Forked streams

Paul Sandoz paul.sandoz at
Fri Nov 29 02:34:28 PST 2013


Recently on this list Stuart mentioned Kirk's blog entry on lambdas, which has some discussion on forked streams:

To get across the issues and have a little fun i wrote a simple stream forker.

The basic approach is to create a recipe to fork a stream multiple times, apply that recipe to a stream, and collect the results. The results of each fork are encapsulated in instances of CompletableFuture. Each fork is executed asynchronously. The stream to fork is operated on using forEach with a consumer that pushes an element to each fork via a LinkedBlockingQueue. A forked stream's source is a special Spliterator covering the elements of the associated LinkedBlockingQueue.

It's not very complicated, quite rough, and i am sure just one of many ways if doing this, but it does highlight the complexities/issues here.

What does it mean for parallel streams? If a parallel forEach is performed on the stream to be forked that means elements will be pushed to the forked streams out of order. Resources for parallel execution of a stream will complete with concurrent execution of the forked streams, which may complication the performance characteristics.

Each fork represents a separate computation, which may be formulated differently for parallel execution. The computation of toArray is different to that of findFirst.

I can think of improvements to the current API that could enable better sequential forking, but for parallel forking it will be particularly tricky to fork the whole computation across thread boundaries and a questionable endeavour IMHO. 

So in general this can be complex, there are different models of execution competing for attention, and streams has currently opted for a simpler model that is easy to grok that with a bit of work can be utilized with forking if one is careful. There are probably ways we could improve that for any further streams work, perhaps with an SPI supporting user-defined operations where one could write a fork operation that behaves like a hybrid intermediate/terminal operation. 

Anyway, that's all speculation right now; hope some people find this a fun exercise too.


package stream.fork;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Spliterator;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.function.Consumer;
import java.util.function.Function;

public class ForkExample {

    public static void main(String[] args) throws Exception {
        // Set up the forks
        StreamForker<String> forker = new StreamForker<>();
        StreamForker.Memento<String> mr1 = forker.addFork(
                s ->;
        StreamForker.Memento<Integer> mr2 = forker.addFork(
                s -> s.mapToInt(Integer::valueOf).sum());
        StreamForker.Memento<Map<Integer, Long>> mr3 = forker.addFork(
                s -> s.collect(Collectors.groupingBy(String::length, Collectors.counting())));
        StreamForker.Memento<Optional<String>> mr4 = forker.addFork(
                s -> s.skip(10).findFirst());

        // Fork an existing stream 4 times
        Stream<String> p = IntStream.range(0, 100).mapToObj(Integer::toString);
        ForkedStreamResults rs = forker.fork(p);

        // Wait for all forks to complete

        // Get the results of each fork
        CompletableFuture<String> r1 = rs.get(mr1);
        CompletableFuture<Integer> r2 = rs.get(mr2);
        CompletableFuture<Map<Integer, Long>> r3 = rs.get(mr3);
        CompletableFuture<Optional<String>> r4 = rs.get(mr4);

     * A forker of streams
     * @param <T> the type of elements output from the forked stream.
    public static class StreamForker<T> {

         * A memento to a fork
         * @param <R> the type of result of the fork
        static class Memento<R> {

        private Map<Memento<?>, Function<Stream<T>, ?>> forks = new HashMap<>();

         * Add a fork
         * @param f the fork function to apply to a forked stream. The stream is
         * forked, and the function is applied to that forked stream to produce
         * a result.
         * @param <R> the type of result of the fork
         * @return a typed memento associated with the fork that can be used to
         * obtain the result returned by the fork function
        public <R> Memento<R> addFork(Function<Stream<T>, R> f) {
            Memento<R> m = new Memento<>();
            forks.put(m, f);
            return m;

         * Fork a stream
         * <p>The stream will be forked N times where N is the number of forks
         * added.
         * @param s the stream to fork
         * @return the results of forking
        public ForkedStreamResults fork(Stream<T> s) {
            // @@@ Obtain the spliterator from the stream so as to
            // get the characteristics that can be passed to the
            // LinkedBlockingQueueSpliterator, then re-create the
            // to-be-forked stream from that Spliterator

            ForkingStreamConsumer<T> consumer = build();
            try {
                // @@@ If the stream is parallel then the encounter order,
                // if any, will not be preserved, in addition the parallel
                // execution will compete with the execution of the forked
                // streams
            finally {
            return consumer;

        ForkingStreamConsumer<T> build() {
            List<LinkedBlockingQueue<T>> queues = new ArrayList<>();
            Map<Memento<?>, CompletableFuture<?>> actions = new HashMap<>();
            for (Map.Entry<Memento<?>, Function<Stream<T>, ?>> e : forks.entrySet()) {
                LinkedBlockingQueue<T> queue = new LinkedBlockingQueue<>();

                // Forked representation of the stream
                Stream<T> source =
                        new LinkedBlockingQueueSpliterator<>(queue), false);
                Function<Stream<T>, ?> f = e.getValue();
                CompletableFuture<?> action = CompletableFuture.supplyAsync(
                        () -> f.apply(source));
                actions.put(e.getKey(), action);

            return new ForkingStreamConsumer<>(queues, actions);

     * The results of forking a stream
    public static interface ForkedStreamResults {
         * @return a completable future encapsulating the futures of all forks
         * that is completed when all forks complete.
        public CompletableFuture<Void> all();

         * Get the completable future encapsulating the result of a fork
         * @param m the memento associated with the fork
         * @param <R> the type of results of the fork
         * @return the completable future encapsulating the result of the fork
        public <R> CompletableFuture<R> get(StreamForker.Memento<R> m);

    static class ForkingStreamConsumer<T> implements Consumer<T>, ForkedStreamResults {
        // Object element marking the end of the stream
        static final Object SENTINAL = new Object();

        private final List<LinkedBlockingQueue<T>> queues;
        private final Map<StreamForker.Memento<?>, CompletableFuture<?>> actions;

        ForkingStreamConsumer(List<LinkedBlockingQueue<T>> queues, Map<StreamForker.Memento<?>,
                CompletableFuture<?>> actions) {
            this.queues = queues;
            this.actions = actions;

        public void accept(T t) {
            // @@@ Buffering issues, can barf if queue is full
            // i.e. producer is faster than consumer
            queues.forEach(q -> q.add(t));

        public CompletableFuture<Void> all() {
            return CompletableFuture.allOf(

        public <R> CompletableFuture<R> get(StreamForker.Memento<R> m) {
            return (CompletableFuture<R>) actions.get(m);

        void finish() {
            accept((T) SENTINAL);

    static class LinkedBlockingQueueSpliterator<T> implements Spliterator<T> {
        private final LinkedBlockingQueue<T> q;

        private boolean finished;

        LinkedBlockingQueueSpliterator(LinkedBlockingQueue<T> q) {
            this.q = q;

        public boolean tryAdvance(Consumer<? super T> action) {
            if (!finished) {
                T t;
                while (true) {
                    try {
                        t = q.take();
                    catch (InterruptedException e) {

                if (t != ForkingStreamConsumer.SENTINAL) {
                    return true;

                finished = true;
            return false;

        public Spliterator<T> trySplit() {
            // @@@ Support limited splitting, using buffering with say
            // q.drainTo
            return null;

        public long estimateSize() {
            // @@@ Support size if known by forking stream
            return 0;

        public int characteristics() {
            // @@@ Inherit characters from forking stream
            return 0;

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