Multithreading is a powerful feature in Java that enables developers to build responsive, scalable, and high-performance applications. However, it’s also one of the most misunderstood and error-prone areas of Java development.
In this tutorial, we’ll explore the most common mistakes developers make when working with threads and concurrency in Java—and how to avoid them using best practices, Java’s concurrency utilities, and clear design patterns.
🚦 What is Multithreading in Java?
Multithreading refers to the concurrent execution of two or more threads (lightweight processes) to maximize CPU utilization. It's widely used in scenarios like:
- Background processing
- Web servers handling multiple requests
- GUI responsiveness
- Parallel data processing
Java provides built-in support for multithreading via the Thread class and Runnable, and advanced support through java.util.concurrent.
🔄 Thread Lifecycle Overview
Understanding the thread lifecycle is crucial to avoid bugs like improper synchronization or zombie threads.
NEW → RUNNABLE → BLOCKED/WAITING/TIMED_WAITING → TERMINATED
- NEW: Thread is created but not started.
- RUNNABLE: Thread is eligible for execution.
- BLOCKED: Waiting for monitor lock.
- WAITING/TIMED_WAITING: Waiting via
wait()/sleep()/join(). - TERMINATED: Thread has completed or thrown an exception.
⚠️ Common Mistakes in Java Multithreading
1. ❌ Calling run() instead of start()
Thread t = new Thread(() -> System.out.println("Running"));
t.run(); // Wrong - runs on current thread
t.start(); // Correct - runs on new thread
2. ❌ Not Using Thread-Safe Collections
Avoid ArrayList, HashMap in multithreaded code.
✅ Use ConcurrentHashMap, CopyOnWriteArrayList, etc.
3. ❌ Ignoring Synchronization
public synchronized void increment() { counter++; }
Or use ReentrantLock for finer control.
4. ❌ Using synchronized on Wrong Objects
Locking on this or string literals can create unintended side effects. Prefer private final lock objects.
private final Object lock = new Object();
synchronized (lock) {
// critical section
}
5. ❌ Race Conditions
Multiple threads read and write shared data simultaneously, leading to unpredictable results.
🔧 Use synchronization or atomic variables like AtomicInteger.
6. ❌ Deadlocks
Two or more threads wait on each other to release locks.
// A locks X then waits for Y, B locks Y then waits for X
✅ Avoid nested locks, use timeout-based locking (e.g., tryLock())
7. ❌ Blocking the Main Thread
Long-running tasks should run in background threads.
✅ Use ExecutorService for background execution.
8. ❌ Thread Leaks
Not shutting down thread pools causes resource leaks.
✅ Always call shutdown() or shutdownNow() on ExecutorService.
9. ❌ Inefficient Thread Pools
Don't use newFixedThreadPool() blindly. Use ThreadPoolExecutor with tuning.
10. ❌ Poor Exception Handling
Unhandled exceptions can silently kill threads.
✅ Use Thread.setUncaughtExceptionHandler().
🔧 Java Concurrency Utilities to the Rescue
ExecutorService and ThreadPoolExecutor
Thread pool = team of workers. Efficient and reusable.
Callable, Future, CompletableFuture
Handle return values, exceptions, and async composition.
ReentrantLock, ReadWriteLock, StampedLock
Advanced locking with timeout and condition support.
BlockingQueue, ConcurrentHashMap
Safe inter-thread communication and shared data access.
🧩 Design Patterns in Multithreading
| Pattern | Description |
|---|---|
| Worker Thread | Pool of threads to process requests |
| Future Task | Handles async task execution with results |
| Thread-per-message | Creates thread per request, less scalable |
| Producer-Consumer | Separation of task production and consumption |
💣 Anti-Patterns to Avoid
- Overuse of synchronized blocks
- Ignoring memory visibility (
volatile) - Spawning unbounded threads
- Busy-waiting with
while(true)
📘 Real-World Scenarios
1. Producer-Consumer
Use BlockingQueue to decouple producers and consumers.
2. File Processing
Split files into chunks and process in parallel using ExecutorService.
📌 What's New in Java Versions?
Java 8
- Lambdas with
Runnable,Callable CompletableFutureand async APIs- Parallel streams
Java 9
Flow API(Reactive Streams)
Java 11
- Small improvements to
CompletableFuture
Java 21
- Virtual Threads (Project Loom)
- Structured Concurrency
- Scoped Values for safe thread-local replacement
✅ Best Practices Summary
- Prefer
ExecutorServiceover manual threads - Always handle exceptions in threads
- Shut down executors properly
- Use
volatileor synchronization for shared variables - Avoid premature optimization—profile first
❓ FAQ – Expert-Level Q&A
Q1: Why can't I call run() instead of start()?
Because run() executes on the current thread. Only start() spawns a new thread.
Q2: What is false sharing?
When threads on different cores write to variables on the same cache line—causing performance degradation.
Q3: What’s the difference between wait() and sleep()?
wait()releases the lock and waits to be notified.sleep()holds the lock and just pauses the thread.
Q4: When to use volatile?
For visibility without locking. But not sufficient for atomic operations.
Q5: How does ThreadLocal work?
Each thread gets its own isolated copy of the variable.
Q6: What causes lock contention?
Too many threads competing for the same lock—causes CPU spikes and reduced throughput.
Q7: What is structured concurrency?
A Java 21 feature where child threads are managed in a block, ensuring proper lifecycle management.
Q8: When should I prefer CompletableFuture?
For async composition, chaining, and non-blocking behavior.
Q9: What’s a thread-safe alternative to HashMap?
ConcurrentHashMap.
Q10: What is a thread dump?
A snapshot of all thread states—useful for debugging deadlocks or stuck threads.
🎯 Conclusion and Key Takeaways
Multithreading in Java is both powerful and dangerous. With great power comes great responsibility—and a higher chance of bugs. By understanding the pitfalls, using proper concurrency tools, and following best practices, you can write scalable, responsive, and safe multithreaded applications.
✅ Focus on correctness before optimization.
✅ Use high-level concurrency constructs.
✅ Test thoroughly under concurrent load.