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!");
}
}
Popular Pragmatic Bookshelf topics
page 37
ANTLRInputStream input = new ANTLRInputStream(is);
as of ANTLR 4 .8 should be:
CharStream stream = CharStreams.fromStream(i...
New
Working through the steps (checking that the Info,plist matches exactly), run the demo game and what appears is grey but does not fill th...
New
Title: Web Development with Clojure, Third Edition, pg 116
Hi - I just started chapter 5 and I am stuck on page 116 while trying to star...
New
Title: Hands-On Rust (Chapter 11: prefab)
Just played a couple of amulet-less games. With a bit of debugging, I believe that your can_p...
New
Hi! I know not the intentions behind this narrative when called, on page XI:
mount() |> handle_event() |> render()
but the correc...
New
The book has the same “Problem space/Solution space” diagram on page 18 as is on page 17. The correct Problem/Solution space diagrams ar...
New
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
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64)
If you follow the defau...
New
I am using Android Studio Chipmunk | 2021.2.1 Patch 2
Build #AI-212.5712.43.2112.8815526, built on July 10, 2022
Runtime version: 11.0....
New
Hello faithful readers! If you have tried to follow along in the book, you are asked to start up the dev environment via dx/build and ar...
New
Other popular topics
If it’s a mechanical keyboard, which switches do you have?
Would you recommend it? Why?
What will your next keyboard be?
Pics always w...
New
Ruby, Io, Prolog, Scala, Erlang, Clojure, Haskell. With Seven Languages in Seven Weeks, by Bruce A. Tate, you’ll go beyond the syntax—and...
New
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
Small essay with thoughts on macOS vs. Linux:
I know @Exadra37 is just waiting around the corner to scream at me “I TOLD YOU SO!!!” but I...
New
In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby :man_facepalming: do this first:
asdf plugin-upd...
New
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
If you get Can't find emacs in your PATH when trying to install Doom Emacs on your Mac you… just… need to install Emacs first! :lol:
bre...
New
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
New
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
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /python
- /js
- /rails
- /security
- /go
- /swift
- /vim
- /java
- /clojure
- /emacs
- /haskell
- /typescript
- /svelte
- /onivim
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /elm
- /vscode
- /flutter
- /html
- /ash
- /deepseek
- /zig
- /opensuse
- /centos
- /php
- /scala
- /react-native
- /lisp
- /sublime-text
- /textmate
- /debian
- /nixos
- /agda
- /deno
- /django
- /kubuntu
- /arch-linux
- /nodejs
- /spring
- /ubuntu
- /revery
- /manjaro
- /julia
- /lua
- /diversity
- /quarkus
- /markdown









