What is volatile Keyword in Java?

The volatile keyword in Java is used to ensure visibility of changes to variables across multiple threads. It tells the JVM that a variable’s value may be modified by different threads, so it should always be read from main memory, not from a thread’s local cache.

 volatile Keyword in Java

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🔹 Why do we need volatile?

In multithreading, each thread can keep a local copy of variables in its CPU cache.
This may cause inconsistent data when one thread updates a variable but other threads still see the old value.

👉 volatile solves this visibility problem.

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🔹 Example Without volatile

class Test {
    static boolean flag = true;

    public static void main(String[] args) {
        new Thread(() -> {
            while(flag) {
                // loop runs forever
            }
            System.out.println("Stopped");
        }).start();

        flag = false;
    }
}

❌ Problem:

• The thread may never stop because it reads cached value true.

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🔹 Example With volatile

class Test {
    static volatile boolean flag = true;

    public static void main(String[] args) {
        new Thread(() -> {
            while(flag) {
            }
            System.out.println("Stopped");
        }).start();

        flag = false;
    }
}

✅ Now:

• Every read happens from main memory

• Thread immediately sees updated value.

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🔹 Key Features of volatile

✔ Guarantees visibility between threads
✔ Prevents instruction reordering (partial ordering guarantee)
✔ Lightweight compared to synchronization
❌ Does NOT provide mutual exclusion (no locking)

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🔹 volatile vs synchronized




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🔹 When to Use volatile

✅ Status flags (stop/start signals)
✅ Configuration variables
✅ One writer, multiple readers scenario

❌ Avoid when:

• Multiple operations must be atomic (increment, decrement, etc.)

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🔹 Important Interview Point

volatile ensures visibility, not thread safety.

Example:

volatile int count = 0;
count++;   // NOT thread-safe

Because increment involves multiple steps (read → modify → write).

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