Question

I need to perform 2 tasks independently.

First Task

Once per minute it should check whether there is any file in a specific folder. If there is, it should add the names of the files to a queue.

This can be done as follows:

import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;


public class schedulerExample extends Thread{

    public void checkFile()
    {
        System.out.println("checking whether file exist in folder");

    }

    public void getFiles()
    {
        System.out.println("getting the file names");
    }

    public void enqueueFiles()
    {
        System.out.println("add files to queue");
    }
    public static void main(String[] args) {

    final schedulerExample obj = new schedulerExample();
    ScheduledExecutorService executor = new ScheduledThreadPoolExecutor(1);
    executor.scheduleAtFixedRate(new Runnable() {

        @Override
        public void run() {

            obj.checkFile();
            obj.getFiles();
            obj.enqueueFiles();

        }
    }, 0, 1, TimeUnit.MINUTES);


    }

}

Second Task

If the queue is empty , sleep for one minute or else process the file from the queue one-by-one.

public class processModel extends Thread{

    public static void getQueueSize(int size)
    {
        System.out.println("getting queue size");

    }
    public void dequeue()
    {

        // dequeue the queue
        System.out.println("dequeue");

    }

    public void processFile()
    {
        // process the file
        System.out.println("process file");
    }

    public static void main(String[] args) {
        final boolean flag = true;
        final int size = 9;
        final processModel obj = new processModel();
        Thread t1 = new Thread(){
            public void run()
            {
                while(flag)
                {
                obj.dequeue();
                obj.processFile();
                getQueueSize(size);
                    if(size == 0)
                    {
                        try
                        {
                        Thread.sleep(60000);
                        }
                        catch(InterruptedException e)
                        {

                        }
                    }
                }

            }

        };

        t1.start();

    }

}

Now I need to do both in a single class concurrently. Is that possible?

One thread should be fetching the files once per minute. Another thread should execute files one-by-one.. if no files are there it waits for a minute and checks again. In the second method I have used an infinite loop — instead of that, is there a way that I can perform things one-by-one?

Answer 1

Synchronize on the queue object when you get file from it and when you add file to it.

In the thread that reads, call wait() if the queue is empty.
In the thread that checks for new files, call notify() after you added the new file to the queue.

This is how it’s usually done.

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You should also prevent adding file that is being processed to the queue.