FULL PRODUCT VERSION :


A DESCRIPTION OF THE PROBLEM :
When a FileLock obtained from a FileChannel gets GC'd (garbage collect'd) the underlying file lock is released. This behavior is very surprising. Since a developer normally wouldn't expect the need to maintain a reference to the obtained FileLock object himself/herself and may easily assume the reference is maintained automatically in the FileChannel object.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run the code attached. See code-level documentation for more detail.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Throw an OverlappingFileLockException.
ACTUAL -
File lock released unexpectedly. No OverlappingFileLockException thrown.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.File;
import java.io.RandomAccessFile;

public class FileLockBug {

    /**
     * This class demonstrates a bug in which a FileLock object gets GC'd
     * and results in unexpected release of the file lock.
     */
    public static void main(String[] args) throws Exception {
        
        final File f = new File("/tmp/file");

        final RandomAccessFile raf1 = new RandomAccessFile(f, "rw");

        // acquire an exclusive lock
        // the returned FileLock has no reference to it
        // (candidate for GC)
        raf1.getChannel().tryLock();

        // force GC and remove the FileLock object created above
        System.gc();

        final RandomAccessFile raf2 = new RandomAccessFile(f, "rw");

        // expected: an OverlappingFileLockException
        // (since a lock has already been acquired on the same file)
        // actual: lock acquired successfully due to this bug
        raf2.getChannel().tryLock();

        raf2.close();
        raf1.close();
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Always maintain a reference of the FileLock obtained to avoid it being GC'd.

For example:

    FileLock fileLock = randomAccessFile.getChannel().tryLock();

...as opposed to

    randomAccessFile.getChannel().tryLock();