Java multi threading when multiple threads updating same variable

Question

This is the program

public class Thread2 implements Runnable {

    private static int runTill = 10000;
    private static int count = 0;
    @Override
    public void run() {
        for(int i=0;i<runTill;i++) {
            count++;
        }
    }
    
    public static void main(String s[]) {
        int iteration = 10;
        for(int i = 0; i < iteration ;i++) {
            Thread t = new Thread(new Thread2());
            t.start();
        }
        
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        
        System.out.println("Expected : "+(iteration * runTill));
        System.out.println("Actual : "+count);
    }

}

At the end I want count to be equal to (Expected : 100000). How can I achieve this?

Probably the smartest way is to use AtomicInteger.getAndIncrement(). docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/… Is there a reason why you are not using the java.util.concurrent package API? — markspace
– markspace, Commented Jan 29, 2021 at 15:06
Besides AtomicInteger, you can also consider synchronization and locks. especially if you want to manage more than just an integer (see winterbe.com/posts/2015/04/30/…) — Angel Koh
– Angel Koh, Commented Jan 29, 2021 at 15:08
Also, don't simply sleep to wait for the thread to complete: put the threads into a list, and use Thread.join() to wait for their completion. — Andy Turner
– Andy Turner, Commented Jan 29, 2021 at 15:15
Yes, I want to reinforce "don't use sleep()". Calling sleep like this to wait for threads is a terrible idea. Please call Thread.join() on each thread, or use something like a CountDownLatch — markspace
– markspace, Commented Jan 29, 2021 at 15:18
@Abra No, I think the OP is asking how to update a shared variable. His final result (100,000) is too large to be one single thread updating a instance variable 10,000 times. — markspace
– markspace, Commented Jan 29, 2021 at 15:35

M A · Accepted Answer · 2021-01-29 15:06:20Z

2

A call to count++ is not atomic: it first has to load count, increment it and then store the new value in the variable. Without synchronization in place, threads will interleave during execution of this operation.

A simple way to get what you want is to use an AtomicInteger:

private static AtomicInteger count = new AtomicInteger();

@Override
public void run() {
    for(int i=0;i<runTill;i++) {
        count.incrementAndGet();
    }
}

answered Jan 29, 2021 at 15:06

M A

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Comments

Angel Koh · Accepted Answer · 2021-01-29 15:17:27Z

0

use "compare and set" instead of "increment and get"

private static AtomicInteger count = new AtomicInteger();

@Override
public void run() {
    for(int i=0;i<runTill;i++) {

        //note: another thread might reach this point at the same time when i is 9,999
        // (especially if you have other codes running prior to the increment within the for loop)
        // then count will be added 2x if you use incrementAndGet
        boolean isSuccessful = count.compareAndSet(i, i+1);

        if(!isSuccessful) 
            System.out.println("number is not increased (another thread already updated i)");
    }
}

answered Jan 29, 2021 at 15:17

Angel Koh

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1 Comment

markspace Over a year ago

I don't think compare and set is a good match to the OP's (implied) use case. Also this is not a great way to handle an unsuccessful update.

tevemadar · Accepted Answer · 2021-01-29 15:33:51Z

As the comments suggest, besides the need for synchronizing access (to count, became an AtomicInteger here), threads should be waited to complete using Thread.join(), instead of "guessing" their runtime:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

public class Thread2 implements Runnable {

    private static int runTill = 10000;
    private static AtomicInteger count = new AtomicInteger();

    @Override
    public void run() {
        for (int i = 0; i < runTill; i++) {
            count.incrementAndGet();
        }
    }

    public static void main(String s[]) {
        int iteration = 10;
        List<Thread> threads = new ArrayList<Thread>();
        for (int i = 0; i < iteration; i++) {
            Thread t = new Thread(new Thread2());
            threads.add(t);
            t.start();
        }

        try {
            for (Thread t : threads)
                t.join();
        } catch (InterruptedException ie) {
            ie.printStackTrace();
        }

        System.out.println("Expected : " + (iteration * runTill));
        System.out.println("Actual : " + count);
    }
}

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