Best Practices for Writing Concurrent Java Code

Concurrency is at the heart of modern software development. Whether you're writing backend services, UI apps, or real-time systems, writing correct and efficient concurrent Java code is crucial for performance, scalability, and maintainability.

In this guide, we’ll walk through the best practices every Java developer should follow when writing multithreaded code—complete with code samples, analogies, and real-world use cases.

🚀 What is Concurrent Programming?

Concurrent programming is the ability to run multiple tasks at overlapping times. In Java, this means using threads to perform parallel computations, process requests, or handle asynchronous events.

It’s about managing task execution, shared resources, and synchronization to achieve higher efficiency and responsiveness.

🔁 Thread Lifecycle Overview

NEW → RUNNABLE → BLOCKED/WAITING → TERMINATED

States

NEW: Thread is created but not started.
RUNNABLE: Eligible for execution.
BLOCKED: Waiting for a lock.
WAITING/TIMED_WAITING: Paused via wait(), sleep(), or join().
TERMINATED: Execution finished or exception thrown.

Understanding these states helps avoid issues like zombie threads, deadlocks, and race conditions.

🧵 Best Practice 1: Prefer `ExecutorService` over Manual Threads

Instead of this:

new Thread(() -> doWork()).start();

Use:

ExecutorService executor = Executors.newFixedThreadPool(4);
executor.submit(() -> doWork());
executor.shutdown();

✅ Benefits:

Thread pooling and reuse
Better control and monitoring
Easier to scale

💥 Best Practice 2: Always Shutdown Executors

Forgetting this causes thread leaks and app hangups.

executor.shutdown(); // graceful
executor.shutdownNow(); // forceful

🧠 Best Practice 3: Avoid Shared Mutable State

Mutable shared data is the root cause of most concurrency bugs.

✅ Use:

AtomicInteger, AtomicReference
Immutable objects
ConcurrentHashMap, CopyOnWriteArrayList

🔒 Best Practice 4: Lock Smartly

Don’t do this:

synchronized (this) { ... }

Do this:

private final Object lock = new Object();
synchronized (lock) { ... }

✅ Use ReentrantLock for more flexibility:

Try-locking
Interruptibility
Fairness policy

👀 Best Practice 5: Understand Memory Visibility

Without volatile, changes made by one thread may not be visible to others.

private volatile boolean running = true;

Or use synchronized to enforce happens-before relationships.

⛔ Best Practice 6: Avoid Busy Waiting

Instead of:

while (!condition) { }

Use BlockingQueue, wait()/notify(), or CountDownLatch.

🧮 Best Practice 7: Favor High-Level Concurrency Utilities

ExecutorService for thread pools
ForkJoinPool for divide-and-conquer
CompletableFuture for async flows
BlockingQueue for producer-consumer
Semaphore, CountDownLatch, CyclicBarrier for coordination

⚠️ Best Practice 8: Handle Exceptions in Threads

Uncaught exceptions silently kill threads.

Thread t = new Thread(task);
t.setUncaughtExceptionHandler((th, ex) -> log(ex));
t.start();

🔀 Best Practice 9: Use Thread-Safe Collections

Don't use ArrayList or HashMap in multithreaded code.

✅ Use:

ConcurrentHashMap
CopyOnWriteArrayList
ConcurrentLinkedQueue

🧵 Best Practice 10: Use Thread Naming and Monitoring

Thread t = new Thread(task, "FileWorker-1");
System.out.println(t.getName());

✅ Helps in debugging and logging.

🧪 Best Practice 11: Test with Concurrency in Mind

Use stress tests
Include timeouts
Test interleavings using tools like JCStress, JMH

🌐 Best Practice 12: Use Virtual Threads for Lightweight Tasks (Java 21+)

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> fetchData());
}

✅ Perfect for I/O-bound operations
❌ Avoid blocking synchronizations inside virtual threads

🧠 Design Patterns That Encourage Safe Concurrency

Pattern	Use Case
Worker Thread	Task queue with a fixed number of workers
Thread-per-message	Spawn thread per event
Future Task	Asynchronous computation with result
Producer-Consumer	Decouple producer and consumer
Balking	Avoid duplicate thread work

📌 What's New in Java Versions?

Java 8

Lambdas for Runnable, Callable
CompletableFuture
parallelStream()

Java 9

Flow API (Reactive Streams)

Java 11

Minor CompletableFuture updates

Java 21

Virtual Threads
Structured Concurrency
Scoped Values

🚫 Common Anti-Patterns

Thread.sleep() for coordination
Not shutting down executor services
Overusing synchronized
Shared mutable state without locks
Unbounded thread creation
Holding locks while doing I/O

🧠 Expert FAQ

Q1: What’s the difference between `synchronized` and `ReentrantLock`?

ReentrantLock provides advanced capabilities like timeout, interruptibility, and fairness policies.

Q2: Why should I avoid `Thread.sleep()`?

It’s not reliable for coordination. Use wait(), join(), or concurrency utilities.

Q3: Can `volatile` ensure atomicity?

No. It ensures visibility, not atomicity. Use atomic classes or locks.

Q4: When should I use virtual threads?

For high-throughput I/O-bound operations with lightweight concurrency needs.

When multiple threads modify variables in the same CPU cache line, degrading performance.

Q6: What is a memory barrier?

A CPU instruction that ensures memory visibility across threads.

Q7: Is `ConcurrentHashMap` 100% safe?

It’s safe for concurrent reads/writes but compound operations still need synchronization.

Q8: How does work-stealing improve performance?

Idle threads steal tasks from busy ones, balancing load dynamically in ForkJoinPool.

Q9: Can I mix virtual threads with executors?

Yes, with Executors.newVirtualThreadPerTaskExecutor().

Q10: Why not call `run()` directly on a thread?

It executes in the current thread instead of starting a new one.

✅ Conclusion and Key Takeaways

Writing safe and efficient concurrent Java code is a matter of understanding the tools, avoiding common pitfalls, and using best practices:

Avoid reinventing thread management—use ExecutorService
Favor immutability and high-level utilities
Always think about visibility and atomicity
Monitor and test for concurrency bugs
Leverage modern Java features like virtual threads and structured concurrency