Java FileLock for Reading and Writing

Question

I have a process that will be called rather frequently from cron to read a file that has certain move related commands in it. My process needs to read and write to this data file - and keep it locked to prevent other processes from touching it during this time. A completely separate process can be executed by a user to (potential) write/append to this same data file. I want these two processes to play nice and only access the file one at a time.

The nio FileLock seemed to be what I needed (short of writing my own semaphore type files), but I'm having trouble locking it for reading. I can lock and write just fine, but when attempting to create lock when reading I get a NonWritableChannelException. Is it even possible to lock a file for reading? Seems like a RandomAccessFile is closer to what I need, but I don't see how to implement that.

Here is the code that fails:

FileInputStream fin = new FileInputStream(f);
FileLock fl = fin.getChannel().tryLock();
if(fl != null) 
{
  System.out.println("Locked File");
  BufferedReader in = new BufferedReader(new InputStreamReader(fin));
  System.out.println(in.readLine());
          ...

The exception is thrown on the FileLock line.

java.nio.channels.NonWritableChannelException
 at sun.nio.ch.FileChannelImpl.tryLock(Unknown Source)
 at java.nio.channels.FileChannel.tryLock(Unknown Source)
 at Mover.run(Mover.java:74)
 at java.lang.Thread.run(Unknown Source)

Looking at the JavaDocs, it says

Unchecked exception thrown when an attempt is made to write to a channel that was not originally opened for writing.

But I don't necessarily need to write to it. When I try creating a FileOutpuStream, etc. for writing purposes it is happy until I try to open a FileInputStream on the same file.

Answer 1

(a) you are aware that locking the file won't keep other processes from touching it unless they also use locks? (b) You have to lock via a writable channel. Get the lock via a RandomAccessFile in "rw" mode and then open your FileInputStream. Make sure to close both!

Answer 2

It would be better if you created the lock using tryLock(0L, Long.MAX_VALUE, true).

This creates a shared lock which is the right thing to do for reading.

tryLock() is a shorthand for tryLock(0L, Long.MAX_VALUE, false), i.e. it requests an exclusive write-lock.