File locking and delete

Question

I'm making a program in java that monitors and backup a directory. From time to time I have to upload modified files to the repository or download if there is a new version of it. In order to do this I have to lock the file so that the user is unable to change the contents or delete it. Currently I'm using this code to lock the file:

        file = new RandomAccessFile("C:\\Temp\\report.txt", "rw");

        FileChannel fileChannel = file.getChannel();
        fileLock = fileChannel.tryLock();
        if (fileLock != null) {
            System.out.println("File is locked");

            try{

            //Do what i need    

            }catch (Exception e){//Catch exception if any
                System.err.println("Error: " + e.getMessage());
            }
        }
        else{
            System.out.println("Failed");
        }
    } catch (FileNotFoundException e) {
        System.out.println("Failed");
    }finally{
        if (fileLock != null){
            fileLock.release();
        }

However if there is a new version I have to delete the old file and replace with new one. But File lock does not allow me to delete the file.

Should I unlock and delete it write away, trusting that the user wont write in file? Or is there any other way of doing this?

Solution

You could truncate the file:

fileChannel.truncate(0);

and afterwards write the new version over it, this wouldn't create the time gap in which the user can create the file again.

From documentation:

If the given size is less than the file's current size then the file is truncated, discarding any bytes beyond the new end of the file. If the given size is greater than or equal to the file's current size then the file is not modified. In either case, if this channel's file position is greater than the given size then it is set to that size.

http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#truncate%28long%29

Comments

[user1308768]

Unless I'm missing the big picture, that only erases the contents of file. So that's not what i needed.

[Francisco Spaeth]

Should I unlock and delete it write away, trusting that the user wont write in file? Or is there any other way of doing this? I answered with truncate that you can use after the file is locked to clear the content and write the new content over it, so you don't have the time gap between delete the file and create the new one based on the file to be synched.

[Francisco Spaeth]

I just add some additional explanation... please let me know if you need more clarification.

[user1308768]

ah ok. It's indeed a solution. What i had in mind it was the move operation (docs.oracle.com/javase/7/docs/api/java/nio/file/…), since it is faster than copy the content of file. But until now what you it's the best solution