What is the volatile keyword useful for?
How does the volatile keyword work in Java, and what makes it important in multithreading? What problems does it solve when multiple threads access and update the same variable?
The volatile keyword in Java is mainly used in multithreading to ensure visibility of changes to variables across different threads. Normally, when multiple threads work on a shared variable, each thread may cache a copy of that variable. This can lead to inconsistencies because one thread might not immediately see the latest value updated by another thread. That’s where volatile comes in.
When you declare a variable as volatile, it tells the JVM and compiler:
- Always read the variable’s value directly from the main memory.
- Always write updates to the variable back to the main memory immediately.
- This ensures that all threads are working with the most recent and consistent value.
Some key points about volatile:
- Visibility Guarantee: It ensures visibility but not atomicity. For example, volatile int count guarantees that threads see the latest count, but operations like count++ are not atomic.
- Use Case: Useful for flags, state checks, or simple status updates shared across threads (e.g., a volatile boolean isRunning to stop a thread).
- Performance: Lighter than using synchronized, since it avoids full locking but still ensures visibility.
- Limitation: Does not prevent race conditions in compound operations (like incrementing). For those, synchronization or classes from java.util.concurrent.atomic are better.
- In short, volatile is useful when you need thread-safe visibility of a variable’s value, but not when complex operations or atomicity are required. It’s a simple yet powerful tool for multithreaded programming.