java 27 lines · 6 steps

Bounding concurrency with a Semaphore in Java

A Semaphore caps how many tasks run at once and rejects callers that wait too long.

Explained by highlit

Intermediate concurrency semaphore rate-limiting backpressure resource-guard

1public class RequestThrottler {
2 
  private final Semaphore permits;
  private final long acquireTimeoutMillis;
5 
  public RequestThrottler(int maxInFlight, long acquireTimeoutMillis) {
      this.permits = new Semaphore(maxInFlight, true);
      this.acquireTimeoutMillis = acquireTimeoutMillis;
  }
10 
  public <T> T execute(Callable<T> task) throws Exception {
      boolean acquired = permits.tryAcquire(acquireTimeoutMillis, TimeUnit.MILLISECONDS);
      if (!acquired) {
          throw new RejectedExecutionException(
                  "Request rejected: " + permits.getQueueLength() + " callers waiting, no permits available");
      }
      try {
          return task.call();
      } finally {
          permits.release();
      }
  }
23 
  public int availablePermits() {
      return permits.availablePermits();
  }
27}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1A fair Semaphore is a clean primitive for capping in-flight work without spinning up thread pools.
2Bounded acquire timeouts turn silent queuing into fast, explicit rejection under overload.
3Releasing in a finally block guarantees permits return even when the task throws.

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