Java Multithreading make threads end in same order they started but run at same time

Question

I have to make a program that searches through a bunch of lines from a file and tries to find a given substring. If it finds it, it prints out the line. Each line that I read is created as a thread and each thread searches a single line of the file. Thats not a problem so far. What I need the program to do is print the end results (the line of text) in the same order that the threads were created. I.E. thread 6 shouldnt be printing before thread 2. Its fine that the threads run at the same time, the printing order just needs to be maintained. I can not use the join method as I don't want the next one to wait for the other to finish entirely before starting, I do want them to be running at the same time. Any suggestions on doing this? Also, the file can have any number of lines so I cant hardcode the number of threads.

THE THREADS SHOULD DO THEIR OWN PRINTING. THE MAIN DOES NOT DO THE PRINTING.

Answer 1

First of all the order of threaded applications is hard to define. See here: unwanted output in multithreading

If you want the output in a particular order then you probably should use an ExecutorService which will return a Future. You submit Callable<String> classes to the service each which return the results. The submit returns a Future<String>. You then can call get() from the Future in the same order that you submitted the jobs to the service.

// create a thread pool with 10 workers
ExecutorService threadPool = Executors.newFixedThreadPool(10);
// or you can create an open-ended thread pool
// ExecutorService threadPool = Executors.newCachedThreadPool();
// define your jobs somehow as a Callable that returns the line (prolly a string)
List<Future<String>> futures = threadPool.invokeAll(jobsToDo);
// once we have submitted all jobs to the thread pool, it should be shutdown
threadPool.shutdown();
// now we can go through the futures and get the results in order
for (Future<String> future : futures) {
    // this returns the results from the `call()` in order or it throws
    String resultFromCall = future.get();
}

You job Callable class would look something like:

public class MyCallable implements Callable<String> {
    private String input;
    public MyCallable(String input) {
        this.input = input;
    }
    public String call() {
        // search the input string
        String result = search(input);
        return result;
    }
}

Answer 2

Actually, you can use join(). But you do need to call it at the appropriate time and place for your requirements. Updated for your new requirement "Main thread should not print the results, the worker threads should print the results".

NOTE: This type of program is actually better written using the utility classes in java.util.concurrent. However, I wrote this example using the threading "base classes" to help with learning. Please note reggert's comment (above) -- blindly creating one thread per line of text in a file may result in so many threads that your program crashes or OS gets overwhelmed. This kind of work delegation is better done with a thread pool (e.g. java.util.concurrent.Executors).

public class MultiThreadedLineSearcher {

    public static void main(String[] args) throws Exception {
        Thread previousThread = null;
        for (int i = 0; i < LINES.length; i++) {
            JobRunnable job = new JobRunnable(i, LINES[i], previousThread);
            Thread thread = new Thread(job, "T-" + i);
            thread.start();
            previousThread = thread;
        }
        if (previousThread != null) {
            previousThread.join();
        }
        System.out.println("Program done.");
    }

    public static class JobRunnable implements Runnable {
        private final int _lineIdx;
        private final String _lineText;
        private final Thread _threadToWaitForBeforePrinting;

        public JobRunnable(int lineIdx, String lineText,
                Thread threadToWaitForBeforePrinting) {
            _lineIdx = lineIdx;
            _lineText = lineText;
            _threadToWaitForBeforePrinting = threadToWaitForBeforePrinting;
        }

        public void run() {
            try {
                boolean matched = FIND_ME.matcher(_lineText).find();
                String currentThreadName = Thread.currentThread().getName();
                System.out.println("Thread " + currentThreadName
                        + " is done with its work.");
                if (_threadToWaitForBeforePrinting != null) {
                    System.out.println("Thread " + currentThreadName
                            + " will wait for thread "
                            + _threadToWaitForBeforePrinting.getName()
                            + " before printing its results.");
                    _threadToWaitForBeforePrinting.join();
                }
                System.out.println("RESULT: " + _lineIdx + " matched? "
                        + matched + " (Printed on Thread "
                        + currentThreadName + ")");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }

