Can we use custom code (e.g. java) to extract data in ETL?

I want to extract data from a json file. The file is stored in a directory whose path is fixed. New files are periodically added to this directory. The file name follows a specific naming convention that includes the time stamp and a header in it. I want to extract the timestamp and the header from the file name in order to isolate the latest file that is to be processed and save the header in a database table. This requires to iterate all files in the directory and extract the flie name. Then the file data can be extracted. How I can do this using jasper ETL?