Execute a list of Threads in parallel

Question

I have a list of Script that i want to execute them in parallel, i already find this way in Creating dynamic number of threads concurrently, so i use this piece of code here :

private int i;//<---------------------------------(1)
private String[] listScript = {"Script1", "Script2", "Script3", "Script4"};
private void executeQueries() throws InterruptedException {
 ExecutorService threadPool = Executors.newFixedThreadPool(listScript.length);
 // submit jobs to be executing by the pool
 for (i = 0; i < listScript.length; i++) {
 threadPool.submit(new Runnable() {
 @Override
 public void run() {
 executeQuerie();
 }
 });
 Thread.sleep(500);//<---------------------(2)
 }
 // once you've submitted your last job to the service it should be shut down
 threadPool.shutdown();
 // wait for the threads to finish if necessary
 threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
}
private void executeQuerie() {
 //execute scripts action
 System.out.println(listScript[i]);
}

I'm really not happy with this code, i will explain a little my code so you can understand more :

(1). I use this i variable outside the method so i can know which script i should to execute
(2). I use Thread.sleep(500); to make sure that i execute the right script, without this i can execute the same script multiple time for example it can gives me:

Script1
Script1
Script2
Script2

My question is : how can i make sure that my scripts executed safely in parallel, should i improve this piece of code, or do you have any other safe way?

Answer 1

You should make i a parameter of executeQuery :

private String[] listScript = {"Script1", "Script2", "Script3", "Script4"};
private void executeQueries() throws InterruptedException {
 ExecutorService threadPool = Executors.newFixedThreadPool(listScript.length);
 // submit jobs to be executing by the pool
 for (int i = 0; i < listScript.length; i++) {
 final int j = i;
 threadPool.submit(() -> executeQuery(j));
 }
 // once you've submitted your last job to the service it should be shut down
 threadPool.shutdown();
 // wait for the threads to finish if necessary
 threadPool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
}
private void executeQuery(final int index) {
 System.out.println(listScript[index]);
}

I used lambda here as it's simply less verbose. j is needed as i must be final for the closure to work ;)

Also awaitTermination(Long.MAX_VALUE... isn't a very good idea.... It's over 10 millions years you know ? ^^

• Youcef LAIDANI: but you can't use the i like you do threadPool.submit(() -> executeQuery(i));
• Ronan Dhellemmes: @YCF_L yup I used another variable, I just edited my post with an added comment about this
• Youcef LAIDANI: it still the same problem :(
• Ronan Dhellemmes: @YCF_L ??? it's working properly with j on my computer
• Ronan Dhellemmes: @YCF_L isn't that the point of parrallel execution ? ^^ you can use Thread.sleep if you want them to be ordered but that's not very good looking... if you want a more definitive answer, then this is a question you should check over StackOverflow

Answer 2

If you do not need to have control over the amount of threads used to perform the work, you could consider parallel streams and you are done with these simple lines:

private String[] listScript = {"Script1", "Script2", "Script3", "Script4"};
private void executeQueries() {
 Arrays.asList(listScript).parallelStream().forEach(script -> {
 System.out.println(script + " runs in thread " + Thread.currentThread());
 try { Thread.sleep(5000);} catch (InterruptedException e) {}
 });
}