Java - locking a file for exclusive access

Question

My problem is this: I'm using a WatchService to get notified about new files in a specific folder, now if a file gets moved/copied or created in said folder an event gets triggered and the name of the new file gets returned. The problem now is, if I try to access the file and it is not fully there yet (e.g. the copy is still in progress) an exception gets raised. What i tried was to do something like this:

RandomAccessFile raf = new RandomAccessFile(fileName, "rw"); FileChannel fc = raf.getChannel(); FileLock lck = fc.lock();

But even if a lock gets acquired, sometimes still an Exception gets raised if I try to write to the file because another process has still an open handle to it.

Now, how can a file in Java be locked for truly exclusive access?

sethu · Accepted Answer · 2011-12-30 10:24:53Z

For me, the statement

RandomAccessFile raf = new RandomAccessFile(fileName, "rw");

returns a FileNotFoundException if I cannot acquire a lock on the file. I catch the filenotfound exception and treat it...

public static boolean isFileLocked(String filename) { boolean isLocked=false; RandomAccessFile fos=null; try { File file = new File(filename); if(file.exists()) { fos=new RandomAccessFile(file,"rw"); } } catch (FileNotFoundException e) { isLocked=true; }catch (Exception e) { // handle exception }finally { try { if(fos!=null) { fos.close(); } }catch(Exception e) { //handle exception } } return isLocked; }

you could run this in a loop and wait until you get a lock on the file. Wouldn't the line

FileChannel fc = raf.getChannel();

never reach if the file is locked? You will get a FileNotFoundException thrown..

TanvirChowdhury · Accepted Answer · 2020-11-22 05:50:31Z

its better not to use classes in thejava.io package, instead use the java.nio package . The latter has a FileLock class that you can use for a lock to a FileChannel.

try { // Get a file channel for the file File file = new File("filename"); FileChannel channel = new RandomAccessFile(file, "rw").getChannel(); // Use the file channel to create a lock on the file. // This method blocks until it can retrieve the lock. FileLock lock = channel.lock(); /* use channel.lock OR channel.tryLock(); */ // Try acquiring the lock without blocking. This method returns // null or throws an exception if the file is already locked. try { lock = channel.tryLock(); } catch (OverlappingFileLockException e) { // File is already locked in this thread or virtual machine } // Release the lock - if it is not null! if( lock != null ) { lock.release(); } // Close the file channel.close(); } catch (Exception e) { }

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