Java post multipart/form-data

在Java中发送multipart/form-data类型的POST请求，可以通过使用Java的HttpURLConnection类或Apache的HttpClient库来实现。

使用HttpURLConnection类的示例代码如下：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultipartFormDataExample {
    public static void main(String[] args) {
        try {
            // 创建URL对象
            URL url = new URL("http://example.com/upload");

            // 打开连接
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            // 设置请求方法为POST
            connection.setRequestMethod("POST");

            // 设置Content-Type为multipart/form-data
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");

            // 允许输入输出流
            connection.setDoInput(true);
            connection.setDoOutput(true);

            // 创建请求体
            String boundary = "----WebKitFormBoundary7MA4YWxkTrZu0gW";
            String CRLF = "\r\n"; // 回车换行符

            // 设置请求参数
            String data = "--" + boundary + CRLF +
                    "Content-Disposition: form-data; name=\"file\"; filename=\"example.txt\"" + CRLF +
                    "Content-Type: text/plain" + CRLF + CRLF +
                    "This is the content of the file" + CRLF +
                    "--" + boundary + "--" + CRLF;

            // 获取输出流
            OutputStream outputStream = connection.getOutputStream();

            // 写入请求体
            outputStream.write(data.getBytes());

            // 关闭输出流
            outputStream.close();

            // 获取响应状态码
            int responseCode = connection.getResponseCode();

            // 处理响应结果
            if (responseCode == HttpURLConnection.HTTP_OK) {
                // 获取响应输入流
                InputStream inputStream = connection.getInputStream();

                // 读取响应内容
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
                String line;
                StringBuilder response = new StringBuilder();
                while ((line = bufferedReader.readLine()) != null) {
                    response.append(line);
                }

                // 关闭输入流
                inputStream.close();

                // 打印响应内容
                System.out.println(response.toString());            } else {
                System.out.println("POST request failed with response code: " + responseCode);
            }

            // 关闭连接
            connection.disconnect();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

注意，在代码中需要替换URL、boundary、filename和请求体中的内容为实际的值。

Apache HttpClient库也提供了发送multipart/form-data类型请求的功能，使用方法请参考相关文档。