Why my cyclicBarrier is null?

Question

Im learnign about ciclycbarrier and im trying to create a little app. The constructor of my app is the follow:

public FileDownloader(String line, int maxDownload){
    this.position = 0;
    this.line = line;
    this.maxDownload = maxDownload;
    this.urls = new ArrayList<String>();
    this.ths = new ArrayList<Thread>();
    this.exm = new Semaphore(1);
    this.GenerateURLS();
    final CyclicBarrier cb = new CyclicBarrier(this.maxDownload, new Runnable(){
        @Override
        public void run(){


            System.out.println("All download are finished");
            //Mergear cuando se termina
            //Borrar cuando se termina
        }

    });

    for(int i=0; i<this.maxDownload;i++){
        ths.add(new Thread(new Task(this.cb),"Hilo"+i));
    }
    for(Thread th: ths){
        th.start();
    }

}

In the constructor i create my Cyclicbarrier, setting a maxDownload number and a new Runnable. After That, y create all my thread setting a Task(setting the cyclicbarrier. The tasks implements Runnable). The code of my task is the follow:

class Task implements Runnable{
    private CyclicBarrier barrier;
    public static int position;
    public Task(CyclicBarrier cb){
        this.barrier = cb;

    }

    public void run(){
        try {

            FileDownloader.DownloadManager();
            this.barrier.await();
        } catch (InterruptedException | BrokenBarrierException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            System.out.println(e.getStackTrace());
        }
    }
}

But the problem is when the method DownloadFile(Inside Run of my task) ends, and its time to do cb.await, i have the next error:

Exception in thread "Hilo1" java.lang.NullPointerException
    at Prac2.Task.run(FileDownloader.java:23)
    at java.lang.Thread.run(Thread.java:745)

and debugging i can see that the cyclicbarrier(barrier) in my task is always null, but the cb not.

what can be the problem?

Answer 1

Look closely on your code.

You create local variable cb.

 final CyclicBarrier cb = new CyclicBarrier(this.maxDownload, new Runnable(){
    @Override
    public void run(){
        System.out.println("All download are finished");
        //Mergear cuando se termina
        //Borrar cuando se termina
    }

});

But here you access class-level variable.

for(int i=0; i<this.maxDownload;i++){
    ths.add(new Thread(new Task(this.cb),"Hilo"+i));
}

I mean following:

this.cb

Be attentive.