Return null in case of no matching part even for array/collection parameter

Closes gh-25819

spring-web/src/main/java/org/springframework/web/multipart/support/MultipartResolutionDelegate.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.
@@ -99,37 +99,53 @@ public final class MultipartResolutionDelegate {
 		boolean isMultipart = (multipartRequest != null || isMultipartContent(request));
 
 		if (MultipartFile.class == parameter.getNestedParameterType()) {
-			if (multipartRequest == null && isMultipart) {
+			if (!isMultipart) {
+				return null;
+			}
+			if (multipartRequest == null) {
 				multipartRequest = new StandardMultipartHttpServletRequest(request);
 			}
-			return (multipartRequest != null ? multipartRequest.getFile(name) : null);
+			return multipartRequest.getFile(name);
 		}
 		else if (isMultipartFileCollection(parameter)) {
-			if (multipartRequest == null && isMultipart) {
+			if (!isMultipart) {
+				return null;
+			}
+			if (multipartRequest == null) {
 				multipartRequest = new StandardMultipartHttpServletRequest(request);
 			}
-			return (multipartRequest != null ? multipartRequest.getFiles(name) : null);
+			List<MultipartFile> files = multipartRequest.getFiles(name);
+			return (!files.isEmpty() ? files : null);
 		}
 		else if (isMultipartFileArray(parameter)) {
-			if (multipartRequest == null && isMultipart) {
-				multipartRequest = new StandardMultipartHttpServletRequest(request);
-			}
-			if (multipartRequest != null) {
-				List<MultipartFile> multipartFiles = multipartRequest.getFiles(name);
-				return multipartFiles.toArray(new MultipartFile[0]);
-			}
-			else {
+			if (!isMultipart) {
 				return null;
 			}
+			if (multipartRequest == null) {
+				multipartRequest = new StandardMultipartHttpServletRequest(request);
+			}
+			List<MultipartFile> files = multipartRequest.getFiles(name);
+			return (!files.isEmpty() ? files.toArray(new MultipartFile[0]) : null);
 		}
 		else if (Part.class == parameter.getNestedParameterType()) {
-			return (isMultipart ? request.getPart(name): null);
+			if (!isMultipart) {
+				return null;
+			}
+			return request.getPart(name);
 		}
 		else if (isPartCollection(parameter)) {
-			return (isMultipart ? resolvePartList(request, name) : null);
+			if (!isMultipart) {
+				return null;
+			}
+			List<Part> parts = resolvePartList(request, name);
+			return (!parts.isEmpty() ? parts : null);
 		}
 		else if (isPartArray(parameter)) {
-			return (isMultipart ? resolvePartList(request, name).toArray(new Part[0]) : null);
+			if (!isMultipart) {
+				return null;
+			}
+			List<Part> parts = resolvePartList(request, name);
+			return (!parts.isEmpty() ? parts.toArray(new Part[0]) : null);
 		}
 		else {
 			return UNRESOLVABLE;

spring-web/src/test/java/org/springframework/web/method/annotation/RequestParamMethodArgumentResolverTests.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.
@@ -199,6 +199,17 @@ public class RequestParamMethodArgumentResolverTests {
 		assertThat(result).isEqualTo(Arrays.asList(expected1, expected2));
 	}
 
+	@Test
+	public void resolveMultipartFileListMissing() throws Exception {
+		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest();
+		request.addFile(new MockMultipartFile("other", "Hello World 3".getBytes()));
+		webRequest = new ServletWebRequest(request);
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(List.class, MultipartFile.class);
+		assertThatExceptionOfType(MissingServletRequestPartException.class).isThrownBy(() ->
+				resolver.resolveArgument(param, null, webRequest, null));
+	}
+
 	@Test
 	public void resolveMultipartFileArray() throws Exception {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest();
@@ -220,6 +231,17 @@ public class RequestParamMethodArgumentResolverTests {
 		assertThat(expected2).isEqualTo(parts[1]);
 	}
 
+	@Test
+	public void resolveMultipartFileArrayMissing() throws Exception {
+		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest();
+		request.addFile(new MockMultipartFile("other", "Hello World 3".getBytes()));
+		webRequest = new ServletWebRequest(request);
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(MultipartFile[].class);
+		assertThatExceptionOfType(MissingServletRequestPartException.class).isThrownBy(() ->
+				resolver.resolveArgument(param, null, webRequest, null));
+	}
+
 	@Test
 	public void resolvePart() throws Exception {
 		MockHttpServletRequest request = new MockHttpServletRequest();
@@ -257,6 +279,19 @@ public class RequestParamMethodArgumentResolverTests {
 		assertThat(result).isEqualTo(Arrays.asList(expected1, expected2));
 	}
 
+	@Test
+	public void resolvePartListMissing() throws Exception {
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.setMethod("POST");
+		request.setContentType("multipart/form-data");
+		request.addPart(new MockPart("other", "Hello World 3".getBytes()));
+		webRequest = new ServletWebRequest(request);
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(List.class, Part.class);
+		assertThatExceptionOfType(MissingServletRequestPartException.class).isThrownBy(() ->
+				resolver.resolveArgument(param, null, webRequest, null));
+	}
+
 	@Test
 	public void resolvePartArray() throws Exception {
 		MockHttpServletRequest request = new MockHttpServletRequest();
@@ -280,6 +315,19 @@ public class RequestParamMethodArgumentResolverTests {
 		assertThat(expected2).isEqualTo(parts[1]);
 	}
 
+	@Test
+	public void resolvePartArrayMissing() throws Exception {
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.setMethod("POST");
+		request.setContentType("multipart/form-data");
+		request.addPart(new MockPart("other", "Hello World 3".getBytes()));
+		webRequest = new ServletWebRequest(request);
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(Part[].class);
+		assertThatExceptionOfType(MissingServletRequestPartException.class).isThrownBy(() ->
+				resolver.resolveArgument(param, null, webRequest, null));
+	}
+
 	@Test
 	public void resolveMultipartFileNotAnnot() throws Exception {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest();