FuturesB.java Example of using Futures for nested calls showing how it blocks inefficiently.

FuturesB.java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class FuturesB {

    /**
     * Demonstrate how futures can easily become blocking and prevent other work
     * from being performed asynchronously.
     */
    public static void run() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            // get f3 with dependent result from f1
            Future<String> f1 = executor.submit(new CallToRemoteServiceA());
            Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));

            /* The work below can not proceed until f1.get() completes even though there is no dependency */

            // also get f4/f5 after dependency f2 completes 
            Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());
            Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
            Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));

            System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
        } finally {
            executor.shutdownNow();
        }
    }

    /**
     * Demonstrates how reordering of Future.get() can improve the situation
     * but that it still doesn't address differing response latencies of f1 and f2.
     */
    public static void run2() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            // kick of f1/f2 in parallel
            Future<String> f1 = executor.submit(new CallToRemoteServiceA());
            Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());

            // get f3 with dependent result from f1 (blocks)
            Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));

            /* The work below can not proceed until f1.get() completes even if f2.get() is done. */

            // get f4/f5 after dependency f2 completes (blocks)
            Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
            Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));

            System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
        } finally {
            executor.shutdownNow();
        }
    }

    /**
     * Demonstrate how changing where threads are injected can solve the issue of run2()
     * at the cost of incidental complexity being added to the code.
     * <p>
     * This same example could be accomplished by refactoring CallToRemoteServiceC
     * to accept a Future<String> instead of String but the principle is the same.
     */
    public static void run3() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            // kick of f1/f2 in parallel
            final Future<String> f1 = executor.submit(new CallToRemoteServiceA());
            Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());

            // spawn in another thread so waiting on f1 for f3 doesn't block f4/f5
            Future<String> f3 = executor.submit(new Callable<String>() {

                @Override
                public String call() throws Exception {
                    // get f3 with dependent result from f1 (blocks)
                    return new CallToRemoteServiceC(f1.get()).call();
                }

            });

            /* The following can now proceed as soon as f2.get() completes even if f1.get() isn't done yet */

            // get f4/f5 after dependency f2 completes (blocks)
            Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
            Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));

            System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
        } finally {
            executor.shutdownNow();
        }
    }

    /**
     * Demonstrate typical handling of responding to Futures as they complete.
     * <p>
     * This successfully executes multiple calls in parallel but then synchronously handles
     * each response in the order they were put in the list rather than the order they complete.
     */
    public static void run4() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            List<Future<?>> futures = new ArrayList<Future<?>>();

            // kick off several async tasks
            futures.add(executor.submit(new CallToRemoteServiceA()));
            futures.add(executor.submit(new CallToRemoteServiceB()));
            futures.add(executor.submit(new CallToRemoteServiceC("A")));
            futures.add(executor.submit(new CallToRemoteServiceC("B")));
            futures.add(executor.submit(new CallToRemoteServiceC("C")));
            futures.add(executor.submit(new CallToRemoteServiceD(1)));
            futures.add(executor.submit(new CallToRemoteServiceE(2)));
            futures.add(executor.submit(new CallToRemoteServiceE(3)));
            futures.add(executor.submit(new CallToRemoteServiceE(4)));
            futures.add(executor.submit(new CallToRemoteServiceE(5)));

            // as each completes do further work

            for (Future<?> f : futures) {
                /* this blocks so even if other futures in the list complete earlier they will wait until this one is done */
                doMoreWork(f.get());
            }
        } finally {
            executor.shutdownNow();
        }
    }

    private static void doMoreWork(Object s) {
        // do work
        System.out.println("do more work => " + s);
    }

    /**
     * Demonstrate polling approach to handling Futures as they complete.
     * <p>
     * This becomes unwieldy and error prone quickly.
     */
    public static void run5() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            List<Future<?>> futures = new ArrayList<Future<?>>();

            // kick off several async tasks
            futures.add(executor.submit(new CallToRemoteServiceA()));
            futures.add(executor.submit(new CallToRemoteServiceB()));
            futures.add(executor.submit(new CallToRemoteServiceC("A")));
            futures.add(executor.submit(new CallToRemoteServiceC("B")));
            futures.add(executor.submit(new CallToRemoteServiceC("C")));
            futures.add(executor.submit(new CallToRemoteServiceD(1)));
            futures.add(executor.submit(new CallToRemoteServiceE(2)));
            futures.add(executor.submit(new CallToRemoteServiceE(3)));
            futures.add(executor.submit(new CallToRemoteServiceE(4)));
            futures.add(executor.submit(new CallToRemoteServiceE(5)));

            // as each completes do further work

            // keep polling until all work is done
            while (futures.size() > 0) {
                // use an iterator so we can remove from it
                Iterator<Future<?>> i = futures.iterator();
                while (i.hasNext()) {
                    Future<?> f = i.next();
                    if (f.isDone()) {
                        // only do work if the Future is done
                        doMoreWork(f.get());
                        i.remove();
                    }
                    // otherwise we continue to the next Future
                }
            }
        } finally {
            executor.shutdownNow();
        }
    }

    public static void main(String args[]) {
        try {
            long start = System.currentTimeMillis();
            run();
            System.out.println("Finished in: " + (System.currentTimeMillis() - start) + "ms");

            run2();
            run3();
            run4();
            run5();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static final class CallToRemoteServiceA implements Callable<String> {
        @Override
        public String call() throws Exception {
            // simulate fetching data from remote service
            Thread.sleep(100);
            return "responseA";
        }
    }

    private static final class CallToRemoteServiceB implements Callable<Integer> {
        @Override
        public Integer call() throws Exception {
            // simulate fetching data from remote service
            Thread.sleep(40);
            return 100;
        }
    }

    private static final class CallToRemoteServiceC implements Callable<String> {

        private final String dependencyFromA;

        public CallToRemoteServiceC(String dependencyFromA) {
            this.dependencyFromA = dependencyFromA;
        }

        @Override
        public String call() throws Exception {
            // simulate fetching data from remote service
            Thread.sleep(60);
            return "responseB_" + dependencyFromA;
        }
    }

    private static final class CallToRemoteServiceD implements Callable<Integer> {

        private final Integer dependencyFromB;

        public CallToRemoteServiceD(Integer dependencyFromB) {
            this.dependencyFromB = dependencyFromB;
        }

        @Override
        public Integer call() throws Exception {
            // simulate fetching data from remote service
            Thread.sleep(140);
            return 40 + dependencyFromB;
        }
    }

    private static final class CallToRemoteServiceE implements Callable<Integer> {

        private final Integer dependencyFromB;

        public CallToRemoteServiceE(Integer dependencyFromB) {
            this.dependencyFromB = dependencyFromB;
        }

        @Override
        public Integer call() throws Exception {
            // simulate fetching data from remote service
            Thread.sleep(55);
            return 5000 + dependencyFromB;
        }
    }
}

Thanks Ben! This is an excellent example on Java Future and how things can go pretty ugly.

@benjchristensen have you considered using the thenCompose() of the CompletableFuture class instead?

    public static void run2() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            // kick of f1/f2 in parallel
            Future<String> f1 = executor.submit(new CallToRemoteServiceA());
            Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());

            // get f3 with dependent result from f1
            Future<String> f3 = f1.thenCompose(f1Result -> executor.submit(new CallToRemoteServiceC(f1Result)));

            // get f4/f5 after dependency f2 completes
            Future<Integer> f4 = f2.thenCompose(f2Result -> executor.submit(new CallToRemoteServiceD(f2Result)));
            Future<Integer> f5 = f2.thenCompose(f2Result -> executor.submit(new CallToRemoteServiceE(f2Result)));

            // first blocking command
            System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
        } finally {
            executor.shutdownNow();
        }
    }

@Turbo87 true that...completable future is available with java 8, rxjava started earlier

I am a bit confused, what about scala's Future and chaining using onSuccess etc... will that help?

don't understand why in the run(), f2,f4,f5 can not proceed until f1.get() completes. Doesn't the ThreadPoolExecutor can hold 4 threads at most?

I got it, because f1.get() blocks.

Is that only because it is Java future? Scala Future works better with nesting until you do Awaits.result()

very handsome!

Concise example with point to point explanation!
In line 55 does the call to f5 is blocked, until f4 is finished? both of them depend on f2, so once f2 is available, can both f4, f5 proceed simultaneously?

@Turbo87 true that...completable future is available with java 8, rxjava started earlier

that’s all?