Using Callable in java runs code sequentially

Question

I am making a server in java wanted to perform certain tasks in parallel. I have a list of a certain type I want 20 threads to deal with elements of the list.

for( Item item : result ) {
    Callable< List< IResult > > callable = new HTMLParser( item ); 
    Future< List< IResult > > future = pool.submit( callable );
    set.add( future );
}

for( Future< List< IResult > >  future : set ) {
    List< IResult > result = future.get( ); // wait for a processor to complete
    if( result != null && !result.isEmpty( ) )
        FResults.addAll( result ); 
    }
}

Only running this hiccup, analyzing the logs seems to me to be running the threads sequentially

2016-12-06 14:17:02.106 DEBUG 18229 --- [pool-1-thread-5] ......
2016-12-06 14:17:02.106 DEBUG 18229 --- [pool-1-thread-5] ....
2016-12-06 14:17:02.106 DEBUG 18229 --- [pool-1-thread-5] .....
2016-12-06 14:17:02.106 DEBUG 18229 --- [pool-1-thread-5] .......
2016-12-06 14:17:02.106  INFO 18229 --- [pool-1-thread-5] .....
2016-12-06 14:17:02.243 DEBUG 18229 --- [ool-1-thread-19] .....
2016-12-06 14:17:02.243  INFO 18229 --- [ool-1-thread-19] ....
2016-12-06 14:17:02.587 DEBUG 18229 --- [ool-1-thread-16] .....
2016-12-06 14:17:02.587 DEBUG 18229 --- [ool-1-thread-16] ....
2016-12-06 14:17:02.587 DEBUG 18229 --- [ool-1-thread-16] .....
2016-12-06 14:17:02.587 DEBUG 18229 --- [ool-1-thread-16] .....
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] .....
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] ......
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] .....
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] ....
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] ......
2016-12-06 14:17:02.588 DEBUG 18229 --- [ool-1-thread-16] ...

The logs of a thread appears sequentially all in a row, it doesn’t make sense to me. Any idea ?

Answer 1

In fact, it makes perfect sense:

List< IResult > result = future.get( ); // wait for a processor to complete

You’re calling get in each Future, which is bloating, thus making a sequential execution once the next getwill only be called when the current is finished running.

The API of Futureonly gives you the method isDone, which is not block and will return true if that computation is completed.

Instead of passing the Callableone by one you can use invokeAll:

List<Future<Whatever>> futures = pool.invokeAll(callables);

Then iterate on the calling Futures getwhen you know that it has already completed processing:

for(Future<List<Whatever>> f : futures) {
    if(f.isDone()) {
        f.get() // aqui você faz a mágica acontecer
    }
}

Finally, you don’t need to get stuck to the loop that checks isDone, you can periodically check the result.