dtonhofer

dtonhofer

Functional Programming in Java, Second Edition: Chapter 9, p.164 addendum to "parallel stream"

In Chapter 9, p.164 the stream is parallelized in 1 step.

This inspired my to write some test some code to call a task “in parallel”:

  • Not in parallel, inside a loop
  • Using ThreadGroups (essentially ‘temporary worker pools’) to run “slices” of the list the tasks in parallel
  • Using a Java 8 ForkJoinPool
  • Using a Java 8 parallel stream.

Not sure whether this is of interest, the most interesting part is the handling of checked and unchecked exceptions.

package chapter9;

import org.junit.jupiter.api.Test;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class RunningInParallel {

    // This could also implement Runnable instead.

    public static class DoSomething {

        private final int index;

        public DoSomething(int index) {
            this.index = index;
        }

        // This method should not throw any checked exceptions

        public void doSomething() {
            try {
                // Math.random is synchronized, so we can use it here
                final long sleep_ms = (long) (Math.random() * 1000.0);
                System.out.println("Thread " + index + " starts on thread '" + Thread.currentThread().getName() + "', sleeping for " + sleep_ms + " ms");
                Thread.sleep(sleep_ms);
            } catch (InterruptedException ex) {
                // Someone told us to stop sleeping, so we do!
                // Set the "interrupted" bit again and get out.
                Thread.currentThread().interrupt();
            }
        }

    }

    private List<DoSomething> createElements() {
        return IntStream.rangeClosed(1, 20)
                .mapToObj(DoSomething::new)
                .collect(Collectors.toList());
    }

    private static ThreadGroup startThreads(final int sliceStart, final int sliceEndIncl, final List<DoSomething> elements) {
        final int slizeSize = sliceEndIncl - sliceStart + 1;
        ThreadGroup tGroup = new ThreadGroup("slice [" + sliceStart + "," + sliceEndIncl + "] of size " + slizeSize);
        for (int threadIndex = 0; threadIndex < slizeSize; threadIndex++) {
            final int elementIndex = sliceStart + threadIndex;
            // no need to retain reference to the Thread, we will get it back from the ThreadGroup
            new Thread(tGroup, new Runnable() {
                @Override
                public void run() {
                    // If doSomething() threw a checked exception, we would have to catch it here
                    // If doSomething() throws a RuntimeException, the Exception is left up the stack here,
                    // terminating the worker thread.
                    elements.get(elementIndex).doSomething();
                }
            }).start();
        }
        return tGroup;
    }

    private static void waitForThreadEnd(final ThreadGroup tGroup, final int slizeSize) throws InterruptedException {
        final Thread[] threads = new Thread[slizeSize];
        final int count = tGroup.enumerate(threads);
        assert count <= slizeSize; // some threads may already have finished
        for (int i = 0; i < count; i++) {
            try {
                // Dangerous, as infinite waiting may follow, there should be a timeout value!!
                System.out.println("Joining thread " + (i + 1) + " of " + count);
                threads[i].join();
            } catch (InterruptedException ex) {
                // What should we do here? Just set the interrupt flag and throw...
                Thread.currentThread().interrupt();
                throw ex;
            }
        }
    }

    @Test
    void preJava8_singleThread() {
        final List<DoSomething> elements = Collections.unmodifiableList(createElements());
        for (DoSomething elem : elements) {
            // If doSomething() threw a checked exception, we would have to catch it here.
            // If doSomething() throws a RuntimeException, the Exception is left up the stack here.
            elem.doSomething();
        }
        System.out.println("DONE!");
    }

    @Test
    void preJava8_rollYourOwnMultipleThreads() throws InterruptedException {
        final List<DoSomething> elements = Collections.unmodifiableList(createElements());
        final int threadCount = 7;
        // Iterate over "slices" of "threadCount" threads.
        int sliceIndex = 0;
        while (sliceIndex * threadCount < elements.size()) {
            final int sliceStart = sliceIndex * threadCount;
            final int sliceEndIncl = Math.min(sliceStart + threadCount - 1, elements.size() - 1);
            final int slizeSize = sliceEndIncl - sliceStart + 1;
            ThreadGroup tGroup = startThreads(sliceStart, sliceEndIncl, elements);
            waitForThreadEnd(tGroup, slizeSize);
            System.out.println("Done with ThreadGroup '" + tGroup.getName() + "' running " + slizeSize + " threads");
            sliceIndex++;
        }
        System.out.println("DONE!");
    }

