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The Java Interview Handbook: 300+ Interview Questions

Mutexes vs. Semaphores

Explore the key differences between mutexes and semaphores in Java multithreading. Learn how mutexes enforce exclusive access and how semaphores manage limited concurrent resource access. Understand practical implementations using ReentrantLock and Semaphore classes, and how these concepts apply to scalable, thread-safe applications.

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Concurrent applications often need to limit the number of threads that can access a shared resource at the same time. Exclusive access can be implemented with synchronized or Lock, while Semaphore allows a limited number of threads to access a resource or resource pool concurrently through a fixed number of permits.

Let’s explore the differences between them and how Java implements these concepts.

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What is the difference between a mutex and a semaphore?

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To see mutual exclusion in practice, we can look at a standard implementation using a ReentrantLock.

Java
import java.util.concurrent.locks.ReentrantLock;
class MutexDemonstration {
    private final ReentrantLock lock = new ReentrantLock();
    private int sharedCounter = 0;
    public void safelyIncrement() {
        lock.lock();
        try {
            sharedCounter++;
        } finally {
            lock.unlock();
        }
    }
}

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