Java - Is this method multithreaded?

Question

First of all, thanks for taking time to read this. The code is written and executed as a JUnit test, so I dont know whether that affects the answer.

@Test
public void generate___() {
    long startTime = System.nanoTime();
    for (File file : getResultsFromFolder("C:\\temp\\....")) {
        class runnableClass implements Runnable{
            public void run() {
                // do something with file

            }
        }
        new runnableClass().run();
    }

    long endTime = System.nanoTime();
    System.out.println("total took: " + (endTime - startTime) / 1000000); //divide by 1000000 to get milliseconds.
}

Depends if `getResultsFromFolder` is multithreaded. Otherwise, I don't see the creation of any threads. — Jacob G., Apr 19 '18 at 14:10
I don't agree with Jacob; Since you are creating and running a new thread (Runnable) for each file you get from `getResultsFromFolder("C:\\temp\\....")`, it is multithreaded — Arnauld Alex, Apr 19 '18 at 14:14
http://javarevisited.blogspot.fr/2012/01/difference-thread-vs-runnable-interface.html Ok it's hard to see the difference but ok. — Arnauld Alex, Apr 19 '18 at 14:18
@ArnauldAlex, the article you linked has a misleading title. It does not explain the difference between what a `Thread` _is_, and what a `Runnable` is. A `Thread` is an object with a magic method `start()` that creates a new thread. `Runnable` is just an interface: It doesn't _do_ anything at all. If you create a class that _implements_ runnable, then you will be allowed to store it in `Runnable` variables and pass it to functions that take a `Runnable` argument. But the compiler will require your class to have a `run()` method. — Solomon Slow, Apr 19 '18 at 17:48

OldCurmudgeon · Answer 1 · 2018-04-19T14:25:50.203

2

No it is not.

new runnableClass().run();

This calls the run method directly as defined above it.

If you want this code to be multithreaded you will need to use:

new Thread(new runnableClass()).start();

edited Apr 19 '18 at 14:25

answered Apr 19 '18 at 14:13

OldCurmudgeon

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xingbin · Answer 2 · 2018-04-19T14:32:03.323

2

No, the runnable will be executed by the main caller thread, just like this:

for (File file : getResultsFromFolder("C:\\temp\\....")) {
     // do something with file
}

To make it multi-threaded, you can create new threads and call start():

for (final File file : getResultsFromFolder("C:\\temp\\....")) {
    class runnableClass implements Runnable{
        public void run() {
            // do something with file
        }
    }
    new Thread(new runnableClass()).start();
}

edited Apr 19 '18 at 14:32

answered Apr 19 '18 at 14:13

xingbin

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Thank you very much for the answer! – Allen H. Apr 19 '18 at 14:41
@AllenH. no problem :) – xingbin Apr 19 '18 at 14:41
@AllenH. chech this for more https://stackoverflow.com/questions/8579657/whats-the-difference-between-thread-start-and-runnable-run – xingbin Apr 19 '18 at 14:42
I am able to get this to multithread, but the code block in the run() method does not behave as expected. Any clues on where I can start to look? – Allen H. Apr 19 '18 at 16:32
@AllenH. Better to create a new question and post more details. – xingbin Apr 19 '18 at 16:33
Thanks for the suggestion! New question here: https://stackoverflow.com/questions/49926689/java-multithreaded-method-not-behaving-as-expected – Allen H. Apr 19 '18 at 17:15

Garreth Golding · Accepted Answer · 2018-04-19T14:32:58.530

2

This method is not multi threaded due to the creation of Runnable instance in your method.

Example to showcase answer:

Code

 public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Runnable myRunnable = new Runnable() {
                @Override
                public void run() {
                    System.out.println(Thread.currentThread().getName());
                }
            };
            myRunnable.run();
        }
    }

Output

main
main
main
main
main
main
main
main
main
main

To make this method multi threaded you could use an ExecutorService.

Code

 public static void main(String[] args) {

    ExecutorService executorService = Executors.newFixedThreadPool(2);

    for (int i = 0; i < 10; i++) {
        Runnable myRunnable = new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName());
            }
        };

        executorService.execute(myRunnable);
    }

    executorService.shutdown();
}

Output

pool-1-thread-1
pool-1-thread-2
pool-1-thread-1
pool-1-thread-2
pool-1-thread-1
pool-1-thread-2
pool-1-thread-1
pool-1-thread-2
pool-1-thread-1
pool-1-thread-2

edited Apr 19 '18 at 14:32

answered Apr 19 '18 at 14:17

Garreth Golding

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Thanks for demonstrating this. Is there a quick tweak to change it to be multithreaded? – Allen H. Apr 19 '18 at 14:20
I've updated my answer to showcase the use of an ExecutorService to make this multi threaded. Please accept the answer if it helps. – Garreth Golding Apr 19 '18 at 14:26
Thanks! This exactly matches the implementation of some colleagues. ExecutorService.execute is the trick! – Allen H. Apr 19 '18 at 14:40
I am able to get this to multithread, but the code block in the run() method does not behave as expected. Any clues on where I can start to look? – Allen H. Apr 19 '18 at 16:32
Can you update your question with the code in the run method and I can try and help. – Garreth Golding Apr 19 '18 at 16:54
Thanks!! New question posted here https://stackoverflow.com/questions/49926689/java-multithreaded-method-not-behaving-as-expected – Allen H. Apr 19 '18 at 17:15

score 1 · Answer 4 · edited Feb 02 '21 at 02:45

1

As a complement see this tutorial, it explains well what you want to know. And it has example

https://www.tutorialspoint.com/java/java_multithreading.htm

edited Feb 02 '21 at 02:45

xingbin

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answered Apr 19 '18 at 14:16

Arnauld Alex

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Java - Is this method multithreaded?

4 Answers4