🔐 What is a Semaphore?
A Semaphore maintains a set of permits — threads must acquire a permit before proceeding, and release it when done.
💡 If no permits are available, the thread blocks (waits) until one is released.
✅ Real-Life Analogy:
Imagine:
- A parking lot with 3 spots.
- Only 3 cars can park at once.
- A car has to wait if all spots are full.
🎯 The 3 parking spots = 3 permits in a
Semaphore
🔧 Java API: java.util.concurrent.Semaphore
Semaphore semaphore = new Semaphore(3); // 3 permits
✅ Example
Semaphore semaphore = new Semaphore(2); // Only 2 threads can run at once
Runnable task = () -> {
try {
semaphore.acquire(); // Get a permit
System.out.println(Thread.currentThread().getName() + " acquired permit");
Thread.sleep(1000); // Simulate work
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
System.out.println(Thread.currentThread().getName() + " released permit");
semaphore.release(); // Release permit
}
};
for (int i = 0; i < 5; i++) {
new Thread(task).start();
}
🧠 What Happens:
- Only 2 threads run at the same time.
- Others wait until a permit is released.
📘 Key Methods
| Method | Description |
|---|---|
acquire() | Acquires a permit (waits if none are available) |
release() | Releases a permit |
tryAcquire() | Attempts to acquire a permit without waiting |
availablePermits() | Shows how many permits are available right now |
🚦 Binary Semaphore = Mutex
Semaphore mutex = new Semaphore(1); // Only one thread at a time
This behaves like a lock — only one thread can enter the critical section at a time.
🔄 Semaphore vs Lock
| Feature | Semaphore | Lock / synchronized |
|---|---|---|
| Multiple access? | ✅ Yes (N threads) | ❌ No (1 thread at a time) |
| Resource control | ✅ Ideal for managing limited resources | ❌ Basic mutual exclusion |
| Fairness option | ✅ Yes (new Semaphore(permits, true)) | ✅ with ReentrantLock |
🎯 Use Cases
- Limiting concurrent database connections
- Controlling access to a pool of resources
- Implementing rate limiting
- Simulating barriers or entry gates