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

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
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
joepstender
The generated iex result below should list products instead of product for the metadata. (page 67) iex&gt; product = %Product{} %Pento....
New
alanq
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
New
leba0495
Hello! Thanks for the great book. I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
New
brian-m-ops
#book-python-testing-with-pytest-second-edition Hi. Thanks for writing the book. I am just learning so this might just of been an issue ...
New
jonmac
The allprojects block listed on page 245 produces the following error when syncing gradle: “org.gradle.api.GradleScriptException: A prob...
New
creminology
Skimming ahead, much of the following is explained in Chapter 3, but new readers (like me!) will hit a roadblock in Chapter 2 with their ...
New
kolossal
Hi, I need some help, I’m new to rust and was learning through your book. but I got stuck at the last stage of distribution. Whenever I t...
New
SlowburnAZ
Getting an error when installing the dependencies at the start of this chapter: could not compile dependency :exla, "mix compile" failed...
New

Other popular topics Top

Devtalk
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
1031 17377 381
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
Exadra37
On modern versions of macOS, you simply can’t power on your computer, launch a text editor or eBook reader, and write or read, without a ...
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
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
mafinar
This is going to be a long an frequently posted thread. While talking to a friend of mine who has taken data structure and algorithm cou...
New
AstonJ
We’ve talked about his book briefly here but it is quickly becoming obsolete - so he’s decided to create a series of 7 podcasts, the firs...
New
New
sir.laksmana_wenk
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc. However, I don’t...
New
AnfaengerAlex
Hello, I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: