Question

I have a multithreaded execution and I want to track and print out the execution time, but when I execute the code, the child threads takes longer than the main execution, thus the output is not visible nor it prints the right value, since it is terminating earlier.

Here is the code:

public static void main(String[] args) throws CorruptIndexException, IOException, LangDetectException, InterruptedException {

    /* Initialization */
    long startingTime = System.currentTimeMillis();
    Indexer main = new Indexer(); // this class extends Thread
    File file = new File(SITES_PATH);
    main.addFiles(file);

    /* Multithreading through ExecutorService */
    ExecutorService es = Executors.newFixedThreadPool(4);
    for (File f : main.queue) {
        Indexer ind = new Indexer(main.writer, main.identificatore, f);
        ind.join();
        es.submit(ind);
    }

    es.shutdown();

    /* log creation - code I want to execute when all the threads execution ended */
    long executionTime = System.currentTimeMillis()-startingTime;
    long minutes = TimeUnit.MILLISECONDS.toMinutes(executionTime);
    long seconds = TimeUnit.MILLISECONDS.toSeconds(executionTime)%60;
    String fileSize = sizeConversion(FileUtils.sizeOf(file));

    Object[] array = {fileSize,minutes,seconds};
    logger.info("{} indexed in {} minutes and {} seconds.",array);
}

I tried several solutions such as join(), wait() and notifyAll(), but none of them worked.

I found this Q&A on stackoverflow which treats my problem, but join() is ignored and if I put

es.awaitTermination(timeout, TimeUnit.SECONDS);

actually the executor service never executes threads.

Which can be the solution for executing multithreading only in ExecutorService block and finish with main execution at the end?

Answer 1

Given your user case you might as well utilize the invokeAll method. From the Javadoc:

Executes the given tasks, returning a list of Futures holding their
status and results when all complete. Future.isDone() is true for each
element of the returned list. Note that a completed task could have
terminated either normally or by throwing an exception. The results of
this method are undefined if the given collection is modified while
this operation is in progress.

To use:

final Collection<Indexer> tasks = new ArrayList<Indexer>();
for(final File f: main.queue) {
    tasks.add(new Indexer(main.writer, main.identificatore, f));
}

final ExecutorService es = Executors.newFixedThreadPool(4);
final List<Future<Object>> results = es.invokeAll(tasks);

This will execute all supplied tasks and wait for them to finish processing before proceeding on your main thread. You will need to tweak the code to fit your particular needs, but you get the gist. A quick note, there is a variant of the invokeAll method that accepts timeout parameters. Use that variant if you want to wait up to a maximum amount of time before proceeding. And make sure to check the results collected after the invokeAll is done, in order to verify the status of the completed tasks.

Good luck.