java发送multipart/form-data格式的数据 的demo

以下是Java发送multipart/form-data格式数据的示例代码：

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultipartFormDataDemo {
    public static void main(String[] args) {
        String url = "http://example.com/upload";
        String charset = "UTF-8";
        String boundary = Long.toHexString(System.currentTimeMillis());

        try {
            URL uploadUrl = new URL(url);
            HttpURLConnection connection = (HttpURLConnection) uploadUrl.openConnection();

            connection.setDoOutput(true);
            connection.setRequestMethod("POST");
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

            DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream());

            // 添加上传文件
            File file = new File("path/to/file");
            FileInputStream fileInputStream = new FileInputStream(file);
            byte[] buffer = new byte[(int) file.length()];
            fileInputStream.read(buffer);
            fileInputStream.close();
            outputStream.writeBytes("--" + boundary + "\r\n");
            outputStream.writeBytes("Content-Disposition: form-data; name=\"file\"; filename=\"" + file.getName() + "\"\r\n");
            outputStream.writeBytes("Content-Type: application/octet-stream\r\n\r\n");
            outputStream.write(buffer);
            outputStream.writeBytes("\r\n");

            // 添加其他参数
            outputStream.writeBytes("--" + boundary + "\r\n");
            outputStream.writeBytes("Content-Disposition: form-data; name=\"param1\"\r\n\r\n");
            outputStream.writeBytes("value1\r\n");
            outputStream.writeBytes("--" + boundary + "\r\n");
            outputStream.writeBytes("Content-Disposition: form-data; name=\"param2\"\r\n\r\n");
            outputStream.writeBytes("value2\r\n");
            outputStream.writeBytes("--" + boundary + "--\r\n");
            outputStream.flush();
            outputStream.close();

            // 读取服务器响应
            BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
            connection.disconnect();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

注意事项：

- Content-Type 头部必须设置为 multipart/form-data，并且包含一个 boundary 参数，boundary 是一个随机字符串，用于分隔不同的部分。
- 文件和其他参数都需要使用 multipart/form-data 格式进行发送。
- 每个部分必须以 --boundary 开始，以 --boundary-- 结束，并且每个部分之间需要用 \r\n 分隔。