Readers-Writers Lock (Java)

A Java implementation of the classic second readers-writers problem, using semaphores to allow concurrent read access while giving writers priority (preventing writer starvation).

Design

• S — binary semaphore guarding the shared resource.
• mutex — binary semaphore protecting the reader count.
• serviceQueue — a fair (FIFO) binary semaphore ("turnstile") that readers must also pass through before acquiring mutex — this is what prevents writers from starving under a steady stream of readers.
• readCount — tracks how many readers currently hold the resource; only the first reader acquires S, and only the last reader releases it.

API

• readLock() / readUnLock() — acquire/release read access
• writeLock() / writeUnLock() — acquire/release exclusive write access

Tech stack

Java (java.util.concurrent.Semaphore)