Java写一段附件上传

这里是Java写一段附件上传的示例代码：

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.UUID;

public class FileUploader {
    
    private static final String BOUNDARY = "---------------------------" + UUID.randomUUID().toString().replaceAll("-", "");

    public static void main(String[] args) throws IOException {
        File file = new File("path/to/file");
        String url = "http://example.com/upload";
        String contentType = "application/octet-stream";
        upload(file, url, contentType);
    }
    
    public static void upload(File file, String url, String contentType) throws IOException {
        HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
        conn.setRequestMethod("POST");
        conn.setDoOutput(true);
        conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);
        
        String fileName = file.getName();
        String fieldName = "file";
        
        StringBuilder sb = new StringBuilder();
        sb.append("--").append(BOUNDARY).append("\r\n")
          .append("Content-Disposition: form-data; name=\"").append(fieldName).append("\"; filename=\"").append(fileName).append("\"\r\n")
          .append("Content-Type: ").append(contentType).append("\r\n")
          .append("\r\n");
        byte[] headerBytes = sb.toString().getBytes();
        
        InputStream is = new FileInputStream(file);
        byte[] dataBytes = is.readAllBytes();
        
        byte[] footerBytes = ("\r\n--" + BOUNDARY + "--\r\n").getBytes();
        
        conn.setRequestProperty("Content-Length", String.valueOf(headerBytes.length + dataBytes.length + footerBytes.length));
        conn.getOutputStream().write(headerBytes);
        conn.getOutputStream().write(dataBytes);
        conn.getOutputStream().write(footerBytes);
        
        int responseCode = conn.getResponseCode();
        System.out.println("Response code: " + responseCode);
    }
}

使用时，需要替换以下部分：

- `path/to/file`：文件路径
- `http://example.com/upload`：上传接口地址
- `application/octet-stream`：文件类型

该代码使用`HttpURLConnection`发送POST请求，将文件作为multipart/form-data形式的请求体发送。请求体的格式如下：

--boundary
Content-Disposition: form-data; name="field_name"; filename="file_name"
Content-Type: content_type

file_data
--boundary--

其中，`boundary`为随机生成的字符串，用于分隔请求体的各个部分；`field_name`为文件字段名，可以自定义；`file_name`为文件名；`content_type`为文件类型，根据实际情况设置；`file_data`为文件内容。最后，需要计算请求体的长度，并设置`Content-Length`请求头。上传完成后，可以根据响应码判断上传是否成功。