I started okay on visibility, said something about flushing to main memory and every thread reading fresh values.
Start by defining volatile's core guarantee: visibility of writes and reads to that variable across threads, and the happens-before ordering it establishes. Then explain how it restricts reordering, and contrast with synchronized/locks by highlighting atomicity and mutual exclusion gaps. Use a concrete example like a flag or double-checked locking to illustrate.
Pro tip: Emphasize that volatile is not a substitute for synchronization when compound actions or multiple variables are involved, and mention that it's often used for status flags or one-time publication. Showing awareness of the Java Memory Model's happens-before rules and the cost trade-offs (volatile vs. locks) demonstrates depth.
Explain that a write to a volatile variable is immediately visible to other threads, and a read always sees the most recent write. This prevents stale cached values in CPU registers or caches.
Describe how volatile establishes a happens-before relationship: a write to a volatile field happens-before every subsequent read of that field. This ensures that actions before the write are visible after the read.
Mention that volatile prevents certain instruction reorderings: reads/writes cannot be reordered with volatile accesses in ways that break the happens-before guarantee. This acts as a memory barrier.
Point out that volatile does not provide atomicity for compound operations (e.g., i++), nor mutual exclusion. Synchronized/locks provide both visibility and atomicity, and can protect multiple variables.
Use examples like a volatile boolean flag to stop a thread, or double-checked locking (with volatile) for lazy initialization. Discuss performance: volatile is lighter than locks but limited in scope.
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