CopyOnWriteArrayList concurrent access problem on removing

Question

I am struggling to find how to remove an Element of a CopyOnWriteAccess without getting a thread exception.

Exception in thread "Thread-3649" java.lang.ArrayIndexOutOfBoundsException: Index 4 out of bounds for length 4
    at java.base/java.util.concurrent.CopyOnWriteArrayList.elementAt(CopyOnWriteArrayList.java:386)
    at java.base/java.util.concurrent.CopyOnWriteArrayList.remove(CopyOnWriteArrayList.java:479)
    at com.mycompany.playmatehunter.Downloader.init(Downloader.java:28)
    at com.mycompany.playmatehunter.Downloader.run(Downloader.java:19)
    at java.base/java.lang.Thread.run(Thread.java:834)

I have the following Downloader class

public class Downloader extends Thread{
    private CopyOnWriteArrayList<String> url_list;
    private String chemin;

    public Downloader( CopyOnWriteArrayList<String> l, String chemin)
    {
        this.url_list = l;
        this.chemin = chemin;
    }

    public void run()
    {
        init();   
    }

    public synchronized void init() // the list is already synchronized
    {
        if (!url_list.isEmpty())
        {
            int alea = (int)(Math.random()*url_list.size());
            System.out.println(this.getName()+"removes the link nbr "+url_list.get(alea));
            url_list.remove(alea);
        }

    }


}

And inside the main :

CopyOnWriteArrayList<String> index = new CopyOnWriteArrayList( FileToList("/index/index.txt") );

        while( index.size() != 0)
        {
            List<Thread> tg = new ArrayList<Thread>();            
            for(int i=0;i<7;i++)
            {
                Thread t=new Thread(new Downloader(index, ""));
                t.start();
                tg.add(t);
            }

            for(Thread t: tg) 
                t.join();
        }

Do you know how to get rid of ThreadException? Thank you

Answer 1

Your access to the list is not synchronized. You have multiple threads, and even though the init() method is synchronized, the synchronization is on the thread instance, not on a common object, so it is useless. If you need to ensure mutual exclusion among threads, you have to synchronize on a common object:

 public void init() // No synchronization here
    {
       synchronized(url_list) { // synchronize on a common object
        if (!url_list.isEmpty())
        {
            int alea = (int)(Math.random()*url_list.size());
            System.out.println(this.getName()+"removes the link nbr "+url_list.get(alea));
            url_list.remove(alea);
        }
      }

    }