How does the volatile keyword in Java work?

The volatile keyword in Java ensures all threads "see" the latest changes to a variable by enforcing specific memory visibility rules under the Java Memory Model (JMM).

Main Memory Access: When a variable is declared volatile, all reads and writes to it go directly to main memory, bypassing thread-local caches (e.g., CPU registers or cache memory). Prevents Caching: Threads cannot store a local copy of a volatile variable, ensuring every read retrieves the most recent value written by any thread. Happens-Before Guarantee: The JMM establishes a "happens-before" relationship, where a write to a volatile variable by one thread is guaranteed to be visible to any subsequent read by another thread. Memory Barrier: volatile writes insert a store barrier, flushing changes to main memory, while reads insert a load barrier, ensuring fresh data is retrieved, preventing stale reads. No Locking: Unlike synchronized, volatile doesn’t use locks, making it lightweight but limited to ensuring visibility, not atomicity for compound operations.

This mechanism ensures all threads see the latest value of a volatile variable, making it ideal for simple state flags or single-variable updates across threads.