Java concurrency file system

Question

Good day,

I need to create a File System Manager (more or less) which can read or write data to files.

My problem is how do I handle concurrency?

I can do something like

public class FileSystemManager {

    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    public byte[] read(String path) {
        readWriteLock.readLock().lock();
        try {
            ...
        } finally {
            readWriteLock.readLock().unlock();
        }
    }
    public void write(String path, byte[] data) {
        readWriteLock.writeLock().lock();
        try {
            ...
        } finally {
            readWriteLock.writeLock().unlock();
        }
    }
}

But this would mean all access to the write (for example) will be locked, even if the first invocation is targeting /tmp/file1.txt and the second invocation is targeting /tmp/file2.txt.

Any ideas how to go about this?

Thanks, Franz

Answer 1

I would look deeply into Java 5 and the java.util.concurrent package. I'd also recommend reading Brian Goetz' "Java Concurrency in Practice".

Answer 2

Can't you create a different object for each Path and then use synchronize blocks and synchronize on "this"?

Answer 3

Suggest Message Passing For Concurrency Not Threads

In general, this kind of locking happens beneath the java level. Are you really planning on reading and writing the same files and directories? Implementing directories?

Right now there is lots of unsafe threading code that may start blowing up as threads start really running together on multicore hardware.

My suggestion would be to manage concurrency via message passing. You can roll your own if this is an educational exercise, or use one of zillions of queuing and message systems out there. In this kind of system you have only one reader/writer process, but possibly many clients. You avoid the need for file and thread concurrency management.

One advantage of the message passing paradigm is that it will be much, much easier to create a distributed system for load balancing.

Answer 4

You can store the ReadWriteLock instances in a map keyed on path, just make sure that you get concurrent access to the map correct (possibly using ConcurrentHashMap).

If you actually care about locking the file using operating system primitives you might try looking into using java.nio.FileChannel. This has support for fine grained locking of file regions among other things. Also check out java.nio.channels.FileLock.