CODEWITHSUNDEEP
javamultithreadingblockingqueue
Rasmus Hjorth Lüdeking
Rasmus Hjorth Lüdeking

Reputation: 770

LinkedBlockingQueue only return one of mupltiple threads

I've made a class that counts words in given files within the same directory. Seeing as the files are very large, I've decided to achieve the count of multiple files using multiple threads.

When running the DriverClass as specified below, it get's stuck at thread one. What am I doing wrong? As I'm iterating over queue.take(), one would expect the parser to wait for something to retrieve and move on. Getting stuck at thread 1 makes me suspect an error when putting() into the queue.

Thank's, in advance!

DriverClass:

public class WordCountTest {
    public static void main(String[] args){
        if (args.length<1){
            System.out.println("Please specify, atleast, one file");
        }
        BlockingQueue<Integer> threadQueue = new LinkedBlockingQueue<>();
        Runnable r;
        Thread t;

        for (int i = 0; i<args.length; i++){
            r = new WordCount(args[i], threadQueue);
            t = new Thread(r);
            t.start();

            int total = 0;
            for (int k = 0; k<args.length; k++){
                try {
                    total += threadQueue.take();
                } catch (InterruptedException e){
                }
            }
            System.out.println("Total wordcount: " + total);
        }
    }
}

WordCountClass: 

public class WordCount implements Runnable {
    private int myId = 0;
    private String _file;
    private BlockingQueue<Integer> _queue;
    private static int id = 0;

    public WordCount(String file, BlockingQueue<Integer> queue){
        _queue = queue;
        _file = file;
        myId = ++id;
    }

    @Override
    public void run() {
        System.out.println("Thread " + myId + " running");
        try {
            _queue.put(countWord(_file));
        } catch (InterruptedException e){

        }
    }

    public int countWord(String file){
        int count = 0;
        try {
            Scanner in = new Scanner(new FileReader(file));
            while (in.hasNext()){
                count++;
                in.next();
            }
        } catch (IOException e){
            System.out.println("File," + file + ",not found");
        }
        return count;
    }
}

Upvotes: 1

Views: 60

Answers (2)

Zim-Zam O&#39;Pootertoot
Zim-Zam O&#39;Pootertoot

Reputation: 18148

The problem is that you're using a nested loop, when you should be using two separate loops: one to start the WordCounts, another to collect the results, something like

public class WordCountTest {
    public static void main(String[] args){
        Queue<Integer> threadQueue = new ConcurrentLinkedQueue<>();
        ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

        CountDownLatch latch = new CountDownLatch(args.length);

        for (int i = 0; i<args.length; i++){
            CompletableFuture.runAsync(new WordCount(args[i], threadQueue), executor)
                .thenRunAsync(latch.countDown(), executor);
        }

        latch.await();

        int sum = 0;
        for(Integer i : threadQueue) {
            sum += i;
        }
    }
}

Or however you want to implement it, the point being that you shouldn't start collecting results until all of the WordCounts have started.

Upvotes: 1

Peter Lawrey
Peter Lawrey

Reputation: 533530

You are waiting for all the results after the first thread is started. Perhaps you intended to wait for the results after all the threads have started.

Note: if you create more threads than you have CPUs its likely to be slower. I suggest using a fixed thread pool instead.

Upvotes: 1

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