Why Does This Code Throw a RuntimeException Despite Using Volatile?

Why does my Java code throw a RuntimeException even when the volatile keyword is used?

public class VolatileExample {
    private static volatile int counter = 0;
    
    public static void main(String[] args) {
        Thread incrementer = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                counter++; // potential issue here
            }
        });
        incrementer.start();
        try {
            incrementer.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final counter value: " + counter);
    }
}

In Java, the volatile keyword is used to indicate that a variable's value will be modified by different threads. The use of volatile provides visibility of changes to variables across threads but does not guarantee atomicity of compound actions, such as incrementing a counter.

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicExample {
    private static AtomicInteger counter = new AtomicInteger(0);
    
    public static void main(String[] args) {
        Thread incrementer = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                counter.incrementAndGet(); // thread-safe increment
            }
        });
        incrementer.start();
        try {
            incrementer.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final counter value: " + counter.get());
    }
}

Causes

• The operation 'counter++' is not atomic; it includes read, increment, and write operations which are not synchronized.
• Even though 'counter' is declared volatile, other threads may not see the immediate update to the variable before the increment operation completes.

Solutions

• To ensure thread safety during increment operations, use synchronization mechanisms like 'synchronized' blocks or 'AtomicInteger'.
• Replace the 'volatile' declaration with 'AtomicInteger' for atomic increments.

Mistake: Using volatile on a non-atomic operation like increment.

Solution: Replace non-atomic operations with atomic alternatives like AtomicInteger.

Mistake: Assuming volatile ensures atomicity.

Solution: Understand that volatile only ensures visibility, not atomicity; use synchronizers for atomic actions.