Question

I’ve made two apps designed to run concurrently (I do not want to combine them), and one reads from a certain file and the other writes to it. When one or the other are running no errors, however if they are both running a get an access is denied error.
Relevant code of the first:

    class MakeImage implements Runnable {
        @Override
        public void run() {
            File file = new File("C:/Users/jeremy/Desktop/New folder (3)/test.png");
            while (true) {


                try{
//make image
                if(image!=null)
                {
                    file.createNewFile();
                ImageIO.write(image, "png", file);
                hello.repaint();}}
                catch(Exception e)
                {
                    e.printStackTrace();
                }

                }
    }
}   

Relevant code of the second:

            BufferedImage image = null;
            try {
                // Read from a file
                image = ImageIO.read(new File("C:/Users/jeremy/Desktop/New folder (3)/test.png"));
            }
            catch(Exception e){
                e.printStackTrace();
            }
            if(image!=null)
            {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ImageIO.write( image, "png", baos );
            baos.flush();
            byte[] imageInByte = baos.toByteArray();
            baos.close();
            returns=Base64.encodeBase64String(imageInByte);
            }

I looked at this: Java: how to handle two process trying to modify the same file, but that is when both are writting to the file where here only one is. I tried the retry later method as suggested in the former’s answer without any luck. Any help would be greatly appreciated.

Answer 1

Unless you use OS level file locking of some sort and check for the locks you’re not going to be able to reliably do this very easily. A fairly reliable way to manage this would be to use another file in the directory as a semaphore, “touch” it when you’re writing or reading and remove it when you’re done. Check for the existence of the semaphore before accessing the file. Otherwise you will need to use a database of some sort to store the file lock (guaranteed consistency) and check for it there.

That said, you really should just combine this into 1 program.