multipart/form-data，多部件请求体。这个请求体比较特殊，它可以拆分为多个部件，每个部件都有自己的header和body，最常用的地方就是：客户端文件上传，因为有多个部件，在上传文件的时候，还可以在body中添加其他的数据。json，form。。。

一般来说，都是客户端发起multipart/form-data请求 ，服务器进行解析。而且这种东西的编码解码工作一般都是由底层的容器/框架完成。开发根本不必关心。但是我最近遇到了一个需求：

服务器响应multipart/form-data(包含了一个二进制文件和其他的文本数据)，客户端来解析

意味着，需要自己完成2个东西

在服务端完成multipart/form-data的数据编码，并且响应给客户端

在客户端获取到响应后，进行数据的解码

multipart/form-data的请求体，看起来像这样(省略了部分 header)

POST /foo HTTP/1.1

Content-Length: 68137

Content-Type: multipart/form-data; boundary=---------------------------974767299852498929531610575

---------------------------974767299852498929531610575

Content-Disposition: form-data; name="description"

some text

---------------------------974767299852498929531610575

Content-Disposition: form-data; name="myFile"; filename="foo.txt"

Content-Type: text/plain

(content of the uploaded file foo.txt)

---------------------------974767299852498929531610575

复制代码

服务端的编码

使用 org.apache.httpcomponents 库进行编码

org.apache.httpcomponents

httpmime

4.5.12

复制代码

Controller

通过 MultipartEntityBuilder， 添加多个部件，每个部件有自己的名字，类型。构建出一个 HttpEntity对象。可以从这个对象中获取到编码后的IO流以及ContentType，直接响应给 客户端就完事儿，比较简单。

import java.io.File;

import java.nio.charset.StandardCharsets;

import javax.servlet.http.HttpServletResponse;

import org.apache.http.HttpEntity;

import org.apache.http.entity.ContentType;

import org.apache.http.entity.mime.MultipartEntityBuilder;

import org.apache.http.entity.mime.content.StringBody;

import org.springframework.web.bind.annotation.GetMapping;

import org.springframework.web.bind.annotation.RequestMapping;

import org.springframework.web.bind.annotation.RestController;

import org.springframework.web.util.UriUtils;

@RestController

@RequestMapping("/test")

public class TestController{

@GetMapping

public void test (HttpServletResponse response) throws Exception{

HttpEntity httpEntity = MultipartEntityBuilder.create()

// 表单 => (部件名称，数据，类型)，要注意uri编码

.addPart("name", new StringBody(UriUtils.encode("SpringBoot中文社区", StandardCharsets.UTF_8), ContentType.APPLICATION_FORM_URLENCODED))

// JSON => (部件名称，JSON，类型)

.addPart("info", new StringBody("{\"site\": \"https://springboot.io\", \"year\": 2019}", ContentType.APPLICATION_JSON))

// 文件 => ( 部件名称，文件，类型，文件名称)

.addBinaryBody("logo", new File("D:\\logo.png"), ContentType.IMAGE_PNG, "logo.png")

.build();

// 设置ContentType

response.setContentType(httpEntity.getContentType().getValue());

// 响应客户端

httpEntity.writeTo(response.getOutputStream());

}

}

复制代码

客户端的解码

使用commons-fileupload 库进行解码

commons-fileupload

commons-fileupload

1.4

复制代码

MultipartTest

看这个代码，会觉得似曾相识。不错，在Servlet3.0以前，HttpServletRequest还没有getPart方法的时候 ，大家都是通过 commons-fileupload来从multipart/form-data请求中解析出数据的。

import java.io.IOException;

import java.io.InputStream;

import java.nio.charset.Charset;

import java.nio.charset.StandardCharsets;

import java.util.Iterator;

import java.util.List;

import org.apache.commons.fileupload.FileItem;

import org.apache.commons.fileupload.FileItemFactory;

import org.apache.commons.fileupload.FileItemHeaders;

import org.apache.commons.fileupload.FileUploadBase;

import org.apache.commons.fileupload.FileUploadException;

import org.apache.commons.fileupload.RequestContext;

import org.apache.commons.fileupload.disk.DiskFileItemFactory;

import org.apache.commons.fileupload.portlet.PortletFileUpload;

import org.springframework.core.io.Resource;

import org.springframework.http.MediaType;

import org.springframework.http.ResponseEntity;

import org.springframework.web.client.RestTemplate;

