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
In Chapter 3, the source for index introduces Config on page 31, followed by more code including tests; Config isn’t introduced until pag...
New
Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’.
In particula...
New
Title: Hands-on Rust: question about get_component (page 295)
(feel free to respond. “You dug you’re own hole… good luck”)
I have somet...
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
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
@noelrappin
Running the webpack dev server, I receive the following warning:
ERROR in tsconfig.json
TS18003: No inputs were found in c...
New
Hi,
I completed chapter 6 but am getting the following error when running:
thread 'main' panicked at 'Failed to load texture: IoError(O...
New
Modern Front-End Development for Rails - application does not start after run bin/setup (page xviii)
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New
Modern front-end development for Rails, second edition - Struggling to get the first chapter to work
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New
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
Hello Devtalk World!
Please let us know a little about who you are and where you’re from :nerd_face:
New
Brace yourself for a fun challenge: build a photorealistic 3D renderer from scratch! In just a couple of weeks, build a ray tracer that r...
New
Write Elixir tests that you can be proud of. Dive into Elixir’s test philosophy and gain mastery over the terminology and concepts that u...
New
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
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
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
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
Fight complexity and reclaim the original spirit of agility by learning to simplify how you develop software. The result: a more humane a...
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
- /clojure
- /java
- /emacs
- /haskell
- /typescript
- /svelte
- /onivim
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /flutter
- /vscode
- /elm
- /ash
- /html
- /deepseek
- /opensuse
- /zig
- /centos
- /php
- /scala
- /react-native
- /lisp
- /textmate
- /sublime-text
- /nixos
- /debian
- /agda
- /deno
- /django
- /kubuntu
- /arch-linux
- /nodejs
- /ubuntu
- /spring
- /revery
- /manjaro
- /lua
- /diversity
- /julia
- /laravel
- /markdown









