Understanding Java CAS and AQS: Unlock Lock-Free Concurrency

CAS and AQS are frequently encountered in Java multithreaded programming and are often examined by major tech companies; the following provides a detailed explanation of both.

CAS

CAS stands for Compare and Swap, an atomic operation used in concurrent programming to achieve synchronization and thread safety.

CAS operations typically involve three parameters: the variable V to be modified, the expected old value A, and the new value B.

When updating a variable, the value is changed to B only if the current value at address V equals the expected value A.

CAS Implementation Principle

The implementation principle is shown in the diagram below:

The diagram shows two threads attempting to modify the value simultaneously using CAS.

Specific Process

In memory address V, a variable with value 7 is stored.

Thread 1 wants to increment the variable; its expected value A=7 and new value B=8.

Thread 2 preemptively updates the memory value to 8.

Thread 1 attempts to commit the update, compares expected A=7 with actual V=8, finds a mismatch, and the update fails.

AQS

AbstractQueuedSynchronizer (AQS) provides a unified low‑level framework for Java’s concurrent synchronizers.

Common synchronizers such as ReentrantLock, Semaphore, CountDownLatch, CyclicBarrier, and ReentrantReadWriteLock are built on AQS.

AQS Principle

The principle is illustrated in the diagram below:

AQS is implemented based on a doubly‑linked list where each node represents a waiting thread.

Main Ideas of AQS

When multiple threads contend for the same resource, only one can acquire it while others are blocked.

After the owning thread releases the resource, other threads can compete for it again.

AQS uses CAS operations and volatile variables to ensure thread‑safety.

Differences Between CAS and AQS

Both CAS and AQS are essential mechanisms for thread safety in Java concurrency, but CAS focuses on lock‑free algorithms, whereas AQS is primarily used to implement locks and higher‑level synchronization constructs.