What's wrong with my usage of ReentrantReadWriteLock?

Question

I have a trouble with ReentrantReadWriteLock. Thread hangs when I trying to wipe file. I have a scheduled read operation and eventual write operation(when user presses the button) which use one instance of ReentrantReadWriteLock. Next code looks unsuitable for use, sorry, I put everything in one place for simplicity.

public class FileDB {
    private static final String ORDERS_FILENAME = "orders.tsv";

    private ReadWriteLock ordersLock;

    private FileDB() {
        ordersLock = new ReentrantReadWriteLock();

        // Swing Timer
        ordersTimer = new Timer(0, (ActionEvent e) -> {
            readFileSplittedByTab("orders.tsv", 5, ordersLock);
        });

        ordersTimer.setDelay(5 * 1000); // 5 sec
        ordersTimer.start();
    }

    private List<String[]> readFileSplittedByTab(String filePath,
            int columns, ReadWriteLock lock) {
        lock.readLock().lock();

        File file = new File(filePath);

        // if file is absent or empty return empty list
        if (!file.exists() || file.length() == 0)
            return new ArrayList<String[]>();

        List<String> lines = null;

        try {
            lines = Files.readAllLines(Paths.get(file.getAbsolutePath()));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            lock.readLock().unlock();
        }

        List<String[]> splittedFile = new ArrayList<>();
        String[] parts = null;

        for (String line : lines) {
            parts = line.split("\t");

            if (parts.length != columns) // skip bad string
                continue;

            splittedFile.add(parts);
        }

        return splittedFile;
    }

    private void wipeFile(String filePath, ReadWriteLock lock) {  
        PrintWriter printWriter = null;

        try {
            lock.writeLock().lock();
            Files.newBufferedWriter(Paths.get(filePath), StandardOpenOption.TRUNCATE_EXISTING).close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            lock.writeLock().unlock();
        }
    }
}

And write operation like that:

wipeFile(ORDERS_FILENAME, ordersLock);

When wipeFile() method fires for the first time everything is working. But starting from the second attempt it hangs on lock.writeLock().lock(); I tried to call wipeFile() method from another thread because guys wrote that write lock should not be used in the one thread with read lock.

Executors.newSingleThreadExecutor().execute(() -> {
                    wipeFile(ORDERS_FILENAME, ordersLock);
});

But it does not help, another thread hangs too.

So, the question is what's wrong with my usage of ReentrantReadWriteLock?

score 2 · Accepted Answer · answered Apr 24 '18 at 20:24

What is wrong is that you never release the readlock after the file is wiped (deleted or file length equal 0):

    lock.readLock().lock();

    File file = new File(filePath);

    // if file is absent or empty return empty list
    if (!file.exists() || file.length() == 0) {
        // lock.readLock().unlock(); // this line is missing
        return new ArrayList<String[]>();
    }

What's wrong with my usage of ReentrantReadWriteLock?

1 Answers1