Lock a file and read it

Question

This is a learning task. I want multiple threads to read files in a directory. If a thread finds an unoccupied file, it reads the file. Otherwise the thread will be terminated.

I'm trying to lock the file for reading and then read it symbol by symbol.

But in the readFile method I get IOException. How can I solve the problem?

This is my code:

private void burden() {

        File mainFolder = new File("C:\\FilesToRead");
        File[] files = mainFolder.listFiles();
        String freeFile;
        boolean jobExists = false;

        for (File file : files) {

            FileChannel channel;
            FileLock lock = null;
            try {
                channel = new RandomAccessFile(file, "rw").getChannel();;
                lock = channel.lock();
                // Ok. We get the lock
                String fl = file.getAbsolutePath();
                readFile(file);
                System.out.println(fl + " is captured.");
                System.out.println("Reading " + file);
            } catch (OverlappingFileLockException e) {
                continue; // File is open by someone else                 
            } catch (FileNotFoundException f) {

            } catch (IOException ex) {

            } catch (NonWritableChannelException n) {
                System.out.println("NonWritableChannelException");
            } finally {
                try {
                    lock.release();
                    System.out.println(file.getName() + " is released");
                } catch (IOException ex) {
                    System.out.println("IOException!");
                }
            }
        }
    } // burden();

    private void readFile(File file) {

        System.out.println("Reading " + file);

        FileReader inputStream = null;
        ArrayList<Integer> list = new ArrayList<>();
        try {
            int c;
            inputStream = new java.io.FileReader(file);

            while ((c = inputStream.read()) != -1) {
                if (Thread.currentThread().isInterrupted()) {
                    return;
                }
                list.add(c);
            }
            
        } catch (IOException e) {
            System.out.println("IOException!");
        } finally {
            if (inputStream != null) {
                try {
                    inputStream.close();
                } catch (IOException ex) {
                    System.out.println("IOException!");
                }
            }
        }

    }

This is the picture (I translated the error message into English):

Added later: this is what I get in the debugger:

at java.io.FileInputStream.readBytes(Native Method)

at java.io.FileInputStream.read(FileInputStream.java:272)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:283)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:325)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:177)
at sun.nio.cs.StreamDecoder.read0(StreamDecoder.java:126)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:112)
at java.io.InputStreamReader.read(InputStreamReader.java:168)
at parallelprogramming.Thrd.readFile(Thrd.java:82)
at parallelprogramming.Thrd.burden(Thrd.java:50)
at parallelprogramming.Thrd.run(Thrd.java:23)
at parallelprogramming.HotThrd.run(HotThrd.java:6)
at parallelprogramming.Thrd.<init>(Thrd.java:18)
at parallelprogramming.HotThrd.<init>(HotThrd.java:3)
at parallelprogramming.ThrdPool.addHotThrd(ThrdPool.java:40)
at parallelprogramming.ThrdPool.<init>(ThrdPool.java:29)
at parallelprogramming.ParallelProgramming.main(ParallelProgramming.java:18)

Answer 1

From the Javadoc

File locks are held on behalf of the entire Java virtual machine. They are not suitable for controlling access to a file by multiple threads within the same virtual machine.

Emphasis mine.