理解HTTP协议中的multipart/form-data

前提

之前在写一个通用HTTP组件的时候遇到过媒体（Media）类型multipart/form-data的封装问题，这篇文章主要简单介绍一下HTTP协议中媒体类型multipart/form-data的定义、应用和简单实现。

multipart/form-data的定义

媒体类型multipart/form-data遵循multipart MIME数据流定义（该定义可以参考Section 5.1 - RFC2046），大概含义就是：媒体类型multipart/form-data的数据体由多个部分组成，这些部分由一个固定边界值（Boundary）分隔。

multipart/form-data请求体布局

multipart/form-data请求体的布局如下：

# 请求头 - 这个是必须的，需要指定Content-Type为multipart/form-data，指定唯一边界值
Content-Type: multipart/form-data; boundary=${Boundary}

# 请求体
--${Boundary}
Content-Disposition: form-data; name="name of file"
Content-Type: application/octet-stream

bytes of file
--${Boundary}
Content-Disposition: form-data; name="name of pdf"; filename="pdf-file.pdf"
Content-Type: application/octet-stream

bytes of pdf file
--${Boundary}
Content-Disposition: form-data; name="key"
Content-Type: text/plain;charset=UTF-8

text encoded in UTF-8
--${Boundary}--

媒体类型multipart/form-data相对于其他媒体类型如application/x-www-form-urlencoded等来说，最明显的不同点是：

• 请求头的Content-Type属性除了指定为multipart/form-data，还需要定义boundary参数
• 请求体中的请求行数据是由多部分组成，boundary参数的值模式--${Boundary}用于分隔每个独立的分部
• 每个部分必须存在请求头Content-Disposition: form-data; name="${PART_NAME}";，这里的${PART_NAME}需要进行URL编码，另外filename字段可以使用，用于表示文件的名称，但是其约束性比name属性低（因为并不确认本地文件是否可用或者是否有异议）
• 每个部分可以单独定义Content-Type和该部分的数据体
• 请求体以boundary参数的值模式--${Boundary}--作为结束标志

{% note warning flat %}

RFC7578中提到两个multipart/form-data过期的使用方式，其一是Content-Transfer-Encoding请求头的使用，这里也不展开其使用方式，其二是请求体中单个表单属性传输多个二进制文件的方式建议换用multipart/mixed（一个"name"对应多个二进制文件的场景）

{% endnote %}

特殊地：

• 如果某个部分的内容为文本，其的Content-Type为text/plain，可指定对应的字符集，如Content-Type: text/plain;charset=UTF-8
• 可以通过_charset_属性指定默认的字符集，用法如下：

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

UTF-8
--ABCDE--
Content-Disposition: form-data; name="field"

...text encoded in UTF-8...
ABCDE--

Boundary参数取值规约

Boundary参数取值规约如下：

• Boundary的值必须以英文中间双横杠--开头，这个--称为前导连字符
• Boundary的值除了前导连字符以外的部分不能超过70个字符
• Boundary的值不能包含HTTP协议或者URL禁用的特殊意义的字符，例如英文冒号:等
• 每个--${Boundary}之前默认强制必须为CRLF，如果某一个部分的文本类型请求体以CRLF结尾，那么在请求体的二级制格式上，必须显式存在两个CRLF，如果某一个部分的请求体不以CRLF结尾，可以只存在一个CRLF，这两种情况分别称为分隔符的显式类型和隐式类型，说的比较抽象，见下面的例子：

# 请求头
Content-type: multipart/data; boundary="--abcdefg"

--abcdefg
Content-Disposition: form-data; name="x"
Content-type: text/plain; charset=ascii

It does NOT end with a linebreak # <=== 这里没有CRLF，隐式类型
--abcdefg
Content-Disposition: form-data; name="y"
Content-type: text/plain; charset=ascii

It DOES end with a linebreak # <=== 这里有CRLF，显式类型

--abcdefg

## 直观看隐式类型的CRLF
It does NOT end with a linebreak CRLF --abcdefg

## 直观看显式类型的CRLF
It DOES end with a linebreak CRLF CRLF --abcdefg

实现multipart/form-data媒体类型的POST请求

这里只针对低JDK版本的HttpURLConnection和高JDK版本内置的HttpClient编写multipart/form-data媒体类型的POST请求的HTTP客户端，其他如自定义Socket实现可以依照类似的思路完成。先引入org.springframework.boot:spring-boot-starter-web:2.6.0做一个简单的控制器方法：

@RestController
public class TestController {

    @PostMapping(path = "/test")
    public ResponseEntity<?> test(MultipartHttpServletRequest request) {
        return ResponseEntity.ok("ok");
    }
}

