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
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
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
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64)
If you follow the defau...
New
Is the book’s epub format available to read on Google Play Books?
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
It seems the second code snippet is missing the code to set the current_user:
current_user: Accounts.get_user_by_session_token(session["...
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
@mfazio23
I’m following the indications of the book and arriver ad chapter 10, but the app cannot be compiled due to an error in the Bas...
New
I got this error when executing the plot files on macOS Ventura 13.0.1 with Python 3.10.8 and matplotlib 3.6.1:
programming_ML/code/03_...
New
Other popular topics
I am thinking in building or buy a desktop computer for programing, both professionally and on my free time, and my choice of OS is Linux...
New
I’m thinking of buying a monitor that I can rotate to use as a vertical monitor?
Also, I want to know if someone is using it for program...
New
This is going to be a long an frequently posted thread.
While talking to a friend of mine who has taken data structure and algorithm cou...
New
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig.
General-purpose programming language and toolchain for maintaini...
New
Hello,
I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New
This is cool!
DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON
We just witnessed something incredible: the largest open-s...
New
Overarching tutorial for Python beginner and intermediate developers that teaches web development with the Flask framework.
Miguel Gr...
New
Hair Salon Games for Girls Fun
Girls Hair Saloon game is mainly developed for kids. This game allows users to select virtual avatars to ...
New
A concise guide to MySQL 9 database administration, covering fundamental concepts, techniques, and best practices.
Neil Smyth
MySQL...
New
As digital systems increasingly run the world, mastery of the recurring patterns of software development risk is the key to fast and effe...
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
- /ubuntu
- /spring
- /revery
- /manjaro
- /lua
- /julia
- /diversity
- /quarkus
- /markdown









