How do atomic variables and compare-and-set work in Java?

Explains optimistic atomic updates, retries, contention, immutable state replacement, and the boundary of single-variable safety.

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An atomic variable provides thread-safe operations on one value without requiring callers to hold an explicit lock. Compare-and-set checks whether the current value equals an expected value and replaces it only if the check succeeds as one atomic operation. If another thread changed the value first, the update fails and code can retry using the new value. Methods such as incrementAndGet build on atomic read-modify-write behavior.

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CAS semantics, atomic read-modify-write, retry behavior, and restraint around lock-free designs.

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Overview
Compare-and-set changes a value only if it still matches the expected value observed by the caller.
An atomic variable provides thread-safe operations on one value without requiring callers to hold an explicit lock. Compare-and-set checks whether the current value equals an expected value and replaces it only if the check succeeds as one atomic operation. If another thread changed the value first, the update fails and code can retry using the new value. Methods such as incrementAndGet build on atomic read-modify-write behavior.
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An AtomicReference<State> can replace an immutable state snapshot, allowing several fields to change together because the reference update is one operation. The transformation used in a retry loop should be free of external side effects because it may execute more than once. Heavy contention can cause repeated retries and wasted CPU. Atomics do not make mutable objects stored inside them safe unless updates replace or otherwise coordinate that object correctly.

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Adding side effects inside a retrying update function.
Assuming an AtomicReference makes its object immutable.
Ignoring retry cost under contention.

How do atomic variables and compare-and-set work in Java?

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