Semaphore

A Semaphore maintains a set of permits. Threads block when calling acquire() if no permits are available. This makes semaphores perfect for throttling and managing resource pools.

Binary Semaphore vs. Lock

A semaphore with 1 permit is called a binary semaphore. While it behaves similarly to a Lock (providing mutual exclusion), there is a key difference:

• Lock Ownership: A standard Lock has ownership: the thread that acquires it must release it.
• No Ownership: A Semaphore does not track which thread acquired a permit. Any thread can call release() to add a permit, which is useful for coordination patterns but requires care to avoid logic errors.

Code Walkthrough

This class uses a Semaphore to limit the number of active network calls to a database service.

import java.util.concurrent.Semaphore;
public class DatabaseThrottler {
    private final Semaphore semaphore = new Semaphore(3); // Allow max 3 parallel calls
public void queryDatabase() {
        try {
            semaphore.acquire(); // Blocks if 3 threads are already executing
            try {
                System.out.println(Thread.currentThread().getName() + " executing DB query");
                Thread.sleep(1000); // Simulate network query
            } finally {
                semaphore.release(); // Always release permit!
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

Interview-Relevant Information