executors.java

//Helper Class for read and write operations for a file.

class FileStore {

    private File storeFolder;

    // Locks is used so that only one thread can update the file at a
    // time.
    private ReadWriteLock lock = new ReentrantReadWriteLock();

    /**
     * Construct an FileStore Store
     * @param storeFolder This folder will be used to store the object data.
     */
    FileBasedObjectStore(Path storeFolder) {
        this.storeFolder = storeFolder.toFile();
    }

    /**
     * Reads the file from the file store.
     *
     * @param id - the ID of the object
     * @return the object if it exists or empty if it does not.
     */
    @Override
    public Optional<Object> read(UUID id) {
        lock.readLock().lock();

        File file = new File(storeFolder, id.toString());

        Optional<Object> result;

        try (
            FileInputStream fileIn = new FileInputStream(file);
            ObjectInputStream in = new ObjectInputStream(fileIn)
        ) {
            result = Optional.ofNullable(in.readObject());
        } catch ( IOException | ClassNotFoundException ex) {
            result = Optional.empty();
        } finally {
            lock.readLock().unlock();
        }

        return result;
    }

    /**
     * Writes an object to a file in the file store.
     *
     * @param id - the id of the object being written.
     * @param obj - the object to be written.
     * @return true if the write was successful, false if it failed.
     */
    @Override
    public boolean write(UUID id, Object content) {
        // lock before we can write i file
        lock.writeLock().lock();

        File file = new File(storeFolder, id.toString());

        boolean result;

        try (
            FileOutputStream fileOut = new FileOutputStream(file);
            ObjectOutputStream out = new ObjectOutputStream(fileOut)
        ) {
            out.writeObject(content);
            result = true;
        } catch (IOException ex) {
            result = false;
        } finally {
            lock.writeLock().unlock();
        }

        return result;
    }
}