What is volatile keyword in Java
💡 Concept: Volatile Keyword
The volatile keyword in Java ensures that changes to a variable are always visible to all threads and prevents instruction reordering by the compiler or CPU.
📘 Quick Intro
Marking a variable as volatile ensures it is read from and written directly to main memory, making its latest value visible to all threads instantly.
🧠 Analogy
Imagine a public notice board. If someone writes something on it, anyone passing by can immediately read the latest note. That's what volatile does—it makes sure everyone sees the same, most recent value.
🔧 Technical Explanation
- Ensures visibility of updates to a variable across threads.
- Prevents JVM and CPU from reordering reads/writes for that variable.
- Does NOT guarantee atomicity (e.g., ++var is still unsafe).
- Best used for flags, state indicators, or read-write-only variables.
- Often used in conjunction with
whileloops for polling.
🎯 Use Cases
- ✅ Flags to stop background threads (e.g.,
isRunning). - ✅ Simple state indicators shared across threads.
- ✅ Avoiding stale reads due to caching in multi-core systems.
💻 Example: Volatile Variable
public class VolatileExample {
private static volatile boolean flag = false;
public static void main(String[] args) {
new Thread(() -> {
while (!flag) {
// wait
}
System.out.println("Flag set!");
}).start();
try { Thread.sleep(1000); } catch (InterruptedException e) {}
flag = true;
}
}
❓ Interview Q&A
Q1: What is the purpose of volatile?
A: Ensures visibility of changes across threads.
Q2: Does volatile guarantee atomicity?
A: No.
Q3: Can volatile prevent instruction reordering?
A: Yes.
Q4: Is volatile useful in multithreading?
A: Yes.
Q5: Can volatile be used for compound operations?
A: No, use synchronization instead.
Q6: What happens if you don’t use volatile where needed?
A: Threads may see stale values.
Q7: Can static variables be volatile?
A: Yes.
Q8: Is volatile a memory barrier?
A: It behaves like one for that variable.
Q9: Is volatile available since Java 1?
A: Yes.
Q10: Is volatile faster than synchronized?
A: Yes, but use is limited to specific cases.
📝 MCQs
Q1. What does volatile ensure?
- Atomicity
- Blocking
- Visibility across threads
- Garbage collection
Q2. Does volatile prevent instruction reordering?
- No
- Yes
- Only in Java 5
- Sometimes
Q3. Is volatile good for compound operations?
- Yes
- No
- Sometimes
- Depends
Q4. Can you use volatile for flags?
- No
- Yes
- Sometimes
- Not recommended
Q5. Is volatile keyword thread-safe?
- Yes
- No
- Partially
- Never
Q6. What happens without volatile in multithreaded flags?
- Compile error
- Stale reads
- Always visible
- Infinite loops
Q7. Which is stronger, synchronized or volatile?
- volatile
- final
- synchronized
- transient
Q8. Does volatile affect caching?
- No
- Yes
- Sometimes
- Only on Windows
Q9. Can static fields be volatile?
- No
- Yes
- Only for objects
- Only after Java 11
Q10. Which memory area does volatile interact with?
- Stack
- Thread-local
- Main memory
- Code cache
💡 Bonus Insight
Volatile is often misunderstood. While it provides visibility and ordering guarantees, it is not a replacement for synchronization when atomic operations are required.
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