How and when should we use `AtomicReference` in Java?

General Discussions
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1

Is java AtomicReference necessary for all multithreaded programs, or are there specific scenarios where it is most useful? Can you provide a simple example demonstrating its proper usage?

2

Great question! So, I’ve been working with java AtomicReference for a while now, and I can tell you that it’s one of those tools that you may not always need, but when you do, it makes a big difference. It’s especially useful in concurrent programming where you need to safely update reference types (like objects) without locking. Let’s walk through a basic example where multiple threads need to safely update a shared object without using synchronization.

Here’s a simple example of how you might use AtomicReference for atomic updates:

import java.util.concurrent.atomic.AtomicReference;

class SharedData {
    private final String value;
    
    public SharedData(String value) {
        this.value = value;
    }

    public String getValue() {
        return value;
    }
}

public class AtomicReferenceExample {
    public static void main(String[] args) {
        AtomicReference<SharedData> atomicRef = new AtomicReference<>(new SharedData("Initial"));

        Runnable updateTask = () -> {
            SharedData oldValue, newValue;
            do {
                oldValue = atomicRef.get();
                newValue = new SharedData(oldValue.getValue() + " -> Updated");
            } while (!atomicRef.compareAndSet(oldValue, newValue)); 

            System.out.println(Thread.currentThread().getName() + " updated value to: " + atomicRef.get().getValue());
        };

        Thread t1 = new Thread(updateTask);
        Thread t2 = new Thread(updateTask);

        t1.start();
        t2.start();
    }
}

Why use this?

  • It ensures thread-safe updates without locking mechanisms.
  • By using compareAndSet(), it prevents race conditions.
  • Perfect for cases like caching and state management where multiple threads may modify an object concurrently.

When NOT to use this:

  • If only one thread is modifying the reference, AtomicReference isn’t necessary.
3

Great follow-up! Yanisleidi covered some good ground with thread-safe object updates. Another use case for java AtomicReference that I often come across is when you’re dealing with immutable objects. You might not need to modify an object often, but when you do, you want it to be atomic and lock-free, which is exactly where AtomicReference shines. Here’s an example that shows how to use it for managing an immutable configuration object:

import java.util.concurrent.atomic.AtomicReference;

class Config {
    private final String setting;

    public Config(String setting) {
        this.setting = setting;
    }

    public String getSetting() {
        return setting;
    }
}

public class ImmutableConfigManager {
    private final AtomicReference<Config> configRef = new AtomicReference<>(new Config("Default"));

    public void updateConfig(String newSetting) {
        configRef.set(new Config(newSetting)); // No locking needed
    }

    public String getCurrentConfig() {
        return configRef.get().getSetting();
    }

    public static void main(String[] args) {
        ImmutableConfigManager manager = new ImmutableConfigManager();

        System.out.println("Before update: " + manager.getCurrentConfig());
        manager.updateConfig("New Setting");
        System.out.println("After update: " + manager.getCurrentConfig());
    }
}

Why use this?

  • It prevents an inconsistent state by always replacing the entire object atomically.
  • Lock-free updates mean it’s faster in concurrent applications.
  • Ideal for scenarios like configuration management where changes need to be atomic.

When NOT to use this:

  • If immutability isn’t a requirement, you could just use volatile fields instead.
4

Exactly, Tim! The example you showed is perfect for configurations, but java AtomicReference can be just as useful when you’re managing complex custom objects. For instance, let’s say you’re building a concurrent counter that tracks some custom state. Unlike primitive types, these custom objects need atomic updates as well. You can use AtomicReference for that too, just like this:

import java.util.concurrent.atomic.AtomicReference;

class Counter {
    private final int count;

    public Counter(int count) {
        this.count = count;
    }

    public Counter increment() {
        return new Counter(count + 1);
    }

    public int getCount() {
        return count;
    }
}

public class AtomicReferenceCounter {
    public static void main(String[] args) {
        AtomicReference<Counter> counterRef = new AtomicReference<>(new Counter(0));

        Runnable task = () -> {
            for (int i = 0; i < 5; i++) {
                Counter prev, next;
                do {
                    prev = counterRef.get();
                    next = prev.increment();
                } while (!counterRef.compareAndSet(prev, next));
                
                System.out.println(Thread.currentThread().getName() + " -> Counter: " + next.getCount());
            }
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);

        t1.start();
        t2.start();
    }
}

Why use this?

  • It’s a great way to implement atomic updates for custom objects like a counter.
  • Provides a lock-free alternative to synchronized methods.
  • Useful when multiple threads need to safely modify shared objects.

When NOT to use this?

  • If you’re just updating a simple primitive type like an int, then an AtomicInteger is probably more appropriate. AtomicReference is really useful when you’re dealing with objects.