A lot of Java concurrency bugs come from developers reaching for synchronized and manual wait/notify when the standard library already has a purpose-built class for exactly the coordination problem at hand. java.util.concurrent, added in Java 5 and expanded steadily since, covers most of what application code actually needs: thread-safe collections, atomic primitives, higher-level synchronizers, and executor abstractions. Knowing the package well enough to reach for the right piece by name saves you from reinventing a worse version of it.

Atomic Variables Instead of Synchronized Counters

For a single shared counter or flag, a full synchronized block is usually overkill. AtomicInteger, AtomicLong, and AtomicReference provide lock-free, CAS-based (compare-and-swap) updates.

java
private final AtomicLong requestCount = new AtomicLong();

public void recordRequest() {
    requestCount.incrementAndGet();
}
java
private final AtomicReference<Config> currentConfig = new AtomicReference<>(Config.defaults());

public void reload(Config newConfig) {
    currentConfig.set(newConfig); // safe, visible publication to all threads
}

Under contention, CAS-based updates scale better than locks because there's no thread blocking or OS-level context switch — a losing thread just retries. Under heavy contention on a single atomic, LongAdder scales further still, by striping the counter across multiple cells internally and summing them only when you read the total.

Concurrent Collections

ConcurrentHashMap is the default choice for a shared, mutable map accessed by multiple threads — it uses fine-grained internal locking (and lock-free reads) instead of locking the entire map like a synchronized HashMap wrapper would.

java
private final ConcurrentHashMap<String, Session> sessions = new ConcurrentHashMap<>();

public Session getOrCreate(String id) {
    return sessions.computeIfAbsent(id, key -> new Session(key));
}

computeIfAbsent here is atomic per key — two threads racing to create a session for the same ID will not both succeed; one wins and the other gets the winner's result. That guarantee doesn't hold for the naive if (!map.containsKey(id)) map.put(id, new Session(id)) pattern, which has a race window between the check and the put.

Synchronizers for Coordinating Threads

Class Use it when
CountDownLatch One or more threads must wait until N events complete, one-time use
CyclicBarrier A fixed group of threads must all reach a point before any proceeds, reusable
Semaphore Limiting concurrent access to a resource pool to N permits
ReentrantLock Need explicit lock/unlock, tryLock with timeout, or non-block-structured locking

A CountDownLatch is the common one in tests and startup code — waiting for several background initializers to finish before proceeding:

java
CountDownLatch ready = new CountDownLatch(3);
for (Service service : List.of(cache, database, messageBus)) {
    executor.submit(() -> {
        service.warmUp();
        ready.countDown();
    });
}
ready.await(); // blocks until all three have counted down

Semaphore is the right tool when the constraint isn't mutual exclusion but a bounded pool — for example, capping concurrent outbound calls to a downstream service that can't handle unlimited parallel load:

java
private final Semaphore outboundLimit = new Semaphore(20);

public Response call(Request request) throws InterruptedException {
    outboundLimit.acquire();
    try {
        return client.send(request);
    } finally {
        outboundLimit.release();
    }
}

Executors Over Raw Threads

Creating and managing Thread objects directly doesn't scale past a handful of ad hoc tasks. ExecutorService decouples task submission from thread management, and since JDK 21 includes Executors.newVirtualThreadPerTaskExecutor() for I/O-bound workloads alongside the traditional fixed and cached thread pools for CPU-bound work.

The common thread through all of these classes is the same design principle: pick the primitive whose contract matches your actual coordination need, rather than reaching for synchronized and hand-rolled state checks as a default. Each of these was built, tested, and hardened against the exact race conditions that hand-written coordination code tends to reintroduce.