1. 使用场景，某个文件服务器里面保存了某个图片或者pdf文件，通过url的形式传输到自己这边，然后需要将url承载的文件保存到自己的电脑（服务器）里面。

2. 如果url自带文件名或者文件名加文件后缀那就是最方便的，获取文件名和后缀的时候就从urlAddress.substring来获取，反之这里工具用的是url不带文件名和后缀，需要自己传入destinationDir，并且我这里将文件后缀名字写死了.jpg，可以写一个灵活的后缀

3. 工具类的入口是fileDownload方法urlAddress大致长成"http://www.baidu.com/xxxxxxx"

destinationDir大致为"/User/file/xxx.jpg"

4.工具是灵活的传入的参数也是灵活的，可根据自己的需要修改

@UtilityClass
@Slf4j
public class FileUtil {

    /**
     * SIZE
     */
    private final int size = 1024;

    /**
     * File url
     *
     * @param urlAddress url address
     * @param localFileName local file name
     * @param destinationDir destination dir
     */
    public static void fileUrl(String urlAddress, String localFileName, String destinationDir) {
        OutputStream outputStream = null;
        URLConnection urlConnection = null;
        InputStream inputStream = null;
        try {
            URL url = new URL(urlAddress);
            outputStream = new BufferedOutputStream(Files.newOutputStream(Paths.get(destinationDir + "\\" + localFileName + ".jpg")));
            urlConnection = url.openConnection();
            inputStream = urlConnection.getInputStream();

            byte[] buf = new byte[size];
            int byteRead, byteWritten = 0;
            while ((byteRead = inputStream.read(buf)) != -1) {
                outputStream.write(buf, 0, byteRead);
                outputStream.flush();
                byteWritten += byteRead;
            }
        } catch (Exception e) {
            log.error(e.getMessage());
        } finally {
            try {
                inputStream.close();
                outputStream.close();
            } catch (Exception e) {
                log.error(e.getMessage());
            }
        }
    }

    /**
     * File download
     *
     * @param urlAddress url address
     * @param destinationDir destination dir
     */
    public static void fileDownload(String urlAddress, String destinationDir) {
        int slashIndex = destinationDir.lastIndexOf('/');
        int periodIndex = destinationDir.lastIndexOf('.');

        String fileName = destinationDir.substring(slashIndex + 1, periodIndex);

        if (periodIndex >= 1 && slashIndex >= 0 && slashIndex < urlAddress.length() - 1) {
            fileUrl(urlAddress, fileName, destinationDir);
        } else {
            System.err.println("path or file name.");
        }
    }
}