Copying files using apache fileutil.copyfile

Question

I am using apache util to copy a file to a directory using the fileutil.copyFile(src, dest)

The file I am copying is updated every 2 seconds by an external vendor and I really do not want to lock it (my application is running in windows- this will cause all sorts of problems). I am hoping someone might be able to help me in advising what is the safest way to copy or even read a file without the source file getting locked?

Kind regards

Answer 1

Because you are not explicitly locking the entire file before the copy action the default os file locking mechanism is at work.

I ran a quick test program to see what happens on a Windows machine when you copy a source file while an external process is writing to the file every 2secs.

The process that writes to the file never encountered a problem.

public static void main(String[] args) {
    File f = new File("..\\test.txt");
    long startTime = System.currentTimeMillis();
    long elapsedTime = 0;
    while (elapsedTime < 1000 * 60) {
        try {
            FileUtils.writeStringToFile(f, System.currentTimeMillis()+" : Data Write\r\n", true);
            Thread.sleep(2000);
        } catch (IOException ex) {
            Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
        } catch (InterruptedException ex){
            Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
        }
        elapsedTime = System.currentTimeMillis() - startTime;
    }
}

The process that copies the file will throw an exception if it does not finish the copy before the source file changes length. It appears that this exception is more of a warning that the copied version of the file is incomplete. When I synced timing to keep from reading from the file at the same time writing happens this exception was not thrown.

public static void main(String[] args) {
    File f = new File("..\\test.txt");
    long startTime = System.currentTimeMillis();
    long elapsedTime = 0;
    while (elapsedTime < 1000 * 60) {
        try {
            FileUtils.copyFile(f, new File("..\\test\\test.txt"));
            Thread.sleep(2000);
        } catch (IOException ex) {
            Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
        } catch (InterruptedException ex){
            Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
        }
        elapsedTime = System.currentTimeMillis() - startTime;
    }
}

Based on this test I would not worry about what happens to the writing process. I would do something to handle the case when java.io.IOException: Failed to copy full contents from '..\test.txt' to '..\test\test.txt' is thrown.