Question

I have a Java client (client server app) that does two main things: (1) listen to server to receive files and save them to a local folder and (2) watch that folder for changes and send changes to the server. I want to run each in its own thread. First, is it a good idea to run each task on a separate thread. Second, how do I lock the folder when it’s used by either task to avoid interference?

Answer 1

is it a good idea to run each task on a separate thread

It sounds like a good idea to split your program into threads since the 2 tasks can work asynchronously and concurrently. The 1st thread could be downloading at the same time the 2nd one is uploading.

how do I lock the folder when it’s used by either task to avoid interference?

I wouldn’t do a lock at all. I’d have your 1st thread read a file from the server, write it into the folder, and then add a FileToSend object (or maybe just a File object) to a BlockingQueue. So instead of looking at the directory, your 2nd thread would just be waiting on the BlockingQueue for files to be sent to the server. The LinkedBlockingQueue class should work well for this. The BlockingQueue takes care of the locking for you.

If you do need to share a lock then you could just inject a lock object into your two threads:

private final Object folderLock = new Object();
...
new Thread(new Downloader(folderLock)).start();
new Thread(new Uploader(folderLock)).start();
...

A good pattern would be to define an addFileToUpload(File fileToUpload) method on your Uploader class. Then your Uploader can decide what to do with it. The BlockingQueue could then be local to the Uploader

Uploader uploader = new Uploader();
// pass the uploader into the downloader
new Thread(new Downloader(uploader)).start();
new Thread(uploader).start();