java编写Multipartfile/from-data的http请求

在Java中，当你需要发送一个多部分文件（Multipart/form-data）的HTTP请求时，通常会涉及到`HttpURLConnection`、`HttpClient`或者第三方库如Apache HttpClient或者Spring的`RestTemplate`等。下面是一个基本的例子，展示如何使用`HttpURLConnection`发送这种类型的请求：

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;

import javax.net.ssl.HttpsURLConnection;
import java.util.HashMap;
import java.util.Map;

public class MultipartRequest {
    private static final String BOUNDARY = "-------------14737890123456789";
    
    public void sendMultipartFile(String url, File file) throws IOException {
        HttpURLConnection conn = null;
        try {
            // 创建连接
            conn = (HttpsURLConnection) new URL(url).openConnection();
            conn.setRequestMethod("POST");
            conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);

            // 添加文件数据到请求体
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            addPart(baos, "file", file.getName(), getInputStream(file));
            byte[] requestBody = baos.toByteArray();

            // 设置请求长度
            long length = requestBody.length;
            conn.setFixedLengthStreamingMode(length);

            // 发送请求
            conn.getOutputStream().write(requestBody);
            
            // 获取响应
            int responseCode = conn.getResponseCode();
            System.out.println("Response Code : " + responseCode);

        } finally {
            if (conn != null) {
                conn.disconnect();
            }
        }
    }

    private InputStream getInputStream(File file) throws IOException {
        return Files.newInputStream(file.toPath());
    }

    private void addPart(ByteArrayOutputStream outputStream, String fieldName, String fileName, InputStream inputStream) throws IOException {
        byte[] endLine = ("\r\n--" + BOUNDARY + "\r\n").getBytes();
        byte[] boundaryBytes = ("Content-Disposition: form-data; name=\"" + fieldName + "\"; filename=\"" + fileName + "\"\r\n")
                .getBytes();

        outputStream.write(boundaryBytes);
        outputStream.write("Content-Type: application/octet-stream\r\n".getBytes());
        outputStream.write("\r\n".getBytes());

        byte[] buffer = new byte[1024];
        int bytesRead;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }

        outputStream.write(endLine);
    }
}

在这个例子中，我们首先设置请求头的Content-Type，然后创建一个字节流来添加文件数据。文件数据由文件名、内容类型和文件内容组成，每部分数据之间通过边界标志分隔。