Why volatile Is Not Thread‑Safe: A Java Increment Test

This article demonstrates that the volatile keyword in Java does not provide thread‑safety for compound operations such as incrementing a shared counter.

public class VolatileTest {

    private static volatile int k = 0;

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(new Runnable() {
                public void run() {
                    for (int j = 0; j < 1000; j++) {
                        k++;
                    }
                }
            }).start();
        }
        System.out.println(k);
    }
}

Characteristics

The volatile modifier only guarantees that a thread reads the latest value from main memory; it does not make compound actions like k++ atomic.

Applicable Scenario

Suitable when a single thread performs writes while multiple threads only read the variable.