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 dat

Answer 1

(a) Are you 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

I wrote a test program and bash commands to confirm the effectivity of the file lock:

import java.io.File;
import java.io.RandomAccessFile;
import java.nio.channels.FileLock;

public class FileWriterTest
{
    public static void main(String[] args) throws Exception
    {
        if (args.length != 4)
        {
            System.out.println("Usage: FileWriterTest <filename> <string> <sleep ms> <enable lock>");
            System.exit(1);
        }

        String filename = args[0];
        String data = args[1];
        int sleep = Integer.parseInt(args[2]);
        boolean enableLock = Boolean.parseBoolean(args[3]);

        try (RandomAccessFile raFile = new RandomAccessFile(new File(filename), "rw"))
        {
            FileLock lock = null;
            if (enableLock)
            {
                lock = raFile.getChannel().lock();
            }

            Thread.sleep(sleep);
            raFile.seek(raFile.length());
            System.out.println("writing " + data + " in a new line; current pointer = " + raFile.getFilePointer());
            raFile.write((data+"\n").getBytes());

            if (lock != null)
            {
                lock.release();
            }
        }
    }
}

Run with this bash command to check it works:

for i in {1..1000}
do
java FileWriterTest test.txt $i 10 true &
done

You should see the writing only happening once every 10ms (from the outputs), and in the end all numbers to be present in the file.

Output:

/tmp wc -l test.txt
1000 test.txt
/tmp

The same test without the lock shows data being lost:

for i in {1..1000}
do
java FileWriterTest test.txt $i 10 false &
done

Output:

/tmp wc -l test.txt
764 test.txt
/tmp

It should be easy to modify it to test the tryLock instead.

Answer 3

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.