What is the volatile keyword useful for?

java

memory-model

concurrency

thread-safety

byAlex Kataev·Sep 24, 2024

The volatile keyword in Java ensures changes to a variable are immediately visible to all threads, mandating real-time synchronicity spanning all threads. It serves as a lightweight synchronization technique to avert the local caching of a variable by a thread's local memory.

public class FlagController {
    volatile boolean flag = false;

    public void raiseFlag() {
        flag = true;  // Making sure all threads fly with the same flag
    }

    public boolean isFlagRaised() {
        return flag;  // Peeking at the current flag status in real-time
    }
}

Unravelling volatile

Let's dive into the workings of volatile. The Java memory model dictates the regulations for thread interactions with memory. When using volatile variables, it assures visibility of writes to variables by other threads.

Volatile: Gain with little Pain

Consistent Visibility: Ensures that a field is consistently updated visible through all threads.
Avoiding Obsolete caches: In the case of a volatile field, it's always read directly from the main memory, preventing threads from storing the field's value.
Synchronized Lite: volatile provides a simpler, lightweight alternative to full-blown synchronization blocks for certain patterns like single writes and multiple reads.

However, volatile isn't the universal fix for all concurrency dilemmas. volatile can't replace synchronized methods or blocks when operations must be accomplished atomically.

Volatile: Know where to Pull the Trigger

Signal Firing: Control a thread by resetting a volatile boolean flag.
Singleton Order: Regulate instance creation in singleton designs via double-checked locking.
Checks, not Balances: Verify the running state of an operation or resource without disrupting the flow.

Volatile: Watch Your Step

Atomic Absence: volatile doesn't provide atomicity for combined actions (e.g., incrementing a value).
Performance Misconception: volatile does have a cost associated; it might affect performance because it often humors certain compiler optimizations.

Volatile for thread communication

By providing memory visibility, volatile is vital for thread communication. Lack of it might lead to stale data views and incorrect behavior. Besides, volatile acts as a 'memory barrier', forcing all the reads and writes pre and post volatile access to be performed in the specified order.

Threads, Volatile and you

volatile helps in implementing fail-safe mechanisms like thread communication using a volatile counter. It reduces the complexities associated with locks when updates aren't necessarily in order.

Pacing forward with alternatives

While volatile is handy, for robust concurrency control situations, it's wise to explore the java.util.concurrent package for classes that offer atomic operations.

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