在 Java 中使用附加参数将文件上传到 HTTP 服务器

将文件上传到 HTTP 服务器是许多应用程序的常见需求。但是，有时还需要随文件一起传递附加参数。这是一个允许您在不使用外部库的情况下发送文件和参数的解决方案：

java.net.URLConnection 和 Multipart/Form-Data

发送文件和参数，您将利用 java.net.URLConnection 并采用 multipart/form-data 编码。 Multipart/form-data 允许您在单个 HTTP 请求中混合二进制数据（文件）和字符数据（参数）。

示例代码：

String url = "http://example.com/upload";
String charset = "UTF-8";
String param = "value";
File textFile = new File("/path/to/file.txt");
File binaryFile = new File("/path/to/file.bin");
String boundary = Long.toHexString(System.currentTimeMillis());
String CRLF = "\r\n";

URLConnection connection = new URL(url).openConnection();
connection.setDoOutput(true);
connection.setRequestProperty("Content-Type", "multipart/form-data; boundary="   boundary);

try (
    OutputStream output = connection.getOutputStream();
    PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, charset), true);
) {
    // Send normal param.
    writer.append("--"   boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"param\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset="   charset).append(CRLF);
    writer.append(CRLF).append(param).append(CRLF).flush();

    // Send text file.
    writer.append("--"   boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"textFile\"; filename=\""   textFile.getName()   "\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset="   charset).append(CRLF);
    writer.append(CRLF).flush();
    Files.copy(textFile.toPath(), output);
    output.flush(); 
    writer.append(CRLF).flush();

    // Send binary file.
    writer.append("--"   boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"binaryFile\"; filename=\""   binaryFile.getName()   "\"").append(CRLF);
    writer.append("Content-Type: "   URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
    writer.append("Content-Transfer-Encoding: binary").append(CRLF);
    writer.append(CRLF).flush();
    Files.copy(binaryFile.toPath(), output);
    output.flush(); 
    writer.append(CRLF).flush();

    // End of multipart/form-data.
    writer.append("--"   boundary   "--").append(CRLF).flush();
}

// Request is lazily fired whenever you need to obtain information about response.
int responseCode = ((HttpURLConnection) connection).getResponseCode();
System.out.println(responseCode); 

附加说明：

• 确保为每个多部分请求提供唯一的边界值。
• 文件必须采用指定的字符集发送 Content-Type 标头。
• Apache Commons HttpComponents Client 可以进一步简化该过程，但这不是必需的。

参考：

• [使用 java.net.URLConnection 触发和处理 HTTP 请求](https://docs.oracle.com/javase/tutorial/networking/urls/creating-urls.html)