CODEWITHSUNDEEP
javamultithreadingthread-safety
kobra
kobra

Reputation: 1315

Strange behaviour of synchronized

class TestSync {
public static void main(String[] args) throws InterruptedException {
    Counter counter1 = new Counter();
    Counter counter2 = new Counter();
    Counter counter3 = new Counter();
    Counter counter4 = new Counter();

    counter1.start();
    counter2.start();
    counter3.start();
    counter4.start();

    counter1.join();
    counter2.join();
    counter3.join();
    counter4.join();

    for (int i = 1; i <= 100; i++) {
        if (values[i] > 1) {
            System.out.println(String.format("%d was visited %d times", i, values[i]));
        } else if (values[i] == 0) {
            System.out.println(String.format("%d wasn't visited", i));
        }
    }
}

public static Integer count = 0;
public static int[] values = new int[105];

static {
    for (int i = 0; i < 105; i++) {
        values[i] = 0;
    }
}

public static void incrementCount() {
    count++;
}

public static int getCount() {
    return count;
}

public static class Counter extends Thread {
    @Override
    public void run() {
        do {
            synchronized (count) {
                incrementCount();

                values[getCount()]++;
            }
        } while (getCount() < 100);
    }
}

}

That is a code from one online course. My task is to make this code visit each element of array only once (only for elements from 1 to 100). So I have added simple synchronized block to run method. In case of using values inside of that statement everything works. But with count it doesn't want to work.

What the difference? Both of this objects are static fields inside of the same class. Also I have tried to make count volatile but it hasn't helped me.

PS: a lot of elements are visited 2 times and some of them even 3 times. In case of using values in synchronized all elements are visited only once!!!

Upvotes: 0

Views: 50

Answers (1)

SMA
SMA

Reputation: 37023

Integer is immutable. The moment you call increment method, You get a new object and reference of count variable gets changed and hence leads to an issue.

Upvotes: 2

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