废话不多说，直接上代码
一、异常捕捉类

public class BusinessException extends RuntimeException {public BusinessException(String msg){super(msg);}
}

二、转换类

package com.example.answer_system.utils;import org.springframework.mock.web.MockMultipartFile;
import org.springframework.web.multipart.MultipartFile;import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;//文件转换工具
public class FileUtils {/*** 将网络文件转换为文件流* @param imageUrl* @return*/public static MultipartFile fileUrlConvertToMultipartFile(String imageUrl) {try {// 将在线图片地址转换为URL对象URL url = new URL(imageUrl);// 打开URL连接URLConnection connection = url.openConnection();// 转换为HttpURLConnection对象HttpURLConnection httpURLConnection = (HttpURLConnection) connection;// 获取输入流InputStream inputStream = httpURLConnection.getInputStream();// 读取输入流中的数据，并保存到字节数组中ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();byte[] buffer = new byte[1024];int bytesRead;while ((bytesRead = inputStream.read(buffer)) != -1) {byteArrayOutputStream.write(buffer, 0, bytesRead);}// 将字节数组转换为字节数组byte[] bytes = byteArrayOutputStream.toByteArray();// 创建ByteArrayInputStream对象，将字节数组传递给它ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(bytes);// 创建MultipartFile对象，将ByteArrayInputStream对象作为构造函数的参数MultipartFile multipartFile = new MockMultipartFile("file", "filename.jpg", "image/jpg", byteArrayInputStream);return multipartFile;}catch (IOException ex){ex.printStackTrace();throw new BusinessException("附件无效");}}
}