartHttpServletRequest.class);
 
+		String partName = getPartName(parameter);
+		Object arg;
+
 		if (MultipartFile.class.equals(parameter.getParameterType())) {
-			assertMultipartRequest(multipartRequest, request);
+			Assert.notNull(multipartRequest, "Expected MultipartHttpServletRequest: is a MultipartResolver configured?");
 			arg = multipartRequest.getFile(partName);
 		}
 		else if (isMultipartFileCollection(parameter)) {
-			assertMultipartRequest(multipartRequest, request);
+			Assert.notNull(multipartRequest, "Expected MultipartHttpServletRequest: is a MultipartResolver configured?");
 			arg = multipartRequest.getFiles(partName);
 		}
 		else if ("javax.servlet.http.Part".equals(parameter.getParameterType().getName())) {
 			arg = servletRequest.getPart(partName);
 		}
 		else {
-			HttpInputMessage inputMessage = new RequestPartServletServerHttpRequest(servletRequest, partName);
-			arg = readWithMessageConverters(inputMessage, parameter, parameter.getParameterType());
-			if (isValidationApplicable(arg, parameter)) {
-				WebDataBinder binder = binderFactory.createBinder(request, arg, partName);
-				binder.validate();
-				BindingResult bindingResult = binder.getBindingResult();
-				if (bindingResult.hasErrors()) {
-					throw new MethodArgumentNotValidException(parameter, bindingResult);
+			try {
+				HttpInputMessage inputMessage = new RequestPartServletServerHttpRequest(servletRequest, partName);
+				arg = readWithMessageConverters(inputMessage, parameter, parameter.getParameterType());
+				if (isValidationApplicable(arg, parameter)) {
+					WebDataBinder binder = binderFactory.createBinder(request, arg, partName);
+					binder.validate();
+					BindingResult bindingResult = binder.getBindingResult();
+					if (bindingResult.hasErrors()) {
+						throw new MethodArgumentNotValidException(parameter, bindingResult);
+					}
 				}
+			} 
+			catch (MissingServletRequestPartException e) {
+				// handled below
+				arg = null;
 			}
 		}
 
-		if (arg == null) {
-			handleMissingValue(partName, parameter);
+		RequestPart annot = parameter.getParameterAnnotation(RequestPart.class);
+		boolean isRequired = (annot != null) ? annot.required() : true;
+
+		if (arg == null && isRequired) {
+			throw new MissingServletRequestPartException(partName);
 		}
 		
 		return arg;
 	}
 
+	private boolean isMultipartRequest(HttpServletRequest request) {
+		if (!"post".equals(request.getMethod().toLowerCase())) {
+			return false;
+		}
+		String contentType = request.getContentType();
+		return (contentType != null && contentType.toLowerCase().startsWith("multipart/"));
+	}
+	
 	private String getPartName(MethodParameter parameter) {
 		RequestPart annot = parameter.getParameterAnnotation(RequestPart.class);
 		String partName = (annot != null) ? annot.value() : "";
@@ -159,13 +180,6 @@ public class RequestPartMethodArgumentResolver extends AbstractMessageConverterM
 		}
 		return partName;
 	}
-
-	private void assertMultipartRequest(MultipartHttpServletRequest multipartRequest, NativeWebRequest request) {
-		if (multipartRequest == null) {
-			throw new IllegalStateException("Current request is not of type [" + MultipartRequest.class.getName()
-					+ "]: " + request + ". Do you have a MultipartResolver configured?");
-		}
-	}
 	
 	private boolean isMultipartFileCollection(MethodParameter parameter) {
 		Class<?> paramType = parameter.getParameterType();
@@ -178,22 +192,6 @@ public class RequestPartMethodArgumentResolver extends AbstractMessageConverterM
 		return false;
 	}
 	
-	/**
-	 * Invoked if the resolved argument value is {@code null}. The default implementation raises 
-	 * a {@link ServletRequestBindingException} if the method parameter is required.
-	 * @param partName the name used to look up the request part
-	 * @param param the method argument
-	 */
-	protected void handleMissingValue(String partName, MethodParameter param) throws ServletRequestBindingException {
-		RequestPart annot = param.getParameterAnnotation(RequestPart.class);
-		boolean isRequired = (annot != null) ? annot.required() : true;
-		if (isRequired) {
-			String paramType = param.getParameterType().getName();
-			throw new ServletRequestBindingException(
-					"Missing request part '" + partName + "' for method parameter type [" + paramType + "]");
-		}
-	}
-	
 	/**
 	 * Whether to validate the given @{@link RequestPart} method argument. 
 	 * The default implementation return {@code true} if the argument value is not {@code null} 

org.springframework.web.servlet/src/main/java/org/springframework/web/servlet/mvc/support/DefaultHandlerExceptionResolver.java

@@ -40,6 +40,8 @@ import org.springframework.web.bind.ServletRequestBindingException;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestPart;
 import org.springframework.web.method.annotation.support.MethodArgumentNotValidException;
+import org.springframework.web.multipart.MultipartFile;
+import org.springframework.web.multipart.support.MissingServletRequestPartException;
 import org.springframework.web.servlet.ModelAndView;
 import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver;
 import org.springframework.web.servlet.mvc.multiaction.NoSuchRequestHandlingMethodException;
@@ -131,6 +133,9 @@ public class DefaultHandlerExceptionResolver extends AbstractHandlerExceptionRes
 			else if (ex instanceof MethodArgumentNotValidException) {
 				return handleMethodArgumentNotValidException((MethodArgumentNotValidException) ex, request, response, handler);
 			}
+			else if (ex instanceof MissingServletRequestPartException) {
+				return handleMissingServletRequestPartException((MissingServletRequestPartException) ex, request, response, handler);
+			}
 		}
 		catch (Exception handlerException) {
 			logger.warn("Handling of [" + ex.getClass().getName() + "] resulted in Exception", handlerException);
@@ -356,4 +361,20 @@ public class DefaultHandlerExceptionResolver extends AbstractHandlerExceptionRes
 		return new ModelAndView();
 	}
 
+	/**
+	 * Handle the case where an @{@link RequestPart}, a {@link MultipartFile}, 
+	 * or a {@code javax.servlet.http.Part} argument is required but missing. 
+	 * An HTTP 400 error is sent back to the client.
+	 * @param request current HTTP request
+	 * @param response current HTTP response
+	 * @param handler the executed handler
+	 * @return an empty ModelAndView indicating the exception was handled
+	 * @throws IOException potentially thrown from response.sendError()
+	 */
+	protected ModelAndView handleMissingServletRequestPartException(MissingServletRequestPartException ex,
+			HttpServletRequest request, HttpServletResponse response, Object handler) throws IOException {
+ 		response.sendError(HttpServletResponse.SC_BAD_REQUEST);
+		return new ModelAndView();
+	}
+
 }