[FIXED] Java FileLock for Reading and Writing

Issue

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.

Solution

1. Are you aware that locking the file won't keep other processes from touching it unless they also use locks?
2. 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!

Answered By - user207421
Answer Checked By - Senaida (JavaFixing Volunteer)