Question

Works except for when I free the crawler:

public void setCrawlerFree(WebCrawler w)
    {
        synchronized(myFreeCrawlers)
        {
            synchronized(numToGo)
            {
                myFreeCrawlers.add(w);
                myFreeCrawlers.notifyAll();
                numToGo--;
                numToGo.notify();
            }
        }
    }

When the crawler is done, I can add it back on the list. I also want to subtract 1 from the number of things I still need to do. I have one main thread waiting until numToGo is at 0. I’m getting an IllegalMonitorStateException on numToGo.notify() but since its inside of the synchronization block, doesn’t that mean I own it?

Answer 1

Consider rewriting it to ExecutorService.

ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, 
     maximumPoolSize, keepAliveTime, timeUnit, 
     new LinkedBlockingQueue<Runnable>());
executor.submit(new Callable<...>() { ... });

It would greatly simplify your code and eliminate thread synchronization issues.