What is the purpose of the volatile keyword in Java?

Question

Ravi Vishwakarma · Answer

The volatile keyword in Java is used to indicate that a variable's value will be modified by different threads. It ensures that changes to the variable are always visible to other threads, providing a lightweight synchronization mechanism.
Here's a detailed explanation of its purpose and usage:
Visibility: When a variable is declared as volatile, it guarantees that any thread that reads the field will see the most recently written value. This is because the value of a
volatile variable is always read from and written to the main memory, bypassing the local cache.
Atomicity: The volatile keyword ensures visibility but does not guarantee atomicity. Operations on
volatile variables are not atomic, which means operations like incrementing a variable (count++) are not thread-safe with
volatile alone. If you need atomicity, you should use synchronized blocks or classes from the
java.util.concurrent.atomic package (e.g., AtomicInteger).
Instruction Reordering: The Java Memory Model allows the JVM to reorder instructions for performance optimization. However,
volatile prevents certain types of reordering, ensuring a happens-before relationship between the write and read of the
volatile variable. This means that changes made by one thread before writing to a
volatile the variable is visible to other threads that subsequently read that
volatile variable.
Example -
public class VolatileExample {
    private volatile boolean running = true;
    public void stop() {
        running = false;
    }
    public void run() {
        while (running) {
            // perform some work
        }
    }
    public static void main(String[] args) {
        VolatileExample example = new VolatileExample();
        Thread t1 = new Thread(example::run);
        t1.start();
        // Let the thread run for a while
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // Stop the running thread
        example.stop();
    }
}
When to Use volatile
Use volatile when you need to ensure visibility of changes to variables across threads, but only if:
The variable is accessed by multiple threads.The variable is not involved in compound actions (like count++), which require atomicity.You need to avoid the overhead of synchronization for simple flags or state indicators.
and

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What is the purpose of the volatile keyword in Java?

Anubhav Kumar

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When to Use `volatile`

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