Question

I am developing a program that can send http requests to fetch documents.
I have fill a queue with all the requests items:

Queue<RequestItem> requestItems = buildRequest4Docs();

Then,

int threadNum = requestItems.size();
        //ExecutorService exs = Executors.newFixedThreadPool(threadNum);

        for (int i = 0; i < threadNum; i++) {
            ResponseInterface response = new CMSGOResponse();
            RequestTask task = new RequestTask(requestItems.poll(), this, response);
            task.run();
            //exs.execute(new RequestTask(requestItems.poll(), this, response));
        }
        //exs.shutdown();

I am confused here, in the for loop,does the tasks run simultaneously？ Or the tasks run one by one?

Thanks!

Answer 1

In the way you got it now the tasks will be executed one by one. If you uncomment the code you got now as comments and comment the lines RequestTask task = new RequestTask(requestItems.poll(), this, response); and task.run(); you will get a concurrent execution.

So for the concurrent execution it has to look like this:

int threadNum = requestItems.size();
ExecutorService exs = Executors.newFixedThreadPool(threadNum);

for (int i = 0; i < threadNum; i++) {
    ResponseInterface response = new CMSGOResponse();
    exs.execute(new RequestTask(requestItems.poll(), this, response));
}
exs.shutdown();
while (! exs.isTerminated()) {
    try {
        exs.awaitTermination(1L, TimeUnit.DAYS);
    }
    catch (InterruptedException e) {
        // you may or may not care here, but if you truly want to
        // wait for the pool to shutdown, just ignore the exception
        // otherwise you'll have to deal with the exception and
        // make a decision to drop out of the loop or something else.
    }
}

In addition to that I suggest, that you do not bind the amount of threads created with the ExecutorService to the amount of task you got to work. Connecting it to the amount of processors of the host system is usually a better method. To get the amount of processors use: Runtime.getRuntime().availableProcessors()

And in the executor service initialized like this you put the items of your queue. But that works nicely without fetching the total size, rather by polling the Queue until it does not return additional data.

The final result of my proposals could look like this:

final int threadNum = Runtime.getRuntime().availableProcessors();
final ExecutorService exs = Executors.newFixedThreadPool(threadNum);

while (true) {
    final RequestItem requestItem = requestItems.poll();
    if (requestItem == null) {
        break;
    }
    final ResponseInterface response = new CMSGOResponse(); 
    exs.execute(new RequestTask(requestItem , this, response));
}
exs.shutdown();