Wait until child threads completed : Java

Question

Problem description : -

Step 1: Take input FILE_NAME from user at main thread.

Step 2: Perform 10 operations on

Answer 1

You can do:

Thread t = new Thread() {
    public void run() {
        System.out.println("text");
        // other complex code
    }
 };
 t.start();
 t.join();

This way you will wait until the thread finishes and just then continue. You can join multiple threads:

for (Thread thread : threads) {
  thread.join();
}

Answer 2

There are many ways to approach this. Consider CountDownLatch:

import java.util.concurrent.CountDownLatch;

public class WorkerTest {
    final int NUM_JOBS = 3;
    final CountDownLatch countDownLatch = new CountDownLatch(NUM_JOBS);
    final Object mutex = new Object(); 
    int workData = 0;

    public static void main(String[] args) throws Exception {
        WorkerTest workerTest = new WorkerTest();
        workerTest.go();
        workerTest.awaitAndReportData();
    }

    private void go() {
        for (int i = 0; i < NUM_JOBS; i++) {
            final int fI = i;
            Thread t = new Thread() {
                public void run() {
                    synchronized(mutex) {
                        workData++;
                    }
                    try {
                        Thread.sleep(fI * 1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    countDownLatch.countDown();
                }
            };
            t.start();
        }
    }

    private void awaitAndReportData() throws InterruptedException {
        countDownLatch.await();
        synchronized(mutex) {
            System.out.println("All workers done. workData=" + workData);
        }
    }
}

Answer 3

In Java 8 a far better approach is to use parallelStream()

Note: it is far easier to see exactly what these background tasks are doing.

public static void main(String[] args) {
    Stream.<Runnable>of(
         () -> mytest.result.setIntValue(346635),
         () -> mytest.result.setStringValue("Hello hi"),
         () -> mytest.result.setBoolValue(true) )
         .parallel()
         .forEach(Runnable::run);

    System.out.println("main finished");
    System.out.println("Result is : " + mytest.result.toString());
}

I took out the debug information and the sleep as these don't alter the outcome.

Answer 4

Check if all child threads are dead, every n seconds. Simple, yet effective method:

        boolean allDead=false;
        while(! allDead){
            allDead=true;
            for (int t = 0; t < threadCount; t++)
                if(threads[t].isAlive())    allDead=false;
            Thread.sleep(2000);

        }

Answer 5

I would recommend looking at the Executors framework first, and then look into the CompletionService.

Then you can write something like this:

ExecutorService executor = Executors.newFixedThreadPool(maxThreadsToUse);
CompletionService completion = new ExecutorCompletionService(executor);
for (each sub task) {
    completion.submit(new SomeTaskYouCreate())
}
// wait for all tasks to complete.
for (int i = 0; i < numberOfSubTasks; ++i) {
     completion.take(); // will block until the next sub task has completed.
}
executor.shutdown();

Answer 6

You may want to choose CountDownLatch from java.util.concurrent. From JavaDocs:

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

Sample code:

import java.util.concurrent.CountDownLatch;

public class Test {
    private final ChildThread[] children;
    private final CountDownLatch latch;

    public Test() {
        this.children = new ChildThread[4];
        this.latch = new CountDownLatch(children.length);
        children[0] = new ChildThread(latch, "Task 1");
        children[1] = new ChildThread(latch, "Task 2");
        children[2] = new ChildThread(latch, "Task 3");
        children[3] = new ChildThread(latch, "Task 4");
    }

    public void run() {
        startChildThreads();
        waitForChildThreadsToComplete();
    }

    private void startChildThreads() {
        Thread[] threads = new Thread[children.length];

        for (int i = 0; i < threads.length; i++) {
            ChildThread child = children[i];
            threads[i] = new Thread(child);
            threads[i].start();
        }
    }

    private void waitForChildThreadsToComplete() {
        try {
            latch.await();
            System.out.println("All child threads have completed.");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private class ChildThread implements Runnable {
        private final String name;
        private final CountDownLatch latch;

        protected ChildThread(CountDownLatch latch, String name) {
            this.latch = latch;
            this.name = name;
        }

        @Override
        public void run() {
            try {
                // Implementation
                System.out.println(name + " has completed.");
            } finally {
                latch.countDown();
            }
        }
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.run();
    }
}

Output:

Task 1 has completed. Task 4 has completed. Task 3 has completed. Task 2 has completed. All child threads have completed.