recurring check, whether file exists or not

Question

If I would like to check whether file exists or not I will use:

File file = new File("name of File");
if(file.exists()) { //do something}

But what shall i do, If I would like to use it as check of installation: file can be created e.g. after 5 or 10 minutes : so how can file.exists() controls whether file exists or not yet?

E.G.

Installation started, file doesn't exist
Installation continue 5 min file still doesnt exist
After 10 min - file was created - e.g. Installation.chk (file exists already) my if statement is now true

Is it possible to create it in Java? If yes, how?

http://stackoverflow.com/questions/1237235/check-file-exists-java?rq=1 — Tdorno, May 06 '13 at 16:48
Perhaps you should look into directory watchers http://docs.oracle.com/javase/tutorial/essential/io/notification.html — toniedzwiedz, May 06 '13 at 16:51
Very confusing. If i am understanding you, you could possibly just use a while(!file.exists()) loop, and then do whatever actions you're asking about within that loop. — user2277872, May 06 '13 at 16:54
@user2277872 : it's wastage of CPU. That is why there is a thing called Signals in an OS. — Praneeth Peiris, May 06 '13 at 16:58
They are used to monitor Kernel events. (Every request is parsed through the Kernel, so we can listen to them.) Then we don't have to waste CPU by constantly checking the status. Kernel will send a signal when the event is triggered. (Ex : a File gets updated). — Praneeth Peiris, May 06 '13 at 17:18

score 1 · Answer 1 · answered May 06 '13 at 16:55

1

Maybe there is a better way:

http://docs.oracle.com/javase/tutorial/essential/io/notification.html

Instead of looping your exists(), let the OS tell you, if something changes.

answered May 06 '13 at 16:55

desperateCoder

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score 0 · Answer 2 · answered May 06 '13 at 16:50

When dealing with the file-system you have to accept that race-conditions happen and plan for them.

You can only check whether a file exists now, not whether it will continue to exist because disks can be mounted/ejected, super-user accounts can delete files or unlink or make parent directories unreadable.

java.io does not support file-locking either.

If you need to check that something is installed, you can make a best effort, but you cannot prevent a user from uninstalling another piece of software while yours is running.

if(file.exists()) { /*do something*/}

This is rarely useful by itself.

Often, combining a file type and a permissions check identifies more bad states than just testing existence.

file.isFile() && file.canRead()

@Amalgovinus, are you sure? It has to read through directories, and may have to read through symlinks depending on the LinkOptions, but I don't see anything in the doc to suggest that it will check read permissions on a normal file. — Mike Samuel, Jun 20 '18 at 12:42

recurring check, whether file exists or not

2 Answers2