Re: Synchronous write to same file using different file pointers

Alexander Dünisch wrote:
Hi newsgroup,

i have written a piece of code like the following:


// ======= start of code sample ===============

public void writeStuff(byte[] stuff, long location) throws IOException {

RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");
try {
raf.seek(location);
raf.write(stuff);
} finally {
raf.close();
}
}

// ======= end of code sample ===============


This function is being called by multiple threads.
Let's assume it can be guaranteed by the choice of value for location
and the length of array 'stuff', that one thread doesn't overwrite the bytes written by another thread.

Because the RandomAccessFile is local to the method,
each thread would create it's own instance and use the associated
file pointer.

Can this be done safely or do i have to put the call to write in a
synchronized block?
In other words:
Can two or mor threads write synchronously to different locations of
the same file using different RandomAccessFile objects and thus
different file pointers, if there is no risk of one thread overwriting
another's data?

Thanks in advance for your answers.

Alex D.


Probably. There is one concern and that is that if two threads extended the file you might get corrupted data. Also I don't think the performance decrease by synchronizing would be all that great. The disk is the slowest part of this and if by synchronizing you eliminated any partial writes and the associated seeking it might actually go quicker.

I think I would synchronize and see if the performance is too poor. I have running code that uses static methods with local RandomAccessFiles and that use ReentrantReadWriteLocks to synchronize access. In my case the rate of data access is probably well below that which would cause great contention though.

--

Knute Johnson
email s/nospam/linux/

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