    final static String[] LINES = new String[] {
        "Sed ut perspiciatis unde omnis iste natus error sit voluptatem",
        "accusantium doloremque laudantium, totam rem aperiam, eaque ipsa",
        "quae ab illo inventore veritatis et quasi architecto beatae vitae",
        "dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas",
        "sit aspernatur aut odit aut fugit, sed quia consequuntur magni",
        "dolores eos qui ratione voluptatem sequi nesciunt. Neque porro",
        "quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur,",
        "adipisci velit, sed quia non numquam eius modi tempora incidunt",
        "ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad",
        "minima veniam, quis nostrum exercitationem ullam corporis",
        "suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur?",
        "Quis autem vel eum iure reprehenderit qui in ea voluptate velit",
        "esse quam nihil molestiae consequatur, vel illum qui dolorem eum",
        "fugiat quo voluptas nulla pariatur?" };

    // Match only words that are 11 characters or longer
    final static java.util.regex.Pattern FIND_ME = 
            java.util.regex.Pattern.compile("\\w{11,}");
}

And here's how to do it can be done with an Executor:

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

public class MultiThreadedLineSearcherExecutor {

    static final int MAX_THREADS = 10;

    public static void main(String[] args) throws Exception {
        // Create as many threads as there are lines of text,
        // but do not exceed 10 threads
        int lineCount = LINES.length;
        int threadPoolSize = Math.min(MAX_THREADS, lineCount);
        System.out.println("Number of lines = " + lineCount);
        System.out.println("Thread pool size = " + threadPoolSize);
        ExecutorService executor = Executors.newFixedThreadPool(threadPoolSize);
        Future previousFuture = null;
        for (int i = 0; i < lineCount; i++) {
            JobRunnable job = new JobRunnable(i, LINES[i], previousFuture);
            previousFuture = executor.submit(job);
        }
        executor.shutdown();
        executor.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
        System.out.println("Program done.");
    }

    public static class JobRunnable implements Runnable {
        private final int _lineIdx;
        private final String _lineText;
        private final Future _futureToWaitForBeforePrinting;

        public JobRunnable(int lineIdx, String lineText,
                Future previousFuture) {
            _lineIdx = lineIdx;
            _lineText = lineText;
            _futureToWaitForBeforePrinting = previousFuture;
        }

        public void run() {
            try {
                boolean matched = FIND_ME.matcher(_lineText).find();
                String currentThreadName = Thread.currentThread().getName();
                System.out.println("Thread " + currentThreadName
                        + " is done with its work on line " + _lineIdx);
                if (_futureToWaitForBeforePrinting != null) {
                    System.out.println("Thread " + currentThreadName
                            + " will wait for future "
                            + _futureToWaitForBeforePrinting
                            + " before printing its results.");
                    _futureToWaitForBeforePrinting.get();
                }
                System.out.println("RESULT: " + _lineIdx + " matched? "
                        + matched + " (Printed on Thread "
                        + currentThreadName + ")");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }
    }

    final static String[] LINES = new String[] {
        "Sed ut perspiciatis unde omnis iste natus error sit voluptatem",
        "accusantium doloremque laudantium, totam rem aperiam, eaque ipsa",
        "quae ab illo inventore veritatis et quasi architecto beatae vitae",
        "dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas",
        "sit aspernatur aut odit aut fugit, sed quia consequuntur magni",
        "dolores eos qui ratione voluptatem sequi nesciunt. Neque porro",
        "quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur,",
        "adipisci velit, sed quia non numquam eius modi tempora incidunt",
        "ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad",
        "minima veniam, quis nostrum exercitationem ullam corporis",
        "suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur?",
        "Quis autem vel eum iure reprehenderit qui in ea voluptate velit",
        "esse quam nihil molestiae consequatur, vel illum qui dolorem eum",
        "fugiat quo voluptas nulla pariatur?" };

    // Match only words that are 11 characters or longer
    final static java.util.regex.Pattern FIND_ME = 
            java.util.regex.Pattern.compile("\\w{11,}");
}