Concurrent requests via Apache Http Client (and java.util.concurrent) seem to finish correctly but crash my OS

Question

i am currently learning to use the the concurrent features of Java provided by the package java.util.concurrent. As an exercise i tried to write a little program that could be used to performance test a HTTP API. But somehow my program is not terminating correctly very often. It even crashes my OS.

Following is the pseudo code of my program :

• Instantiate Request Objects, that query an HTTP API (In the example i just query one random site).
• Instantiate multiple Callables, where each one represents a represents an Http Call.
• Iterate over the Callables and schedule them via a ScheduledExecutorService (how many requests should be performed per second can be configured at the begin of the code).
• After scheduling all Callables, i am beginning to iterate over the Futures. If a futures is done, retrieve the response. Do this every second. If no new Future was finished, quit the loop.

What problems am i experiencing in detail?

• Lots of times, the program is not finishing correctly. I see all log prints in the console, as if the program is finishing correctly. But actually i am seeing that stop button in eclipse still remains active . If i click it, it says that the program could not be terminated correctly. It does not finish no matter how i long i wait (NOTE: I am starting the program inside eclipse).
• I can provoke the error easily if i am increasing the number of Requests. If am turning up to 2000, this will happen for sure. If it happens my OS even crashes, i can still use eclipse, but other apps do not work anymore.
• My Environment is Eclipse 3.7 on Mac OS X 10.7 with Java 1.6 and Apache httpclient 4.2.2

Do you spot any major erros in my code? Before i have never had such issues in a java program with crashing my OS and seeing no exceptions at all.

The code:

public class ConcurrentHttpRequestsTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(25);

        Integer standardTimeout = 5000;
        Float numberOfRequestsPerSecond = 50.0f;
        Integer numberOfRequests = 500;
        Integer durationBetweenRequests = Math.round(1000 / numberOfRequestsPerSecond);

        // build Http Request
        HttpGet request = null;
        request = new HttpGet("http://www.spiegel.de");
        // request.addHeader("Accept", "application/json");
        HttpParams params = new BasicHttpParams();
        HttpConnectionParams.setConnectionTimeout(params, standardTimeout);
        HttpConnectionParams.setSoTimeout(params, standardTimeout);
        request.setParams(params);

        // setup concurrency logic
        Collection<Callable<Long>> callables = new LinkedList<Callable<Long>>();
        for (int i = 1; i <= numberOfRequests; i++) {
            HttpClient client = new DefaultHttpClient();
            callables.add(new UriCallable(request, client));
        }

        // start performing requests
        int i = 1;
        Collection<Future<Long>> futures = new LinkedList<Future<Long>>();
        for (Callable<Long> callable : callables) {
            ScheduledFuture<Long> future = scheduledExecutorService.schedule(callable, i * durationBetweenRequests, TimeUnit.MILLISECONDS);
            futures.add(future);
            i++;
        }

        // process futures (check wether they are ready yet)
        Integer maximumNoChangeCount = 5;
        boolean futuresAreReady = false;
        int noChangeCount = 0;
        int errorCount = 0;
        List<Long> responses = new LinkedList<Long>();
        while (!futuresAreReady) {
            boolean allFuturesAreDone = true;
            boolean atLeast1FutureIsDone = false;
            Iterator<Future<Long>> iterator = futures.iterator();

            while (iterator.hasNext()) {
                Future<Long> future = iterator.next();
                allFuturesAreDone = allFuturesAreDone && (future.isDone());
                if (future.isDone()) {
                    try {
                        atLeast1FutureIsDone = true;
                        responses.add(future.get());
                        iterator.remove();
                    } catch (Exception e) {
                        // remove failed futures (e.g. timeout)
                        // System.out.println("Reached catch of future.get()" +
                        // e.getClass() + " " + e.getCause().getClass() + " " +
                        // e.getMessage());

                        iterator.remove();
                        errorCount++;
                    }
                }
                if (future.isCancelled()) {
                    // this code is never reached. Just here to make sure that
                    // this is not the cause of problems.
                    System.out.println("Found a cancelled future. Will remove it.");
                    iterator.remove();
                }
            }
            if (!atLeast1FutureIsDone) {
                System.out.println("At least 1 future was not done. Current noChangeCount:" + noChangeCount);
                noChangeCount++;
            } else {
                // reset noChangeCount
                noChangeCount = 0;
            }
            futuresAreReady = allFuturesAreDone;

            // log the current state of responses, errors and remaining futures
            System.out.println("Size of responses :" + responses.size() + "; Size of futures:" + futures.size() + " Errors:" + errorCount);
            if (noChangeCount >= maximumNoChangeCount) {
                System.out.println("Breaking while loop becauce no new future finished in the last " + maximumNoChangeCount + " iterations");
                break;
            }
            // check every second
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        for (Long responsetime : responses) {
            // analyze responsetimes or whatever
        }
        // clean up
        // .shutdown() made even more problems than shutdownNow()
        scheduledExecutorService.shutdownNow();
        System.out.println("Executors have been shutdown - Main Method finished. Will exit System.");
        System.out.flush();
        System.exit(0);
    }

    private static class UriCallable implements Callable<Long> {
        private HttpUriRequest request;
        private HttpClient client;

        public UriCallable(HttpUriRequest request, HttpClient client) {
            super();
            this.request = request;
            this.client = client;
        }

        public Long call() throws Exception {
            Long start = System.currentTimeMillis();
            HttpResponse httpResponse = client.execute(request);
            Long end = System.currentTimeMillis();
            return end - start;
        }
    }

}

Answer 1

Never do this in a loop:

        } catch (InterruptedException e) {
            e.printStackTrace();
        }

It might cause problems on shutdown.

Also, most of your code could be replaced by a single call to ExecutorService.invokeAll() , so try that and see if you have more luck.

Lastly, when you don't know what your Java application is doing, run jconsole , attach to the application, and look at the thread stacks to see what code is currently in progress.