Thread safety if loop is thread safe

Question

The while loop in this question is thread safe: AtomicInteger thread safety. If I insert the randomSum method in the thread safe while loop, is the code still thread safe? Or should the randomSum method be synchronized?

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

public class Print implements Runnable {
    static AtomicInteger atomicInteger = new AtomicInteger(0);
    static Random random = new Random(1);

    public static int randomSum() {
        int sum = 0;
        for (int i = 0; i < 10; i++) {
            sum += random.nextInt();
        }
        return sum;
    }

    @Override
    public void run() {
        while (atomicInteger.getAndIncrement() < 100)
            System.out.println(randomSum() + " " + Thread.currentThread());
    }

    public static void main(String[] args) throws InterruptedException {
        ArrayList<Thread> threads = new ArrayList<>();

        for (int i = 0; i < 5; i++)
            threads.add(new Thread(new Print()));

        for (Thread thread : threads)
            thread.start();

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

Yeah, still thread-safe. No need to synchronize this `randomSum` method since you haven't modify any value outside the method. — Sachith Dickwella, Jun 09 '21 at 06:27
@Sachith Dickwella Well - actually `random.nextInt` modifies random internally, but Random is thread safe. — rghome, Jun 09 '21 at 06:33
This is not exactly a duplicate of https://stackoverflow.com/questions/5819638, but if `Random` is thread-safe, then it follows that your `randomSum` method is thread-safe, since `sum` and `i` are local variables and therefore thread-confined. — Stephen C, Jun 09 '21 at 06:43

score 6 · Answer 1 · edited Jun 09 '21 at 06:50

6

The method randomSum is thread-safe, as is its use.

The only shared object is random. The Java API documentation states that instances of java.util.Random are thread-safe but may encounter performance issues if used concurrently (presumably due to synchronization) and that ThreadLocalRandom should be considered instead if performance is an issue.

edited Jun 09 '21 at 06:50

Basil Bourque

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answered Jun 09 '21 at 06:31

rghome

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Thread safety if loop is thread safe

1 Answers1