When do you use volatile, synchronized, or atomic variables in Java?

Chooses visibility, mutual exclusion, or atomic read-modify-write based on the invariant instead of treating the tools as interchangeable.

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TL;DR

volatile gives visibility and ordering for reads and writes of one field, but it does not make compound operations such as increment atomic. synchronized provides mutual exclusion plus visibility for all state consistently protected by the same monitor. Atomic classes provide operations such as compare-and-set and increment as indivisible updates on one variable. Choose the mechanism that covers the entire state transition.

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Visibility, ordering, atomicity, compound state, and proportionate synchronization choice.

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Core answer
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The decision depends on whether the code publishes one value, updates one atomic variable, or protects a compound invariant.
volatile gives visibility and ordering for reads and writes of one field, but it does not make compound operations such as increment atomic. synchronized provides mutual exclusion plus visibility for all state consistently protected by the same monitor. Atomic classes provide operations such as compare-and-set and increment as indivisible updates on one variable. Choose the mechanism that covers the entire state transition.
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A volatile boolean can be appropriate for a published stop flag when one write represents the whole state. handles a counter update without a lock, but two atomics do not automatically preserve an invariant between them. A bank transfer or state machine transition usually needs one lock, immutable replacement through an atomic reference, or another coordinated design. still loses updates.

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Details

Assuming volatile increment is atomic.
Using separate atomics for one compound invariant.
Treating atomics as always faster than locks.

When do you use volatile, synchronized, or atomic variables in Java?

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