public interface GoodsReturnApiService {
  @Multipart
  @POST(Compares.GOODS_RETURN_POST)  //这里是自己post文件的地址
  Observable<GoodsReturnPostEntity> postGoodsReturnPostEntitys(@PartMap Map<String, RequestBody> map, @Part List<MultipartBody.Part> parts);
}

public class HttpRequestClient {

  public static final String TAG = "HttpRequestClientTAG";

  private static Retrofit retrofit;

  private static OkHttpClient getOkHttpClient() {
    //日志显示级别
    HttpLoggingInterceptor.Level level= HttpLoggingInterceptor.Level.BODY;
    //新建log拦截器
    HttpLoggingInterceptor loggingInterceptor=new HttpLoggingInterceptor(new HttpLoggingInterceptor.Logger() {
      @Override
      public void log(String message) {
        Log.d(TAG, message);
      }
    });
    loggingInterceptor.setLevel(level);
    //定制OkHttp
    OkHttpClient.Builder httpClientBuilder = new OkHttpClient
        .Builder();
    //OkHttp进行添加拦截器loggingInterceptor
    httpClientBuilder.addInterceptor(loggingInterceptor);
    return httpClientBuilder.build();
  }

  public static Retrofit getRetrofitHttpClient(){
    if(null == retrofit){
      synchronized (HttpRequestClient.class){
        if(null == retrofit){
          retrofit = new Retrofit.Builder()
              .client(getOkHttpClient())
              .baseUrl(Compares.URL)
              .addConverterFactory(GsonConverterFactory.create())
              .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
              .build();
        }
      }
    }
    return retrofit;
  }
}

private void postGoodsPicToServer(){
    Map<String,RequestBody> params = new HashMap<>();
    //以下参数是伪代码，参数需要换成自己服务器支持的
    params.put("type", convertToRequestBody("type"));
    params.put("title",convertToRequestBody("title"));
    params.put("info",convertToRequestBody("info");
    params.put("count",convertToRequestBody("count"));

    //为了构建数据，同样是伪代码
    String path1 = Environment.getExternalStorageDirectory() + File.separator + "test1.jpg";
    String path2 = Environment.getExternalStorageDirectory() + File.separator + "test1.jpg";

    List<File> fileList = new ArrayList<>();

    fileList.add(new File(path1));
    fileList.add(new File(path2));

    List<MultipartBody.Part> partList = filesToMultipartBodyParts(fileList);

    HttpRequestClient.getRetrofitHttpClient().create(GoodsReturnApiService.class)
        .postGoodsReturnPostEntitys(params,partList)
        .subscribeOn(Schedulers.newThread())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(new Observer<GoodsReturnPostEntity>() {
          @Override
          public void onSubscribe(@NonNull Disposable d) {

          }

          @Override
          public void onNext(@NonNull GoodsReturnPostEntity goodsReturnPostEntity) {

          }

          @Override
          public void onError(@NonNull Throwable e) {

          }

          @Override
          public void onComplete() {

          }
        });
}

private RequestBody convertToRequestBody(String param){
    RequestBody requestBody = RequestBody.create(MediaType.parse("text/plain"), param);
    return requestBody;
  }

private List<MultipartBody.Part> filesToMultipartBodyParts(List<File> files) {
    List<MultipartBody.Part> parts = new ArrayList<>(files.size());
    for (File file : files) {
      RequestBody requestBody = RequestBody.create(MediaType.parse("image/png"), file);
      MultipartBody.Part part = MultipartBody.Part.createFormData("multipartFiles", file.getName(), requestBody);
      parts.add(part);
    }
    return parts;
  }