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
Some minor things in the paper edition that says “3 2020” on the title page verso, not mentioned in the book’s errata online:
p. 186 But...
New
your book suggests to use Image.toByteData() to convert image to bytes, however I get the following error: "the getter ‘toByteData’ isn’t...
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
Hi @Margaret ,
On page VII the book tells us the example and snippets will be all using Elixir version 1.11
But on page 3 almost the en...
New
I ran this command after installing the sample application:
$ cards add do something --owner Brian
And got a file not found error:
Fil...
New
I’m not quite sure what’s going on here, but I’m unable to have to containers successfully complete the Readiness/Liveness checks. I’m im...
New
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
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
Hi all,
currently I wonder how the Tailwind colours work (or don’t work).
For example, in app/views/layouts/application.html.erb I have...
New
I’ve got to the end of Ch. 11, and the app runs, with all tabs displaying what they should – at first. After switching around between St...
New
Other popular topics
Stop developing web apps with yesterday’s tools. Today, developers are increasingly adopting Clojure as a web-development platform. See f...
New
I know that these benchmarks might not be the exact picture of real-world scenario, but still I expect a Rust web framework performing a ...
New
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
Rust is an exciting new programming language combining the power of C with memory safety, fearless concurrency, and productivity boosters...
New
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
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
Create efficient, elegant software tests in pytest, Python's most powerful testing framework.
Brian Okken @brianokken
Edited by Kat...
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
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
Get the comprehensive, insider information you need for Rails 8 with the new edition of this award-winning classic.
Sam Ruby @rubys
...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /python
- /js
- /rails
- /security
- /go
- /swift
- /vim
- /clojure
- /java
- /emacs
- /haskell
- /typescript
- /svelte
- /onivim
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /flutter
- /elm
- /vscode
- /ash
- /html
- /deepseek
- /opensuse
- /zig
- /centos
- /php
- /scala
- /react-native
- /lisp
- /sublime-text
- /textmate
- /nixos
- /debian
- /agda
- /deno
- /django
- /kubuntu
- /arch-linux
- /nodejs
- /ubuntu
- /spring
- /revery
- /manjaro
- /diversity
- /julia
- /lua
- /markdown
- /laravel









