如何在Spring Feign Client中通过@RequestPart传递多张图片？

尝试使用Feign Client传递多个图片文件时，单个MultipartFile参数配合@RequestPart注解可以正常工作，但使用集合或数组类型的MultipartFile参数时会报错。

Feign Client方法代码：

@ApiResponses(value = { @ApiResponse(code = 200, message = "OK", response = FileInfo.class) })
@PostMapping(
        value = "/upload-multi-files",
        produces = { "application/json" },
        consumes = { "multipart/form-data" }
)
ResponseEntity<FileInfo> uploadMultiFiles(@NotNull @Valid @RequestPart(value = "images", required = true) List<MultipartFile> images);

调用时抛出的错误：

feign.codec.EncodeException: Could not write request: no suitable HttpMessageConverter found for request type [java.util.ArrayList] and content type [multipart/form-data]
    at org.springframework.cloud.openfeign.support.SpringEncoder.encode(SpringEncoder.java:143) ~[spring-cloud-openfeign-core-2.1.0.RELEASE.jar:2.1.0.RELEASE]
    at feign.ReflectiveFeign$BuildEncodedTemplateFromArgs.resolve(ReflectiveFeign.java:372) ~[feign-core-10.1.0.jar:?]
    at feign.ReflectiveFeign$BuildTemplateByResolvingArgs.create(ReflectiveFeign.java:224) ~[feign-core-10.1.0.jar:?]
    ...

请问如何让该方法支持集合或数组类型的参数？

方法一：自定义Feign Encoder扩展集合类型支持 #

Spring Cloud Feign默认的SpringEncoder不支持直接编码List<MultipartFile>类型，需要自定义Encoder处理集合类型的多文件参数：

1. 创建自定义MultipartEncoder类：

import feign.RequestTemplate;
import feign.codec.EncodeException;
import org.springframework.cloud.openfeign.support.SpringEncoder;
import org.springframework.core.io.InputStreamResource;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;

import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class CustomMultipartEncoder extends SpringEncoder {

    public CustomMultipartEncoder(ObjectFactory<HttpMessageConverters> messageConverters) {
        super(messageConverters);
    }

    @Override
    public void encode(Object object, Type bodyType, RequestTemplate template) throws EncodeException {
        if (object instanceof Map) {
            Map<String, Object> paramMap = (Map<String, Object>) object;
            boolean hasMultipartContent = paramMap.values().stream()
                    .anyMatch(val -> val instanceof MultipartFile || val instanceof List);

            if (hasMultipartContent) {
                MultiValueMap<String, Object> multiValueMap = new LinkedMultiValueMap<>();
                paramMap.forEach((key, val) -> {
                    if (val instanceof List) {
                        ((List<?>) val).forEach(item -> {
                            if (item instanceof MultipartFile) {
                                addMultipartFile(multiValueMap, key, (MultipartFile) item);
                            } else {
                                multiValueMap.add(key, item);
                            }
                        });
                    } else if (val instanceof MultipartFile) {
                        addMultipartFile(multiValueMap, key, (MultipartFile) val);
                    } else {
                        multiValueMap.add(key, val);
                    }
                });
                super.encode(new HttpEntity<>(multiValueMap, getMultipartHeaders()), bodyType, template);
                return;
            }
        }
        super.encode(object, bodyType, template);
    }

    private void addMultipartFile(MultiValueMap<String, Object> map, String key, MultipartFile file) {
        HttpHeaders fileHeaders = new HttpHeaders();
        fileHeaders.setContentType(MediaType.parseMediaType(file.getContentType()));
        fileHeaders.setContentDispositionFormData(key, file.getOriginalFilename());
        map.add(key, new HttpEntity<>(new InputStreamResource(file.getInputStream()), fileHeaders));
    }

    private HttpHeaders getMultipartHeaders() {
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.MULTIPART_FORM_DATA);
        return headers;
    }
}

2. 配置Feign Client使用自定义Encoder：

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.context.annotation.Bean;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.beans.factory.ObjectFactory;

@FeignClient(name = "your-service-name", configuration = FileFeignConfig.class)
public interface FileFeignClient {
    // 你的uploadMultiFiles方法
}

class FileFeignConfig {
    @Bean
    public CustomMultipartEncoder feignEncoder() {
        return new CustomMultipartEncoder(new ObjectFactory<HttpMessageConverters>() {
            @Override
            public HttpMessageConverters getObject() {
                return new HttpMessageConverters();
            }
        });
    }
}

方法二：简化参数写法（无需自定义Encoder） #

如果不想自定义Encoder，可以调整参数形式：

• 改用数组类型参数：

@ApiResponses(value = { @ApiResponse(code = 200, message = "OK", response = FileInfo.class) })
@PostMapping(
        value = "/upload-multi-files",
        produces = { "application/json" },
        consumes = { "multipart/form-data" }
)
ResponseEntity<FileInfo> uploadMultiFiles(@NotNull @Valid @RequestPart(value = "images", required = true) MultipartFile[] images);

调用时将List<MultipartFile>转换为数组传入即可。

• 或者将参数包装进DTO，使用@ModelAttribute注解：

// 定义DTO
public class MultiFileUploadDTO {
    private List<MultipartFile> images;

    // getter、setter
}

// Feign方法修改为
ResponseEntity<FileInfo> uploadMultiFiles(@ModelAttribute MultiFileUploadDTO uploadDTO);

这种方式需要服务端接口也对应调整为接收该DTO参数。

内容的提问来源于stack exchange，提问作者mar3g