Postman的模拟请求如下：

后台控制器得到的请求参数如下：

后面编写的客户端可以直接调用此接口进行调试。

封装请求体转换为字节容器的模块

这里的边界值全用显式实现，边界值直接用固定前缀加上UUID生成即可。简单实现过程中做了一些简化：

• 只考虑提交文本表单数据和二进制（文件）表单数据
• 基于上一点，每个部分都明确指定Content-Type这个请求头
• 文本编码固定为UTF-8

编写一个MultipartWriter：

public class MultipartWriter {

    private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
    private static final byte[] FIELD_SEP = ": ".getBytes(StandardCharsets.ISO_8859_1);
    private static final byte[] CR_LF = "\r\n".getBytes(StandardCharsets.ISO_8859_1);
    private static final String TWO_HYPHENS_TEXT = "--";
    private static final byte[] TWO_HYPHENS = TWO_HYPHENS_TEXT.getBytes(StandardCharsets.ISO_8859_1);
    private static final String CONTENT_DISPOSITION_KEY = "Content-Disposition";
    private static final String CONTENT_TYPE_KEY = "Content-Type";
    private static final String DEFAULT_CONTENT_TYPE = "multipart/form-data; boundary=";
    private static final String DEFAULT_BINARY_CONTENT_TYPE = "application/octet-stream";
    private static final String DEFAULT_TEXT_CONTENT_TYPE = "text/plain;charset=UTF-8";
    private static final String DEFAULT_CONTENT_DISPOSITION_VALUE = "form-data; name=\"%s\"";
    private static final String FILE_CONTENT_DISPOSITION_VALUE = "form-data; name=\"%s\"; filename=\"%s\"";

    private final Map<String, String> headers = new HashMap<>(8);
    private final List<AbstractMultipartPart> parts = new ArrayList<>();
    private final String boundary;

    private MultipartWriter(String boundary) {
        this.boundary = Objects.isNull(boundary) ? TWO_HYPHENS_TEXT +
                UUID.randomUUID().toString().replace("-", "") : boundary;
        this.headers.put(CONTENT_TYPE_KEY, DEFAULT_CONTENT_TYPE + this.boundary);
    }

    public static MultipartWriter newMultipartWriter(String boundary) {
        return new MultipartWriter(boundary);
    }

    public static MultipartWriter newMultipartWriter() {
        return new MultipartWriter(null);
    }

    public MultipartWriter addHeader(String key, String value) {
        if (!CONTENT_TYPE_KEY.equalsIgnoreCase(key)) {
            headers.put(key, value);
        }
        return this;
    }

    public MultipartWriter addTextPart(String name, String text) {
        parts.add(new TextPart(String.format(DEFAULT_CONTENT_DISPOSITION_VALUE, name), DEFAULT_TEXT_CONTENT_TYPE, this.boundary, text));
        return this;
    }

    public MultipartWriter addBinaryPart(String name, byte[] bytes) {
        parts.add(new BinaryPart(String.format(DEFAULT_CONTENT_DISPOSITION_VALUE, name), DEFAULT_BINARY_CONTENT_TYPE, this.boundary, bytes));
        return this;
    }

    public MultipartWriter addFilePart(String name, File file) {
        parts.add(new FilePart(String.format(FILE_CONTENT_DISPOSITION_VALUE, name, file.getName()), DEFAULT_BINARY_CONTENT_TYPE, this.boundary, file));
        return this;
    }

    private static void writeHeader(String key, String value, OutputStream out) throws IOException {
        writeBytes(key, out);
        writeBytes(FIELD_SEP, out);
        writeBytes(value, out);
        writeBytes(CR_LF, out);
    }

    private static void writeBytes(String text, OutputStream out) throws IOException {
        out.write(text.getBytes(DEFAULT_CHARSET));
    }

    private static void writeBytes(byte[] bytes, OutputStream out) throws IOException {
        out.write(bytes);
    }

    interface MultipartPart {

        void writeBody(OutputStream os) throws IOException;
    }

    @RequiredArgsConstructor
    public static abstract class AbstractMultipartPart implements MultipartPart {

        protected final String contentDispositionValue;
        protected final String contentTypeValue;
        protected final String boundary;

        protected String getContentDispositionValue() {
            return contentDispositionValue;
        }

        protected String getContentTypeValue() {
            return contentTypeValue;
        }

        protected String getBoundary() {
            return boundary;
        }

        public final void write(OutputStream out) throws IOException {
            writeBytes(TWO_HYPHENS, out);
            writeBytes(getBoundary(), out);
            writeBytes(CR_LF, out);
            writeHeader(CONTENT_DISPOSITION_KEY, getContentDispositionValue(), out);
            writeHeader(CONTENT_TYPE_KEY, getContentTypeValue(), out);
            writeBytes(CR_LF, out);
            writeBody(out);
            writeBytes(CR_LF, out);
        }
    }

    public static class TextPart extends AbstractMultipartPart {

        private final String text;

        public TextPart(String contentDispositionValue,
                        String contentTypeValue,
                        String boundary,
                        String text) {
            super(contentDispositionValue, contentTypeValue, boundary);
            this.text = text;
        }

        @Override
        public void writeBody(OutputStream os) throws IOException {
            os.write(text.getBytes(DEFAULT_CHARSET));
        }

        @Override
        protected String getContentDispositionValue() {
            return contentDispositionValue;
        }

        @Override
        protected String getContentTypeValue() {
            return contentTypeValue;
        }
    }

    public static class BinaryPart extends AbstractMultipartPart {

        private final byte[] content;

        public BinaryPart(String contentDispositionValue,
                          String contentTypeValue,
                          String boundary,
                          byte[] content) {
            super(contentDispositionValue, contentTypeValue, boundary);
            this.content = content;
        }

