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.
File locks are held on behalf of the entire Java virtual machine. They are not suitable for controlling access to a file by multiple threads within the same virtual machine. File-lock objects are safe for use by multiple concurrent threads.
In Java, a file lock can be obtained using FileChannel , which provides two methods — lock() and tryLock() — for this purpose. The lock() method acquires an exclusive lock on entire file, whereas the lock(long position, long size, boolean shared) method can be used to acquire a lock on the given region of a ile.
You can run from Java program the lsof Unix utility that tells you which process is using a file, then analyse its output. To run a program from Java code, use, for example, Runtime , Process , ProcessBuilder classes.
RandomAccessFile
in "rw" mode and then open your FileInputStream
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