前提

之前在寫一個通用HTTP組件的時候遇到過媒體（Media）類型multipart/form-data的封裝問題，這篇文章主要簡單介紹一下HTTP協議中媒體類型multipart/form-data的定義、應用和簡單實現。

multipart/form-data的定義

媒體類型multipart/form-data遵循multipart MIME數據流定義（該定義可以參考Section 5.1 - RFC2046），大概含義就是：媒體類型multipart/form-data的數據體由多個部分組成，這些部分由一個固定邊界值（Boundary）分隔。

multipart/form-data請求體布局

multipart/form-data請求體的布局如下：

# 請求頭 - 這個是必須的，需要指定Content-Type為multipart/form-data，指定唯一邊界值 Content-Type: multipart/form-data; boundary=${Boundary}# 請求體 --${Boundary} Content-Disposition: form-data; name="name of file" Content-Type: application/octet-streambytes of file --${Boundary} Content-Disposition: form-data; name="name of pdf"; filename="pdf-file.pdf" Content-Type: application/octet-streambytes of pdf file --${Boundary} Content-Disposition: form-data; name="key" Content-Type: text/plain;charset=UTF-8text encoded in UTF-8 --${Boundary}--

媒體類型multipart/form-data相對于其他媒體類型如application/x-www-form-urlencoded等來說，最明顯的不同點是：

• 請求頭的Content-Type屬性除了指定為multipart/form-data，還需要定義boundary參數
• 請求體中的請求行數據是由多部分組成，boundary參數的值模式--${Boundary}用于分隔每個獨立的分部
• 每個部分必須存在請求頭Content-Disposition: form-data; name="${PART_NAME}";，這里的${PART_NAME}需要進行URL編碼，另外filename字段可以使用，用于表示文件的名稱，但是其約束性比name屬性低（因為并不確認本地文件是否可用或者是否有異議）
• 每個部分可以單獨定義Content-Type和該部分的數據體
• 請求體以boundary參數的值模式--${Boundary}--作為結束標志

{% note warning flat %} RFC7578中提到兩個multipart/form-data過期的使用方式，其一是Content-Transfer-Encoding請求頭的使用，這里也不展開其使用方式，其二是請求體中單個表單屬性傳輸多個二進制文件的方式建議換用multipart/mixed（一個"name"對應多個二進制文件的場景） {% endnote %}

特殊地：

• 如果某個部分的內容為文本，其的Content-Type為text/plain，可指定對應的字符集，如Content-Type: text/plain;charset=UTF-8
• 可以通過_charset_屬性指定默認的字符集，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參數取值規約

Boundary參數取值規約如下：

• Boundary的值必須以英文中間雙橫杠--開頭，這個--稱為前導連字符
• Boundary的值除了前導連字符以外的部分不能超過70個字符
• Boundary的值不能包含HTTP協議或者URL禁用的特殊意義的字符，例如英文冒號:等
• 每個--${Boundary}之前默認強制必須為CRLF，如果某一個部分的文本類型請求體以CRLF結尾，那么在請求體的二級制格式上，必須顯式存在兩個CRLF，如果某一個部分的請求體不以CRLF結尾，可以只存在一個CRLF，這兩種情況分別稱為分隔符的顯式類型和隱式類型，說的比較抽象，見下面的例子：

# 請求頭 Content-type: multipart/data; boundary="--abcdefg"--abcdefg Content-Disposition: form-data; name="x" Content-type: text/plain; charset=asciiIt does NOT end with a linebreak # <=== 這里沒有CRLF，隱式類型 --abcdefg Content-Disposition: form-data; name="y" Content-type: text/plain; charset=asciiIt DOES end with a linebreak # <=== 這里有CRLF，顯式類型--abcdefg## 直觀看隱式類型的CRLF It does NOT end with a linebreak CRLF --abcdefg## 直觀看顯式類型的CRLF It DOES end with a linebreak CRLF CRLF --abcdefg

實現multipart/form-data媒體類型的POST請求

這里只針對低JDK版本的HttpURLConnection和高JDK版本內置的HttpClient編寫multipart/form-data媒體類型的POST請求的HTTP客戶端，其他如自定義Socket實現可以依照類似的思路完成。先引入org.springframework.boot:spring-boot-starter-web:2.6.0做一個簡單的控制器方法：

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

Postman的模擬請求如下：

后臺控制器得到的請求參數如下：

后面編寫的客戶端可以直接調用此接口進行調試。

封裝請求體轉換為字節容器的模塊

這里的邊界值全用顯式實現，邊界值直接用固定前綴加上UUID生成即可。簡單實現過程中做了一些簡化：

• 只考慮提交文本表單數據和二進制（文件）表單數據
• 基于上一點，每個部分都明確指定Content-Type這個請求頭
• 文本編碼固定為UTF-8

編寫一個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;}@RequiredArgsConstructorpublic 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;}@Overridepublic void writeBody(OutputStream os) throws IOException {os.write(text.getBytes(DEFAULT_CHARSET));}@Overrideprotected String getContentDispositionValue() {return contentDispositionValue;}@Overrideprotected 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;}@Overridepublic 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;}@Overridepublic 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);} }

這個類已經封裝好三種不同類型的部分請求體實現，forEachHeader()方法用于遍歷請求頭，而最終的write()方法用于把請求體寫入到OutputStream中。

HttpURLConnection實現

實現代碼如下（只做最簡實現，沒有考慮容錯和異常處理）：

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");// 設置請求頭writer.forEachHeader(connection::addRequestProperty);connection.setDoInput(true);connection.setDoOutput(true);connection.setConnectTimeout(10000);connection.setReadTimeout(10000);DataOutputStream out = new DataOutputStream(connection.getOutputStream());// 設置請求體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("響應碼:%d,響應內容:%s\n", responseCode, builder);} }

執行響應結果：

響應碼:200,響應內容:ok

可以嘗試加入兩行代碼打印請求體：

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);

控制臺輸出如下；

JDK內置HttpClient實現

JDK11+內置了HTTP客戶端實現，具體入口是java.net.http.HttpClient，實現編碼如下：

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("響應碼:%d,響應內容:%s\n", response.statusCode(), response.body());} }

內置的HTTP組件幾乎都是使用Reactive編程模型，使用的API都是相對底層，靈活性比較高但是易用性不高。

小結

媒體類型multipart/form-data常用于POST方法下的HTTP請求，至于作為HTTP響應的場景相對少見。

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