        @Override
        public void writeBody(OutputStream out) throws IOException {
            out.write(content);
        }
    }

    public static class FilePart extends AbstractMultipartPart {

        private final File file;

        public FilePart(String contentDispositionValue,
                        String contentTypeValue,
                        String boundary,
                        File file) {
            super(contentDispositionValue, contentTypeValue, boundary);
            this.file = file;
        }

        @Override
        public void writeBody(OutputStream out) throws IOException {
            try (InputStream in = new FileInputStream(file)) {
                final byte[] buffer = new byte[4096];
                int l;
                while ((l = in.read(buffer)) != -1) {
                    out.write(buffer, 0, l);
                }
                out.flush();
            }
        }
    }

    public void forEachHeader(BiConsumer<String, String> consumer) {
        headers.forEach(consumer);
    }

    public void write(OutputStream out) throws IOException {
        if (!parts.isEmpty()) {
            for (AbstractMultipartPart part : parts) {
                part.write(out);
            }
        }
        writeBytes(TWO_HYPHENS, out);
        writeBytes(this.boundary, out);
        writeBytes(TWO_HYPHENS, out);
        writeBytes(CR_LF, out);
    }
}

这个类已经封装好三种不同类型的部分请求体实现，forEachHeader()方法用于遍历请求头，而最终的write()方法用于把请求体写入到OutputStream中。

HttpURLConnection实现

实现代码如下（只做最简实现，没有考虑容错和异常处理）：

public class HttpURLConnectionApp {

    private static final String URL = "http://localhost:9099/test";

    public static void main(String[] args) throws Exception {
        MultipartWriter writer = MultipartWriter.newMultipartWriter();
        writer.addTextPart("name", "throwable")
                .addTextPart("domain", "vlts.cn")
                .addFilePart("ico", new File("I:\\doge_favicon.ico"));
        DataOutputStream requestPrinter = new DataOutputStream(System.out);
        writer.write(requestPrinter);
        HttpURLConnection connection = (HttpURLConnection) new java.net.URL(URL).openConnection();
        connection.setRequestMethod("POST");
        connection.addRequestProperty("Connection", "Keep-Alive");
        // 设置请求头
        writer.forEachHeader(connection::addRequestProperty);
        connection.setDoInput(true);
        connection.setDoOutput(true);
        connection.setConnectTimeout(10000);
        connection.setReadTimeout(10000);
        DataOutputStream out = new DataOutputStream(connection.getOutputStream());
        // 设置请求体
        writer.write(out);
        StringBuilder builder = new StringBuilder();
        BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream(), StandardCharsets.UTF_8));
        String line;
        while (Objects.nonNull(line = reader.readLine())) {
            builder.append(line);
        }
        int responseCode = connection.getResponseCode();
        reader.close();
        out.close();
        connection.disconnect();
        System.out.printf("响应码:%d,响应内容:%s\n", responseCode, builder);
    }
}

执行响应结果：

响应码:200,响应内容:ok

可以尝试加入两行代码打印请求体：

MultipartWriter writer = MultipartWriter.newMultipartWriter();
writer.addTextPart("name", "throwable")
        .addTextPart("domain", "vlts.cn")
        .addFilePart("ico", new File("I:\\doge_favicon.ico"));
DataOutputStream requestPrinter = new DataOutputStream(System.out);
writer.write(requestPrinter);

控制台输出如下；

JDK内置HttpClient实现

JDK11+内置了HTTP客户端实现，具体入口是java.net.http.HttpClient，实现编码如下：

public class HttpClientApp {

    private static final String URL = "http://localhost:9099/test";

    public static void main(String[] args) throws Exception {
        HttpClient httpClient = HttpClient.newBuilder()
                .connectTimeout(Duration.of(10, ChronoUnit.SECONDS))
                .build();
        MultipartWriter writer = MultipartWriter.newMultipartWriter();
        writer.addTextPart("name", "throwable")
                .addTextPart("domain", "vlts.cn")
                .addFilePart("ico", new File("I:\\doge_favicon.ico"));
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        writer.write(out);
        HttpRequest.Builder requestBuilder = HttpRequest.newBuilder();
        writer.forEachHeader(requestBuilder::header);
        HttpRequest request = requestBuilder.uri(URI.create(URL))
                .method("POST", HttpRequest.BodyPublishers.ofByteArray(out.toByteArray()))
                .build();
        HttpResponse<String> response = httpClient.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.printf("响应码:%d,响应内容:%s\n", response.statusCode(), response.body());
    }
}

内置的HTTP组件几乎都是使用Reactive编程模型，使用的API都是相对底层，灵活性比较高但是易用性不高。

小结

媒体类型multipart/form-data常用于POST方法下的HTTP请求，至于作为HTTP响应的场景相对少见。

参考资料：

• RFC-7578
• RFC-2046

（本文完 c-1-d e-a-20211226 写完后发现了Boundary前导多加了中横杠，不过看了Postman的请求也多加了很多个，懒得改）