NIO实现从网络URL下载图片，并将File转换为MultipartFile

/**
* description 网络TRL中下载图片
* @param uri
* @param folder 存放文件的文件夹
* @return java.lang.String
*/
private String downloadByUrl(String uri, String folder){
   ReadableByteChannel readableByteChannel = null;
   FileChannel fileChannel = null;
   File file;
   URL url;
   FileOutputStream fileOutputStream = null;
   try {
       url = new URL(uri);
       //首先从 URL stream 中创建一个 ReadableByteChannel 来读取网络文件
       readableByteChannel = Channels.newChannel(url.openStream());
       String fileName = System.currentTimeMillis() + url.getPath().substring(url.getPath().lastIndexOf("."));
       String path = folder + "/" + fileName;
       file = new File(path);
       if (!file.getParentFile().exists() && !file.getParentFile().isDirectory()) {
           file.getParentFile().mkdirs();
       }
       //通过 ReadableByteChannel 读取到的字节会流动到一个 FileChannel 中，然后再关联一个本地文件进行下载操作
       fileOutputStream = new FileOutputStream(file);
       fileChannel = fileOutputStream.getChannel();
       //最后用 transferFrom()方法就可以把 ReadableByteChannel 获取到的字节写入本地文件
       fileChannel.transferFrom(readableByteChannel,0,Long.MAX_VALUE);
       return fileName;
   } catch (Exception e) {
       log.error("下载文件失败" + e.getMessage());
       return "";
   } finally {
       try {
           if (null != readableByteChannel) {
               readableByteChannel.close();
           }
           if (null != fileChannel) {
               fileChannel.close();
           }
           if (null != fileOutputStream) {
               fileOutputStream.close();
           }
       } catch (IOException e) {
           log.error(e.getMessage());
       }
   }
   
/**
 * description 将本地文件转换为MultipartFile
 * @param path
 * @return org.springframework.web.multipart.MultipartFile
 */
private MultipartFile getMultipartFile(String path){
    FileInputStream inputStream = null;
    OutputStream outputStream = null;
    try {
        File file = new File(path);
        FileItem fileItem = new DiskFileItem("formFieldName",//form表单文件控件的名字随便起
                Files.probeContentType(file.toPath()),//文件类型
                false, //是否是表单字段
                file.getName(),//原始文件名
                (int) file.length(),//Interger的最大值可以存储两部1G的电影
                file.getParentFile());//文件会在哪个目录创建
        //为DiskFileItem的OutputStream赋值
        inputStream = new FileInputStream(file);
        outputStream = fileItem.getOutputStream();
        IOUtils.copy(inputStream, outputStream);
        return new CommonsMultipartFile(fileItem);
    } catch (IOException e) {
        log.error("文件类型转换失败" + e.getMessage());
        return null;
    } finally {
        try {
            if (null != inputStream) {
                inputStream.close();
            }

            if (null != outputStream) {
                outputStream.close();
            }
        } catch (IOException e) {
            log.error(">>文件流关闭失败" + e.getMessage());
        }
    }