How to make java.util.ArrayList threadsafe

Question

I have logic similar to below in my program which throws ConcurrentModificationException. How can I make java.util.ArrayList threadsafe other than using CopyOnWriteArrayList (because I have large number of write on this list and as per my understanding writes are expensive on CopyOnWriteArrayList)

import java.util.ArrayList;
import java.util.List;

public class Test extends Thread {

    List<String> list = new ArrayList<String>();

    public static void main(String[] args) {
        Test t = new Test();
        t.start();

        System.out.println("in main thred");

        t.destory();

    }

    public void destory() {
        list.stream().toArray(String[]::new);
        System.out.println("done");

    }

    @Override
    public void run() {
        while(true) {
            list.add("test");
        }

    }
}

Output:

in main thred
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1388)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
    at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
    at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)

https://stackoverflow.com/questions/2444005/how-do-i-make-my-arraylist-thread-safe-another-approach-to-problem-in-java — Sotirios Delimanolis, May 14 '20 at 17:14
https://stackoverflow.com/questions/38845778/thread-safety-when-iterating-through-an-arraylist-using-foreach — Sotirios Delimanolis, May 14 '20 at 17:15
Does this answer your question? [How do I make my ArrayList Thread-Safe? Another approach to problem in Java?](https://stackoverflow.com/questions/2444005/how-do-i-make-my-arraylist-thread-safe-another-approach-to-problem-in-java) — Dracontis, May 14 '20 at 18:48

score 1 · Answer 1 · answered May 14 '20 at 17:09

1

You have to synchronize all accesses to the list:

public void destory() {
    synchronized (list) {
      list.stream().toArray(String[]::new);
    }
    System.out.println("done");

}

and

    while(true) {
      synchronized (list) {
        list.add("test");
      }
    }

Note that it is not sufficient simply to wrap the list in Collections.synchronizedList, since that won't hold the monitor for the entirety of the iteration by the stream.

answered May 14 '20 at 17:09

Andy Turner

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2

But there is no need for using the Stream API here. When replacing the `list.stream().toArray(String[]::new);` with `list.toArray(new String[0]);`, using `synchronizedList` would work. – Holger May 15 '20 at 12:26

How to make java.util.ArrayList threadsafe

1 Answers1