Spring Boot 2.7.5 WebClient Multipart文件上传报400错误，求解决方案

问题 #

基于Spring Boot 2.7.5的WebClient应用调用同版本的多部件文件上传API时，返回400 Bad Request错误，但curl和Angular的POST请求均可正常调用该API。

WebClient端原代码 #

var home = "/home/me";

var uploadFile = new File(home+"/upload.csv");   
var uploadPath = uploadFile.toPath();
var uploadContent = Files.readString(uploadPath);

var multipartBodyBuilder = new MultipartBodyBuilder();
multipartBodyBuilder.part("file", uploadPath);
//multipartBodyBuilder.part("file", uploadFile);
var mimeMultipartData = multipartBodyBuilder.build();

var configuration = new Configuration(/* ... */);

var webClient = WebClient.builder()
        .baseUrl(_baseUrl(configuration.getEndpoint()))
        .filters( filters -> {
            filters.add(authentication(configuration));

            if (configuration.capturePackets()) {
                filters.add(captureRequest());
                filters.add(captureResponse());
            }
        })
        .build();

Mono<HttpStatus> httpStatusMono = webClient
     .post()
     .uri(URI.create("http://localhost:8080/api/uploads"))
     .body(BodyInserters.fromMultipartData(mimeMultipartData))
     .retrieve()
     .bodyToMono(HttpStatus.class);

服务端API代码 #

@PostMapping("/uploads")
public ResponseEntity<?> uploads(@RequestParam(value="file", required=true) MultipartFile file) throws IOException {
    try (InputStream input = file.getInputStream()) {
        doSomething(input);
    }
    return ResponseEntity.ok().build();
}

错误信息 #

org.springframework.web.reactive.function.client.WebClientResponseException$BadRequest: 400 Bad Request from POST http://localhost:8080/api/uploads
    at org.springframework.web.reactive.function.client.WebClientResponseException.create(WebClientResponseException.java:217)
    Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ 400 from POST http://localhost:8080/api/uploads [DefaultWebClient]
Original Stack Trace:
        at org.springframework.web.reactive.function.client.WebClientResponseException.create(WebClientResponseException.java:217)
        ...（省略部分栈信息）

正常调用示例 #

curl命令

curl -X POST -v --user "username:password" -H "Content-Type: multipart/form-data" \
     --form file=@upload.csv  http://localhost:8080/api/uploads

Angular代码

upload(file: File): Observable<any> {
    const formData: FormData = new FormData()

    formData.append('file', file, file.name);

    const observable: Observable<any> = this._http
      .post(url, formData)
      .pipe(
        // map(response => response),
        share()
    );

解决方案 #

问题根源是WebClient构建多部件请求时，直接传入Path对象无法被Spring正确识别为文件资源，导致服务端无法解析出MultipartFile。调整方式如下：

调整后的WebClient代码 #

var home = "/home/me";
var uploadFile = new File(home + "/upload.csv");

var multipartBodyBuilder = new MultipartBodyBuilder();
// 方式1：直接传入File对象，自动处理文件名与内容类型
multipartBodyBuilder.part("file", uploadFile)
                    .contentType(MediaType.MULTIPART_FORM_DATA);

// 方式2：用FileSystemResource包装Path，手动指定文件名（适合需要自定义文件名的场景）
// multipartBodyBuilder.part("file", new FileSystemResource(uploadFile.toPath()))
//                     .filename(uploadFile.getName())
//                     .contentType(MediaType.TEXT_PLAIN); // 根据实际文件类型调整

var mimeMultipartData = multipartBodyBuilder.build();

var configuration = new Configuration(/* ... */);

var webClient = WebClient.builder()
        .baseUrl(_baseUrl(configuration.getEndpoint()))
        .filters( filters -> {
            filters.add(authentication(configuration));

            if (configuration.capturePackets()) {
                filters.add(captureRequest());
                filters.add(captureResponse());
            }
        })
        .build();

// 服务端返回无body响应，用toBodilessEntity()替代bodyToMono(HttpStatus.class)更合理
Mono<ResponseEntity<Void>> responseMono = webClient
     .post()
     .uri("http://localhost:8080/api/uploads")
     .body(BodyInserters.fromMultipartData(mimeMultipartData))
     .retrieve()
     .toBodilessEntity();

关键调整说明 #

1. 正确传递文件资源：必须使用File或FileSystemResource包装文件路径，Spring才能生成符合多部件规范的请求体，包含必要的文件名和内容类型标识。
2. 响应处理优化：服务端返回的是无内容响应，使用toBodilessEntity()可以避免解析空body时的潜在异常。
3. URI简化：直接传入字符串URI即可，无需手动创建URI对象，WebClient会自动处理。

内容的提问来源于stack exchange，提问作者Jan Nielsen