Atomic Variables

In high-concurrency systems, acquiring monitor locks incurs scheduling overhead. Atomic Variables (like AtomicInteger, AtomicLong) use CPU-level Compare-And-Swap (CAS) operations to achieve lock-free thread safety.

Compare-And-Swap (CAS) Mechanism

A CAS operation takes three operands: 1. A memory location (V). 2. The expected old value (A). 3. The new value to write (B).

The CPU atomically updates V to B only if the current value at V equals A. Otherwise, the operation fails. Failed threads retry the operation in loops until they succeed (optimistic locking/spinlocks).

Code Walkthrough

This program compares a thread-safe counter using AtomicInteger against a standard unsynchronized integer.

import java.util.concurrent.atomic.AtomicInteger;
public class AtomicCounterDemo {
    private final AtomicInteger atomicCount = new AtomicInteger(0);
    private int unsafeCount = 0;
public void increment() {
        atomicCount.incrementAndGet(); // Lock-free atomic increment
        unsafeCount++; // Unsafe non-atomic increment
    }
public int getAtomicCount() { return atomicCount.get(); }
    public int getUnsafeCount() { return unsafeCount; }
}

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