When trying to lock a file that is on a SMB or an AFP network filesystem, FileChannel.tryLock() throws an IOException with the message "Operation not supported" .

The reason seems to be that the JRE uses the call fcntl to try to lock files on these network shares. On OS X fcntl does not support locking files on AFP or SMB (I believe NFS works alright, but am not certain). A possible alternative is perhaps the call flock for AFP and SMB.

More general information can be found on http://www.briandwells.com/main/Blog/Entries/2006/10/5_Subversion,_Mac_OS_X,_and_SMB.html

To test the problem, mount an SMB or AFP share on a OS X machine. Then run this code with one of the mounted files as argument:

import java.io.*;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;

public class FileLockTest {

    public static void main(String[] args) throws Exception {
        RandomAccessFile file = null;
        FileLock fileLock = null;
        try
        {
            file = new RandomAccessFile(args[0], "rw");
            FileChannel fileChannel = file.getChannel();

            fileLock = fileChannel.tryLock();
            if (fileLock != null){
                System.out.println("File is locked");
            }
        } finally{
            if (fileLock != null){
                fileLock.release();
            }
        }
    }
}

When working properly, you should see the text "File is locked".
When failing, you see something like:

Exception in thread "main" java.io.IOException: Operation not supported
    at sun.nio.ch.FileDispatcherImpl.lock0(Native Method)
    at sun.nio.ch.FileDispatcherImpl.lock(FileDispatcherImpl.java:90)
    at sun.nio.ch.FileChannelImpl.tryLock(FileChannelImpl.java:1114)
    at java.nio.channels.FileChannel.tryLock(FileChannel.java:1155)
    at FileLockTest.main(FileLockTest.java:15)

Please note that the thrown exception is not very helpful, because it is a plain IOException with some message. As a programmer, I'd like to distinguish between a real failure or an unsupported operation. Even if for some reason proper locking cannot be implemented, please implement a specific exception that is thrown in case a file cannot be locked, because the mechanism isn't properly implemented or supported by the OS. Something like FileLockUnsupportedException (subclass of IOException) with a proper reason that perhaps contains the filesystem type.