我想把MultipartFile转换成File，并把它上传到我的ec2示例的指定目录。

private fun convertMultiPartFileToFile(multipartFile: MultipartFile, name: String): File? {
        val file = multipartFile.originalFilename?.let { File(it) }
        try {
            log.info { file?.canWrite() } // logs 'false'
            log.info { file?.canExecute() } // logs 'false'
            file?.setExecutable(true) // still 'false'
            file?.setWritable(true) // still 'false'

            file.let {
                if (file != null) {
                    FileOutputStream(file).use { outputStream -> outputStream.write(multipartFile.bytes) }
                }
            }
        } catch (e: IOException) {
            throw RuntimeException(e)
        }

        return file
    }

它在本地工作，但是当我在ec2示例上尝试时，出现了错误

java.lang.RuntimeException: java.io.FileNotFoundException: file.png (Permission denied)

我想是因为我没有写指定文件的权限，file?.setWritable(file)和file?.setExecutable(true)返回false怎么解决，我用java -jar path/to/jar运行jar
setWritable方法文档中说

* @return  {@code true} if and only if the operation succeeded.  The
     * operation will fail if the user does not have permission to
     * change the access permissions of this abstract pathname.

那么如何获得对这个抽象路径名的访问权限呢？