how can program guarantee executing multiple threads concurrently using executor framework

Question

I am trying to execute 5 threads start perfectly at same time. But looking at output of below I think they started at same time but not concurrently as the age counter always finish with 5 count. If they executed real concurrently the age output must be same for the bunch. By real concurrently Or perfect Same time I think all 5 threads passing 1 as age and all thread prints same age not increment values. Please correct me.

public class ExecutorServiceTester {

public static void main(String args[]) throws InterruptedException {
    ExecutorServiceTester tester = new ExecutorServiceTester();
    tester.executeTester();
}

private void executeTester() throws InterruptedException {
    ExecutorService executorService = Executors.newFixedThreadPool(10);
    Runnable worker = new MyRunnable(1);
    for (int i = 0; i < 10; i++) {
        executorService.execute(worker);
    } executorService.shutdown();
}

public static class MyRunnable implements Runnable {
    int age = 0;

    public MyRunnable(int count) {
        this.age = count;
    }

    @Override
    public void run() {
        System.out.println(new Timestamp(new Date().getTime())+" ThreadName:"+Thread.currentThread().getName()
                + " Age " + age++);

    }
} }

OUTPUT:

2015-03-23 02:02:18.243 ThreadName:pool-1-thread-1 Age 1
2015-03-23 02:02:18.243 ThreadName:pool-1-thread-5 Age 3
2015-03-23 02:02:18.243 ThreadName:pool-1-thread-3 Age 5
2015-03-23 02:02:18.243 ThreadName:pool-1-thread-2 Age 2
2015-03-23 02:02:18.243 ThreadName:pool-1-thread-4 Age 4

While trying with Executors.newCachedThreadPool(); and increased number to 12, the OUTPUT had something of interest then

2015-03-23 02:17:57.189 **ThreadName:pool-1-thread-4 Age 1**
2015-03-23 02:17:57.189 ThreadName:pool-1-thread-10 Age 3
2015-03-23 02:17:57.189 ThreadName:pool-1-thread-12 Age 2
2015-03-23 02:17:57.189 **ThreadName:pool-1-thread-11 Age 1** ...

I don't understand what you mean by If they executed real concurrently. Can you please explain in more detail which part of the output is confusing to you? — Keppil
– Keppil, Commented Mar 22, 2015 at 20:20
You print the name of the current thread next to each line of output, to ensure those lines aren't output from the same thread — Vince
– Vince, Commented Mar 22, 2015 at 20:21
@VinceEmigh added ThreadName, it looks it happened in different threads. — Sankalp
– Sankalp, Commented Mar 22, 2015 at 20:35
@Keppil By real concurrently I meant all threads starting with same initial value , '1' in this example and all threads end with printing 1 but this looks like each thread is reading another thread updates. — Sankalp
– Sankalp, Commented Mar 22, 2015 at 20:38

Keppil · Accepted Answer · 2015-03-22 20:40:17Z

1

The reason all your threads are using the same counter is that you are feeding them the same Runnable instance. Just create a new one for each thread inside the loop instead:

for (int i = 0; i < 10; i++) {
    Runnable worker = new MyRunnable(1);
    executorService.execute(worker);
}

answered Mar 22, 2015 at 20:40

Keppil

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3 Comments

Sankalp Over a year ago

In this case also the threads don't execute at same time. Could you please suggest how to make sure all start and execute at same time. Although your suggestion is correct for counter but I still see them executing at different timestamp with this.

Sankalp Over a year ago

While trying with Executors.newCachedThreadPool(); and increased number to 12, I found 2 threads actually shared same age (Updated my question output )

Keppil Over a year ago

If you want them to wait for each other, I recommend looking at CountDownLatch.

Zaanzabar · Accepted Answer · 2015-03-22 21:08:26Z

It may not be ideal, but if you need them to be perfectly aligned, I would set some type of Mark. Use System.nanoTime() to get a current time down to the nanosecond. Give it a period of time to wait, say a full second (1_000_000_000) nano seconds. Make sure the delay is long enough (whatever it is) for all threads to be initialized and get to waiting point. Pass that MarkTime to each thread on creation. Then have each thread have a function like this:

while(System.nanoTime() < MarkTime){
    // do nothing
}

Boom, when time is reached, all threads are off and running, coordinated down to the nanosecond.

Is like runners getting ready at the starting line for a race. They all arrive at the line at different times, but all start at the same starters pistol shot.

Also, as @Keppil said, you probably want to create a new instance of the thread for each version, not 5 copies of the same instance. That way you can be sure they run independently, yet at the same time.

Alternatively: Another way to do so, is to set a boolean in the parent class, set it to false, after all threads are active, have them check that boolean. When all are loaded and running, change boolean to true. Same effect, but maybe with a slight drop in precision. How close do you need it?

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