Allow empty list for @RequestPart List<MultipartFile>

spring-cloud-openfeign-core/src/main/java/org/springframework/cloud/openfeign/support/EmptyCollectionWriter.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2013-2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.openfeign.support;
+
+import java.util.Collection;
+
+import feign.codec.EncodeException;
+import feign.form.multipart.Output;
+import feign.form.multipart.Writer;
+
+/**
+ * @author Darren Foong
+ */
+public class EmptyCollectionWriter implements Writer {
+
+	@Override
+	public boolean isApplicable(Object value) {
+		return value instanceof Collection && ((Collection) value).isEmpty();
+	}
+
+	@Override
+	public void write(Output output, String boundary, String key, Object value)
+			throws EncodeException {
+	}
+
+}

spring-cloud-openfeign-core/src/main/java/org/springframework/cloud/openfeign/support/SpringEncoder.java

@@ -29,6 +29,7 @@
 import feign.RequestTemplate;
 import feign.codec.EncodeException;
 import feign.codec.Encoder;
+import feign.form.MultipartFormContentProcessor;
 import feign.form.spring.SpringFormEncoder;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -46,6 +47,7 @@
 import org.springframework.http.converter.protobuf.ProtobufHttpMessageConverter;
 import org.springframework.web.multipart.MultipartFile;
 
+import static feign.form.ContentType.MULTIPART;
 import static org.springframework.cloud.openfeign.support.FeignUtils.getHeaders;
 import static org.springframework.cloud.openfeign.support.FeignUtils.getHttpHeaders;
 
@@ -67,12 +69,20 @@ public class SpringEncoder implements Encoder {
 	public SpringEncoder(ObjectFactory<HttpMessageConverters> messageConverters) {
 		this.springFormEncoder = new SpringFormEncoder();
 		this.messageConverters = messageConverters;
+
+		MultipartFormContentProcessor processor = (MultipartFormContentProcessor) springFormEncoder
+				.getContentProcessor(MULTIPART);
+		processor.addFirstWriter(new EmptyCollectionWriter());
 	}
 
 	public SpringEncoder(SpringFormEncoder springFormEncoder,
 			ObjectFactory<HttpMessageConverters> messageConverters) {
 		this.springFormEncoder = springFormEncoder;
 		this.messageConverters = messageConverters;
+
+		MultipartFormContentProcessor processor = (MultipartFormContentProcessor) springFormEncoder
+				.getContentProcessor(MULTIPART);
+		processor.addFirstWriter(new EmptyCollectionWriter());
 	}
 
 	@Override

spring-cloud-openfeign-core/src/test/java/org/springframework/cloud/openfeign/valid/ValidFeignClientTests.java

@@ -23,6 +23,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
@@ -336,6 +337,19 @@ public void testMultiplePojoRequestParts() {
 		assertThat(response).isEqualTo("abc123hello world 1oi terra 2hello.bin");
 	}
 
+	@Test
+	public void testRequestPartWithEmptyListOfMultipartFiles() {
+		String partNames = this.multipartClient
+				.requestPartListOfMultipartFilesReturnsPartNames(Collections.emptyList());
+		assertThat(partNames).isNull();
+		String fileNames = this.multipartClient
+				.requestPartListOfMultipartFilesReturnsFileNames(Collections.emptyList());
+		assertThat(fileNames).isNull();
+		String count = this.multipartClient
+				.requestPartListOfMultipartFilesReturnsCount(Collections.emptyList());
+		assertThat(count).isEqualTo("0");
+	}
+
 	@Test
 	public void testRequestPartWithListOfMultipartFiles() {
 		List<MultipartFile> multipartFiles = Arrays.asList(
@@ -347,6 +361,19 @@ public void testRequestPartWithListOfMultipartFiles() {
 		String fileNames = this.multipartClient
 				.requestPartListOfMultipartFilesReturnsFileNames(multipartFiles);
 		assertThat(fileNames).contains("hello1.bin", "hello2.bin");
+		String count = this.multipartClient
+				.requestPartListOfMultipartFilesReturnsCount(multipartFiles);
+		assertThat(count).isEqualTo(Integer.toString(multipartFiles.size()));
+	}
+
+	@Test
+	public void testRequestPartWithListOfPojosAndEmptyListOfMultipartFiles() {
+		Hello pojo1 = new Hello(HELLO_WORLD_1);
+		Hello pojo2 = new Hello(OI_TERRA_2);
+		String response = this.multipartClient
+				.requestPartListOfPojosAndListOfMultipartFiles(
+						Arrays.asList(pojo1, pojo2), Collections.emptyList());
+		assertThat(response).isEqualTo("hello world 1oi terra 2");
 	}
 
 	@Test
@@ -444,6 +471,12 @@ String multipartPojo(@RequestPart("hello") String hello,
 				@RequestPart("pojo2") Hello pojo2,
 				@RequestPart("file") MultipartFile file);
 
+		@RequestMapping(method = RequestMethod.POST, path = "/multipartCount",
+				consumes = MediaType.MULTIPART_FORM_DATA_VALUE,
+				produces = MediaType.TEXT_PLAIN_VALUE)
+		String requestPartListOfMultipartFilesReturnsCount(
+				@RequestPart("files") List<MultipartFile> files);
+
 		@RequestMapping(method = RequestMethod.POST, path = "/multipartNames",
 				consumes = MediaType.MULTIPART_FORM_DATA_VALUE,
 				produces = MediaType.TEXT_PLAIN_VALUE)
@@ -798,6 +831,14 @@ String multipartPojo(@RequestPart("hello") String hello,
 					+ file.getOriginalFilename();
 		}
 
+		@RequestMapping(method = RequestMethod.POST, path = "/multipartCount",
+				consumes = MediaType.MULTIPART_FORM_DATA_VALUE,
+				produces = MediaType.TEXT_PLAIN_VALUE)
+		String requestPartListOfMultipartFilesReturnsCount(
+				@RequestPart("files") List<MultipartFile> files) {
+			return Integer.toString(files.size());
+		}
+
 		@RequestMapping(method = RequestMethod.POST, path = "/multipartNames",
 				consumes = MediaType.MULTIPART_FORM_DATA_VALUE,
 				produces = MediaType.TEXT_PLAIN_VALUE)