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

New
iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
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
raul
Page 28: It implements io.ReaderAt on the store type. Sorry if it’s a dumb question but was the io.ReaderAt supposed to be io.ReadAt? ...
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
nicoatridge
Hi, I have just acquired Michael Fazio’s “Kotlin and Android Development” to learn about game programming for Android. I have a game in p...
New
AufHe
I’m a newbie to Rails 7 and have hit an issue with the bin/Dev script mentioned on pages 112-113. Iteration A1 - Seeing the list of prod...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New
bjnord
Hello @herbert ! Trying to get the very first “Hello, Bracket Terminal!" example to run (p. 53). I develop on an Amazon EC2 instance runn...
New
redconfetti
Docker-Machine became part of the Docker Toolbox, which was deprecated in 2020, long after Docker Desktop supported Docker Engine nativel...
New

Other popular topics Top

PragmaticBookshelf
Instantly view the changes you make to an app with stateful hot reload, and define a declarative UI in the same language as the app logic...
New
New
AstonJ
Or looking forward to? :nerd_face:
503 15149 280
New
dasdom
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
New
AstonJ
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
New
New
Exadra37
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
New
PragmaticBookshelf
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
NewsBot
Node.js v22.14.0 has been released. Link: Release 2025-02-11, Version 22.14.0 'Jod' (LTS), @aduh95 · nodejs/node · GitHub
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: