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!");
    }
}