    @Test
    void java8_multipleThreadsWithForkJoinPool() throws InterruptedException {
        final List<DoSomething> elements = Collections.unmodifiableList(createElements());
        final List<ForkJoinTask<?>> tasks = new LinkedList<>();
        for (DoSomething elem : elements) {
            // If doSomething() threw a checked exception, we COULD NOT use it as argument to submit()
            // We would need to wrap doSomething().
            // Note that we use the "common pool" provided by the runtime environment.
            // We could also create our own pool instead, but why bother?
            // Note that some of the tasks will actaully be run by the main thread instead
            // of by a thread from the pool.
            tasks.add(ForkJoinPool.commonPool().submit(elem::doSomething));
        }
        for (ForkJoinTask<?> task : tasks) {
            try {
                task.get();
            } catch (ExecutionException ex) {
                // If doSomething() throws a RuntimeException it will be rethrown as an ExecutionException.
                // The thrown RuntimeException will appear as the "cause".
                System.err.println("Task failed to finish properly, got ExecutionException: '" + ex.getMessage()
                        + "' caused by: '" + ex.getCause() + "'");
            } catch (CancellationException ex) {
                System.err.println("Task was cancelled, hot CancellationException: " + ex.getMessage());
            } catch (InterruptedException ex) {
                // What should we do here? Just set the interrupt flag and throw.
                // Note the ,ethod "doSomething()" does not actually throw it.
                Thread.currentThread().interrupt();
                throw ex;
            }
        }
        System.out.println("DONE!");
    }

    @Test
    void java8_multipleThreadsWithStream() throws InterruptedException {
        final List<DoSomething> elements = Collections.unmodifiableList(createElements());
        // If doSomething() threw a checked exception (a subclass of Exception),
        // we COULD NOT use it as argument to forEach().
        // If doSomething() throws a RuntimeException, the stream pipeline would terminate
        // arbitrarily with any of the exceptions thrown in any of the threads.
        elements.stream().parallel().forEach(DoSomething::doSomething);
        System.out.println("DONE!");
    }
}

Where Next?

Popular Pragmatic Bookshelf topics Top

brianokken
Many tasks_proj/tests directories exist in chapters 2, 3, 5 that have tests that use the custom markers smoke and get, which are not decl...
New
ianwillie
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
New
jamis
The following is cross-posted from the original Ray Tracer Challenge forum, from a post by garfieldnate. I’m cross-posting it so that the...
New
edruder
I thought that there might be interest in using the book with Rails 6.1 and Ruby 2.7.2. I’ll note what I needed to do differently here. ...
New
herminiotorres
Hi @Margaret , On page VII the book tells us the example and snippets will be all using Elixir version 1.11 But on page 3 almost the en...
New
jeremyhuiskamp
Title: Web Development with Clojure, Third Edition, vB17.0 (p9) The create table guestbook syntax suggested doesn’t seem to be accepted ...
New
patoncrispy
I’m new to Rust and am using this book to learn more as well as to feed my interest in game dev. I’ve just finished the flappy dragon exa...
New
jskubick
I’m running Android Studio “Arctic Fox” 2020.3.1 Patch 2, and I’m embarrassed to admit that I only made it to page 8 before running into ...
New
jgchristopher
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ” Excerpt From: Br...
New
oaklandgit
Hi, I completed chapter 6 but am getting the following error when running: thread 'main' panicked at 'Failed to load texture: IoError(O...
New

Other popular topics Top

PragmaticBookshelf
Take your Go skills to the next level by learning how to design, develop, and deploy a distributed service. Start from the bare essential...
New
Exadra37
Please tell us what is your preferred monitor setup for programming(not gaming) and why you have chosen it. Does your monitor have eye p...
New
AstonJ
You might be thinking we should just ask who’s not using VSCode :joy: however there are some new additions in the space that might give V...
New
AstonJ
There’s a whole world of custom keycaps out there that I didn’t know existed! Check out all of our Keycaps threads here: https://forum....
New
AstonJ
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
DevotionGeo
I have always used antique keyboards like Cherry MX 1800 or Cherry MX 8100 and almost always have modified the switches in some way, like...
New
PragmaticBookshelf
Develop, deploy, and debug BEAM applications using BEAMOps: a new paradigm that focuses on scalability, fault tolerance, and owning each ...
New
AstonJ
If you’re getting errors like this: psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
New
PragmaticBookshelf
As digital systems increasingly run the world, mastery of the recurring patterns of software development risk is the key to fast and effe...
New
xiji2646-netizen
Woke up to this today: Claude Code’s complete source code exposed via npm source map. Not a snippet. All 512,000 lines. 1,900 TypeScript ...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: