volatile Keyword in Java

Direct Answer

volatile gives a field Java Memory Model visibility and ordering guarantees. A write to a volatile field happens-before a later read that observes it, so it can safely publish earlier writes. It does not make compound actions such as count++ atomic, and Java does not define it as bypassing CPU caches or always reading physical main memory.

Code Example

class TaskRunner implements Runnable {
    private volatile boolean running = true;

    @Override
    public void run() {
        while (running && !Thread.currentThread().isInterrupted()) {
            processOneUnit();
        }
    }

    void stop() {
        running = false;
    }
}

A volatile write publishes the new flag value under the Java Memory Model. It does not promise instant scheduling or wake a thread blocked in I/O; interruption or a resource-specific cancellation action may also be needed.

Common Mistakes & Pitfalls

• Using volatile for count++: the compound read-modify-write can lose updates.
• Calling volatile a lightweight lock: it provides no mutual exclusion.
• Explaining it as physical main-memory access: the JMM promises ordering and visibility, not one cache algorithm.
• Assuming one writer is a language requirement: several writers are valid when plain last-write-wins semantics are enough.
• Expecting a flag to unblock I/O: a volatile value does not itself wake a blocked operation.