一、express部分（服务器端）

1. 项目的初始化及安装

mkdir app
cd app
npm install -S express
npm install -S multer

2. 代码：

// app.js

const express=require('express')
const http=require('http')
const multer=require('multer')
let upload = multer({ dest: 'uploads/' })
let app=express();

app.post('/upload', upload.single('avatar'), function (req, res, next) {
    let msg={
        body:req.body,
        file:req.file
    }
    res.json(msg)
})

let server=http.createServer(app);
server.listen(8000,function () {
    console.log("start at port 8000")
})

3. 项目运行

node app.js

okhttp部分(java或者android的客户端)

1. 引包

implementation 'com.squareup.okhttp3:okhttp:4.2.1'

2. 上传部分

    @Test
    public void test5() throws Exception{
        String path="/Users/johnyu/Documents/abc.JPG";
        OkHttpClient client=new OkHttpClient();

        //上传文件域的请求体部分
        RequestBody formBody=  RequestBody
                .create(new File(path),MediaType.parse("image/jpeg"));

        //整个上传的请求体部分（普通表单+文件上传域）
        RequestBody requestBody=new MultipartBody.Builder()
                .setType(MultipartBody.FORM)
                .addFormDataPart("title", "Square Logo")
                //filename:avatar,originname:abc.jpg
                .addFormDataPart("avatar", "abc.jpg",formBody)
                .build();

        Request request = new Request.Builder()
                .url("http://localhost:8000/upload")
                .post(requestBody)
                .build();
        Response response = client.newCall(request).execute();

        System.out.println(response.body().string());
    }

三、运行结果

{
  "body": {
    "title": "Square Logo"
  },
  "file": {
    "fieldname": "avatar",
    "originalname": "abc.jpg",
    "encoding": "7bit",
    "mimetype": "image/jpeg",
    "destination": "uploads/",
    "filename": "bbeb89990ed970686eeeb467bb02a211",
    "path": "uploads/bbeb89990ed970686eeeb467bb02a211",
    "size": 2884436
  }
}