/**
* Acquires a lock on the file. Does nothing if the correct lock has already been acquired.
* Otherwise, releases an existing lock.
*
* @param shared shared/exclusive lock
* @return success flag
* @throws IOException I/O exception
*/
private boolean lck(final boolean shared) throws IOException {
if (fl != null && shared == fl.isShared()) return true;
if (fl != null) fl.release();
fl = file.getChannel().tryLock(0, Long.MAX_VALUE, shared);
return fl != null;
}