Java: thread-safe RandomAccessFile

Question

After some serious googleing I found out that the RandomAccessFile-class is not thread-safe. Now I could use one semaphore to lock all reads and writes but I don't think that performs very well. In theory it should be possible to do multiple reads and one write at a time. How can I do this in Java? Is it possible at all?

Thanks!

Answer 1

I could use one semaphore to lock all reads and writes but I don't think that performs very well.

With respect to performance, NEVER think. ALWAYS measure.

That said, java.util.concurrent.locks.ReentrantReadWriteLock is what you are looking for.

Answer 2

Partial locking of a file is a complex business which a lot of operating systems avoid doing. However if you insist on doing it, one way is to design your own locking mechanism object that records which parts of the file are locked. Essentially before reading or writing an object must request a lock for a specific byte range of the file. Locks are considered to clash if they overlap at all in byte range. Read and write locks are treated differently: a read can overlap with any number of read locks safely, but a write lock must overlap with no other locks, read or write. There are a lot of questions about whether to wait or abort if you can't get the lock, and whether to block reads while a write is waiting, but only you can answer them about your application.

Given the complexity of this it may be better to lock the entire file. Check to see if you get adequate performance - and don't forget you can allow multiple reads at once, as long as there are no writes.