Concurrent file write in Java on Windows

Question

What happens when you concurrently open two (or more) FileOutputStreams on the same file?

The Java API says this:

Some platforms, in particular, allow a file to be opened for writing by only one FileOutputStream (or other file-writing object) at a time.

I'm guessing Windows isn't such a platform, because I have two threads that read some big file (each one a different one) then write it to the same output file. No exception is thrown, the file is created and seems to contain chunks from both input files.

Side questions:

• Is this true for Unix, too?
• And since I want the behaviour to be the same (actually I want one thread to write correctly and the other to be warned of the conflict), how can I determine that the file is already opened for writing?

Answer 1

I would be wary of letting the OS determine file status for you (since this is OS-dependent). If you've got a shared resource I would restrict access to it using a Re-entrant lock

Using this lock means one thread can get the resource (file) and write to it. The next thread can check for this lock being held by another thread, and/or block indefinitely until the first thread releases it.

Windows (I think) would restrict two processes writing to the same file. I don't believe Unix would do the same.

Answer 2

If the 2 threads you are talking about are in the same JVM, then you could have a boolean variable somewhere that is accessed by both threads.

Answer 3

Unix allows concurrent writers to the same file.

You shouldn't be attempting to write to the same file more than once. If you are you have a design flaw.