Java发送form-data格式的请求上传multipart文件

在Java中，特别是使用了如Spring框架这样的工具，发送包含multipart文件的form-data格式请求通常涉及到`HttpURLConnection`、`HttpClient`或者第三方库如Apache HttpClient或OkHttp。这里是一个基本的例子，使用`UrlConnection`：

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

public class FileUploadExample {
    private static final String API_URL = "http://example.com/upload";

    public static void main(String[] args) {
        try {
            // 打开文件流
            FileInputStream fileInputStream = new FileInputStream(new File("path_to_your_file"));

            // 创建字节数组输出流
            ByteArrayOutputStream byteOutStream = new ByteArrayOutputStream();

            // 读取并写入文件内容到字节数组
            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = fileInputStream.read(buffer)) != -1) {
                byteOutStream.write(buffer, 0, bytesRead);
            }

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

            // 设置POST请求
            URL url = new URL(API_URL);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("POST");
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");

            // 添加分隔符
            byte[] boundary = "--" + "----WebKitFormBoundary7MA4YWxkTrZu0gW".getBytes();
            connection.getOutputStream().write(boundary);

            // 发送二进制数据
            connection.getOutputStream().write(byteOutStream.toByteArray());

            // 添加文件结束标记
            byte[] endBoundary = new byte[] {'\r', '\n', '--' + '----WebKitFormBoundary7MA4YWxkTrZu0gW--'};
            connection.getOutputStream().write(endBoundary);

            // 获取响应
            int responseCode = connection.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_OK) {
                System.out.println("File uploaded successfully.");
            } else {
                System.err.println("Failed to upload file. Response code: " + responseCode);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}