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 20: … protoc command…
I had to additionally run the following go get commands in order to be able to compile protobuf code using go...
New
As per the title, thanks.
New
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
Hi Jamis,
I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019.
This test doesn’t pass for me:
...
New
Dear Sophie.
I tried to do the “Authorization” exercise and have two questions:
When trying to plug in an email-service, I found the ...
New
On page 78 the following code appears:
<%= link_to ‘Destroy’, product,
class: ‘hover:underline’,
method: :delete,
data: { confirm...
New
@parrt
In the context of Chapter 4.3, the grammar Java.g4, meant to parse Java 6 compilation units, no longer passes ANTLR (currently 4....
New
Disclaimer: I hope all of this is accurate, and not the result of some stupid typo I failed to notice. Anyways…
This post comprises 2 is...
New
@mfazio23
Android Studio will not accept anything I do when trying to use the Transformations class, as described on pp. 140-141. Googl...
New
Is there any plan for volume 2? :slight_smile:
New
Other popular topics
Hello Devtalk World!
Please let us know a little about who you are and where you’re from :nerd_face:
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
Hi folks,
I don’t know if I saw this here but, here’s a new programming language, called Roc
Reminds me a bit of Elm and thus Haskell. ...
New
A few weeks ago I started using Warp a terminal written in rust. Though in it’s current state of development there are a few caveats (tab...
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
Author Spotlight
Jamis Buck
@jamis
This month, we have the pleasure of spotlighting author Jamis Buck, who has written Mazes for Prog...
New
Author Spotlight:
Peter Ullrich
@PJUllrich
Data is at the core of every business, but it is useless if nobody can access and analyze ...
New
Leverage Elixir and the Nx ecosystem to build intelligent applications that solve real-world problems in computer vision, natural languag...
New
Explore the power of Ash Framework by modeling and building the domain for a real-world web application.
Rebecca Le @sevenseacat and ...
New
There’s a GitHub repo at forrestchang/andrej-karpathy-skills that’s sitting at 97.8k stars. It’s a single CLAUDE.md file with four behavi...
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
- /nixos
- /debian
- /agda
- /deno
- /django
- /kubuntu
- /arch-linux
- /nodejs
- /spring
- /ubuntu
- /revery
- /manjaro
- /diversity
- /julia
- /lua
- /quarkus
- /laravel









