MockMvc applies StandardMultipartHttpServletRequest wrapper

This is necessary to correctly process multipart requests and resolve
@RequestPart arguments and MultipartFile arguments.

Closes gh-25602

Author: rstoyanchev

spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -35,6 +35,7 @@
 import org.springframework.web.context.request.async.DeferredResult;
 import org.springframework.web.context.request.async.DeferredResultProcessingInterceptor;
 import org.springframework.web.context.request.async.WebAsyncUtils;
+import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
 import org.springframework.web.servlet.DispatcherServlet;
 import org.springframework.web.servlet.HandlerExecutionChain;
 import org.springframework.web.servlet.ModelAndView;
@@ -67,6 +68,10 @@ public TestDispatcherServlet(WebApplicationContext webApplicationContext) {
 	protected void service(HttpServletRequest request, HttpServletResponse response)
 			throws ServletException, IOException {
 
+		if (!request.getParts().isEmpty()) {
+			request = new StandardMultipartHttpServletRequest(request);
+		}
+
 		registerAsyncResultInterceptors(request);
 
 		super.service(request, response);
@@ -80,8 +85,9 @@ protected void service(HttpServletRequest request, HttpServletResponse response)
 				MockHttpServletRequest mockRequest = WebUtils.getNativeRequest(request, MockHttpServletRequest.class);
 				Assert.notNull(mockRequest, "Expected MockHttpServletRequest");
 				asyncContext = (MockAsyncContext) mockRequest.getAsyncContext();
+				String requestClassName = request.getClass().getName();
 				Assert.notNull(asyncContext, () ->
-						"Outer request wrapper " + request.getClass().getName() + " has an AsyncContext," +
+						"Outer request wrapper " + requestClassName + " has an AsyncContext," +
 								"but it is not a MockAsyncContext, while the nested " +
 								mockRequest.getClass().getName() + " does not have an AsyncContext at all.");
 			}

spring-test/src/main/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -33,7 +33,6 @@
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
-import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
 
 /**
  * Default builder for {@link MockMultipartHttpServletRequest}.
@@ -138,17 +137,9 @@ public Object merge(@Nullable Object parent) {
 	 */
 	@Override
 	protected final MockHttpServletRequest createServletRequest(ServletContext servletContext) {
-
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest(servletContext);
-		this.files.stream().forEach(request::addFile);
+		this.files.forEach(request::addFile);
 		this.parts.values().stream().flatMap(Collection::stream).forEach(request::addPart);
-
-		if (!this.parts.isEmpty()) {
-			new StandardMultipartHttpServletRequest(request)
-					.getMultiFileMap().values().stream().flatMap(Collection::stream)
-					.forEach(request::addFile);
-		}
-
 		return request;
 	}
 

spring-test/src/test/java/org/springframework/test/web/servlet/samples/standalone/MultipartControllerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -230,7 +230,7 @@ public void multipartRequestWithServletParts() throws Exception {
 		MockPart filePart = new MockPart("file", "orig", fileContent);
 
 		byte[] json = "{\"name\":\"yeeeah\"}".getBytes(StandardCharsets.UTF_8);
-		MockPart jsonPart = new MockPart("json", "json", json);
+		MockPart jsonPart = new MockPart("json", json);
 		jsonPart.getHeaders().setContentType(MediaType.APPLICATION_JSON);
 
 		standaloneSetup(new MultipartController()).build()