Java Academy · Lesson

Atomic Variables: Lock-Free Updates

Use AtomicInteger, AtomicLong, and AtomicReference for thread-safe counters without locks.

Lesson 3 of 413 steps

Atomic Variables: Lock-Free Updates is a free Java Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, and your progress syncs across the web and the CoddyKit app. The Java Academy course includes 4 lessons in total.

The Problem with Non-Atomic Operations

Simple increment count++ is NOT thread-safe — it's three operations: read, add, write. Concurrent threads can interleave these, causing lost updates. Atomic variables solve this without locks.

// UNSAFE:
int count = 0;
// Thread A reads count=0, Thread B reads count=0
// Both write 1 — second update lost!
count++; // NOT thread-safe

// SAFE:
import java.util.concurrent.atomic.*;
AtomicInteger atomicCount = new AtomicInteger(0);
atomicCount.incrementAndGet(); // atomic, lock-free

AtomicInteger Basics

AtomicInteger provides integer operations that are guaranteed to be atomic using CPU compare-and-swap (CAS) instructions — no locks needed.

AtomicInteger counter = new AtomicInteger(0);

counter.set(10);                   // set to 10
System.out.println(counter.get()); // 10

int prev = counter.getAndIncrement(); // returns 10, sets to 11
int curr = counter.incrementAndGet(); // sets to 12, returns 12
counter.addAndGet(5);                 // adds 5, returns 17
counter.decrementAndGet();            // subtracts 1, returns 16

compareAndSet: The CAS Operation

compareAndSet(expected, update) atomically updates the value only if it currently equals the expected value. Returns true if successful. This is the foundation of all lock-free algorithms.

AtomicInteger ai = new AtomicInteger(5);

boolean updated = ai.compareAndSet(5, 10); // current==5? set to 10
System.out.println(updated); // true
System.out.println(ai.get()); // 10

boolean failed = ai.compareAndSet(5, 20); // current==10, not 5 → fails
System.out.println(failed); // false
System.out.println(ai.get()); // still 10

AtomicLong for Counters

AtomicLong is the 64-bit equivalent. Common for sequence generators and hit counters:

AtomicLong seq = new AtomicLong(0);

// Thread-safe sequence generator:
long nextId() {
    return seq.incrementAndGet();
}

System.out.println(nextId()); // 1
System.out.println(nextId()); // 2
System.out.println(nextId()); // 3

AtomicBoolean

Thread-safe boolean flag. Commonly used for one-time initialization or shutdown signals:

AtomicBoolean initialized = new AtomicBoolean(false);

void initOnce() {
    // compareAndSet: only first caller succeeds
    if (initialized.compareAndSet(false, true)) {
        System.out.println("Initializing...");
        // perform expensive init
    } else {
        System.out.println("Already initialized");
    }
}

AtomicReference

AtomicReference<V> provides atomic updates to an object reference. Used to implement lock-free data structures:

import java.util.concurrent.atomic.*;

record Config(String host, int port) {}

AtomicReference<Config> configRef =
    new AtomicReference<>(new Config("localhost", 8080));

// Hot-swap the configuration atomically:
Config oldConfig = configRef.get();
Config newConfig = new Config("prod.example.com", 443);

boolean swapped = configRef.compareAndSet(oldConfig, newConfig);
System.out.println(swapped); // true
System.out.println(configRef.get()); // Config[host=prod.example.com, port=443]

updateAndGet / getAndUpdate

Apply a function atomically using a CAS retry loop internally:

AtomicInteger value = new AtomicInteger(10);

// Apply function atomically: square the value
int newVal = value.updateAndGet(x -> x * x);
System.out.println(newVal); // 100

// Get old, then apply:
int old = value.getAndUpdate(x -> x + 1);
System.out.println(old);   // 100
System.out.println(value.get()); // 101

accumulateAndGet

Combine current value with a given value using a function:

AtomicInteger max = new AtomicInteger(0);

// Keep running maximum:
max.accumulateAndGet(42, Math::max); // max(0, 42) = 42
max.accumulateAndGet(15, Math::max); // max(42, 15) = 42
max.accumulateAndGet(99, Math::max); // max(42, 99) = 99

System.out.println(max.get()); // 99

LongAdder for High-Contention Counters

Under high thread contention, AtomicLong's CAS retries can hurt performance. LongAdder uses multiple cells to reduce contention:

import java.util.concurrent.atomic.*;

LongAdder adder = new LongAdder();

// Multiple threads can call add() with minimal contention:
adder.increment();
adder.add(5);
adder.increment();

System.out.println(adder.sum()); // 7
// Note: sum() is NOT atomic with add() — use only when no concurrent adds

Atomic Arrays

AtomicIntegerArray / AtomicLongArray provide atomic operations on individual array elements:

AtomicIntegerArray arr = new AtomicIntegerArray(5);

arr.set(0, 10);
arr.incrementAndGet(0);
arr.compareAndSet(2, 0, 42); // index 2: if 0, set 42

System.out.println(arr.get(0)); // 11
System.out.println(arr.get(2)); // 42

When to Use Atomics vs Locks

Use atomic variables when:

Single-variable updates (counters, flags, references)
Low contention scenarios
Lock-free performance is important

Use locks when:

Multiple variables must be updated atomically
Complex conditional logic is needed
Very high contention (CAS retries hurt)

Quick Check

What does compareAndSet(expected, update) guarantee?

Answer it in the interactive lesson

Recap: Atomic Variables

Key takeaways:

Atomic types use CPU CAS instructions — lock-free and thread-safe
AtomicInteger/Long/Boolean/Reference for single-variable atomicity
compareAndSet is the foundation of lock-free algorithms
updateAndGet/accumulateAndGet for functional updates
LongAdder for high-contention counter scenarios

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Frequently Asked Questions

Is the “Atomic Variables: Lock-Free Updates” lesson free?

Yes — the full text of “Atomic Variables: Lock-Free Updates” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO. The Java Academy course includes 4 lessons in total.

What will I learn in “Atomic Variables: Lock-Free Updates”?

Use AtomicInteger, AtomicLong, and AtomicReference for thread-safe counters without locks. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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All lessons in this course

ReentrantLock vs synchronized
ReadWriteLock for Reader-Writer Scenarios
Atomic Variables: Lock-Free Updates
StampedLock and Optimistic Reads

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