How does CAS work inside AtomicInteger? What happens if the value changes between reading and writing? When does AtomicInteger perform poorly under high contention?

Inside AtomicInteger, a CAS (Compare-And-Swap) operation is used, which atomically compares the current value with an expected value and, if they match, updates the value to a new one. This is implemented at the processor level using special instructions that ensure atomicity.

If the value changes between reading and writing (i.e., the current value does not match the expected), CAS returns false, and the operation is retried (usually in a loop) until the update succeeds.

Under high contention (many threads trying to update the value simultaneously), CAS can perform poorly due to frequent failed attempts, leading to "spin-locks" and increased CPU load. In such cases, it is better to use more complex synchronization structures or algorithms with lower conflict levels.