Question

I have a stateful EJB which calls an EJB stateless method of Web parsing pages.

Here is my stateful code :

@Override
public void parse() {
    while(true) {
        if(false == _activeMode) {
            break;
        }
        for(String url : _urls){
            if(false == _activeMode) {
                break;
            }
            for(String prioritaryUrl : _prioritaryUrls) {
                if(false == _activeMode)
                    break;
                boursoramaStateless.parseUrl(prioritaryUrl);
            }

            boursoramaStateless.parseUrl(url);
        }
    }
}

No problem here.

I have some asynchronously call (with JMS) that add to my _urls variable (a List) some value. Goal is to parse new url inside my infinity loop.

I receive ConcurrentModificationException when I try to add new url in my List via JMS onMessage method but it seems to be working because this new url is parsed.

When I try to wrap a synchronized block :

while(true){
    synchronized(_url){
        // code...
    }
}

My new url is never parsed, I expected to be parsed after a for() loop finished…

So my question is : how can I modify List when it’s accessed inside a loop without having ConcurrentModificationException please ?

I just want 2 threads to modify some shared resource at same time without synchronized block…

Answer 1

You may want a CopyOnWriteArrayList.