/**

* 自己定义一个RequestContext的实现

*/

class SimpleRequestContext implements RequestContext{

private final Charset charset;// 编码

private final MediaType contentType;// contentType

private final InputStream content;// 数据

public SimpleRequestContext(Charset charset, MediaType contentType, InputStream content){

this.charset = charset;

this.contentType = contentType;

this.content = content;

}

@Override

public String getCharacterEncoding(){

return this.charset.displayName();

}

@Override

public String getContentType(){

return this.contentType.toString();

}

@Override

public int getContentLength(){

try {

return this.content.available();

} catch (IOException e) {

}

return 0;

}

@Override

public InputStream getInputStream() throws IOException{

return this.content;

}

}

public class MultipartTest{

public static void main(String[] args) throws IOException, FileUploadException{

// 获取服务器响应的IO流

RestTemplate restTemplate = new RestTemplate();

ResponseEntity responseEntity = restTemplate.getForEntity("http://localhost:8081/test", Resource.class);

// 创建RequestContext对象

RequestContext requestContext = new SimpleRequestContext(StandardCharsets.UTF_8, responseEntity.getHeaders().getContentType(),

responseEntity.getBody().getInputStream());

// 解析器创建

FileUploadBase fileUploadBase = new PortletFileUpload();

FileItemFactory fileItemFactory = new DiskFileItemFactory();

fileUploadBase.setFileItemFactory(fileItemFactory);

fileUploadBase.setHeaderEncoding(StandardCharsets.UTF_8.displayName());

// 解析出所有的部件

List fileItems = fileUploadBase.parseRequest(requestContext);

for (FileItem fileItem : fileItems) {

// 请求头

System.out.println("headers:==========================");

FileItemHeaders fileItemHeaders = fileItem.getHeaders();

Iterator headerNamesIterator = fileItemHeaders.getHeaderNames();

while (headerNamesIterator.hasNext()) { // 迭代name

String headerName = headerNamesIterator.next();

Iterator headerValueIterator = fileItemHeaders.getHeaders(headerName);

while (headerValueIterator.hasNext()) {// 迭代value

String headerValue = headerValueIterator.next();

System.out.println(headerName + ":" + headerValue);

}

}

// 请求体

System.out.println("body:==========================");

if(fileItem.isFormField()) { // 是普通表单项

byte[] data = fileItem.get();

System.out.println(new String(data, StandardCharsets.UTF_8));

} else {// 是文件表单项

String fileName = fileItem.getName();// 文件的原始名称

InputStream inputStream = fileItem.getInputStream();// 文件的IO流

System.out.println("fileName=" + fileName + ", size=" + inputStream.available());

}

System.out.println();

}

}

}

复制代码

完整的日志输出

17:18:55.384 [main] DEBUG org.springframework.web.client.RestTemplate - HTTP GET http://localhost:8081/test

17:18:55.449 [main] DEBUG org.springframework.web.client.RestTemplate - Accept=[application/json, application/*+json, */*]

17:18:56.426 [main] DEBUG org.springframework.web.client.RestTemplate - Response 200 OK

17:18:56.461 [main] DEBUG org.springframework.web.client.RestTemplate - Reading to [org.springframework.core.io.Resource] as "multipart/form-data;boundary=0W40KHiHJTyo5H_n1EIL68aM4tNRhPa-7Vp"

headers:==========================

content-disposition:form-data; name="name"

content-type:application/x-www-form-urlencoded; charset=ISO-8859-1

content-transfer-encoding:8bit

body:==========================

SpringBoot%E4%B8%AD%E6%96%87%E7%A4%BE%E5%8C%BA

headers:==========================

content-disposition:form-data; name="info"

content-type:application/json; charset=UTF-8

content-transfer-encoding:8bit

body:==========================

{"site": "https://springboot.io", "year": 2019}

headers:==========================

content-disposition:form-data; name="logo"; filename="logo.png"

content-type:image/png

content-transfer-encoding:binary

body:==========================

fileName=logo.png, size=2423

复制代码

客户端准确的解析出了服务器响应的 multipart/form-data 数据。