Q50: How do Atomic variables (AtomicInteger) achieve lock-free thread safety using CAS?
Classes in java.util.concurrent.atomic (AtomicInteger, AtomicReference) provide thread-safe operations without locks using Compare-And-Swap (CAS) hardware instructions (CMPXCHG).
- CAS Loop: Reads expected value $E$, computes new value $V$. Invokes atomic CAS CPU instruction. If memory location still matches $E$, updates to $V$. If another thread modified the value, CAS fails and loops to retry.
Key Takeaways
- Lock-free thread safety using native CPU CAS instructions.
- Eliminates thread context-switching overhead under low/moderate contention.
Knowledge Check
What low-level CPU instruction enables lock-free AtomicInteger operations?