Mastering Multithreading in Java: From Threads to Virtual Threads

Multithreading is a powerful feature in Java that allows concurrent execution of two or more parts of a program. It plays a crucial role in performance-critical applications such as web servers, real-time systems, gaming engines, and high-throughput services.

In the modern computing landscape where multi-core processors are the norm, mastering multithreading is essential for building responsive and scalable applications.

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What is Multithreading?

Multithreading refers to the concurrent execution of two or more threads (smaller units of a process). It enables a program to perform multiple operations simultaneously, utilizing CPU cores efficiently.

Real-world analogy

Think of a thread pool like a team of factory workers — each worker (thread) is assigned a task. Once completed, they take the next one from the queue.

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Java Threading API

Core classes and interfaces

• Thread
• Runnable
• Callable<V> (returns result, supports checked exceptions)

Creating threads

public class MyThread extends Thread {
    public void run() {
        System.out.println("Running in MyThread");
    }
}

public class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Running in MyRunnable");
    }
}

ExecutorService executor = Executors.newFixedThreadPool(3);
executor.submit(() -> System.out.println("Running in thread pool"));

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Thread Lifecycle and States

NEW → RUNNABLE → BLOCKED/WAITING/TIMED_WAITING → TERMINATED

Key methods

• start() vs run()
• join()
• sleep(ms)
• yield()
• interrupt()

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Memory Model and Visibility

• Java Memory Model (JMM) defines how threads interact through memory
• volatile ensures visibility (but not atomicity)
• synchronized ensures mutual exclusion and memory visibility

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Thread Coordination

• wait(), notify(), notifyAll()
• join() for waiting on other threads
• sleep() for delaying execution

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Locking and Synchronization

• synchronized blocks and methods
• ReentrantLock
• ReadWriteLock
• StampedLock

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java.util.concurrent Tools

• ExecutorService, ThreadPoolExecutor
• Future, CompletableFuture
• ForkJoinPool, RecursiveTask
• BlockingQueue, ConcurrentHashMap

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Real-World Scenarios

• Producer-Consumer using BlockingQueue
• Thread pools for task management
• Parallel file processing

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Java Version Tracker

📌 What's New in Java Versions?

Java 8

• Lambdas for Runnable
• CompletableFuture
• Parallel streams

Java 9

• Flow API (Reactive Streams)

Java 11+

• Minor enhancements in CompletableFuture

Java 21

• Structured concurrency
• Virtual threads (Project Loom)
• Scoped values

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Best Practices and Common Pitfalls

✅ Best Practices

• Prefer ExecutorService over manual thread creation
• Use immutable objects for shared data
• Use thread-safe collections from java.util.concurrent

🚫 Common Pitfalls

• Race conditions
• Deadlocks
• False sharing
• Uncaught exceptions in threads

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Multithreading Design Patterns

• Worker Thread
• Future Task
• Thread-per-message
• Producer-Consumer

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Conclusion and Key Takeaways

• Multithreading improves performance and responsiveness
• Java provides robust APIs and concurrency tools
• Understand JMM, synchronization, and advanced constructs
• Prefer high-level constructs like thread pools or virtual threads

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FAQs

Q1: Why shouldn't we call run() directly?
A: It executes in the same thread instead of a new one. Use start() to spawn a new thread.

Q2: What is the difference between volatile and synchronized?
A: volatile ensures visibility; synchronized ensures both mutual exclusion and visibility.

Q3: Can synchronized methods lead to deadlocks?
A: Yes, if threads acquire locks in different orders.

Q4: What is the thread-safe way to increment a counter?
A: Use AtomicInteger.incrementAndGet().

Q5: When should I use ReentrantLock over synchronized?
A: When you need try-locking, timed locking, or interruptible locking.

Q6: What is false sharing?
A: When multiple threads modify variables that reside on the same cache line, causing performance degradation.

Q7: How do thread pools help performance?
A: They reduce the overhead of thread creation and reuse existing threads.

Q8: What are virtual threads in Java 21?
A: Lightweight threads managed by the JVM, ideal for I/O-bound workloads.

Q9: What is structured concurrency?
A: A model in Java 21+ to manage thread lifecycles hierarchically for safer concurrency.

Q10: How do I debug race conditions?
A: Use thread dumps, logging, and tools like jconsole or VisualVM to trace thread behavior.

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