Design a Fixed Size Thread Pool in Java | Concurrency LLD Interview

If you’re preparing for Low Level System Design interviews, there’s one concurrency question that appears again and again: Design a Thread Pool. Most developers know how to use Executors.newFixedThreadPool(), but interviews don’t test convenience — they test understanding. In this video, we break down how a thread pool actually works under the hood and build one from scratch to truly understand Java concurrency. In this deep dive, you’ll learn: • How ThreadPoolExecutor really manages threads and queues • The role of corePoolSize, maximumPoolSize, and keepAliveTime • Why a BlockingQueue sits at the heart of thread pool design • How rejection policies provide backpressure in high load systems • The internal worker thread loop that powers Java thread pools We’ll also implement a simplified custom thread pool step by step using a bounded BlockingQueue and worker threads. This will help you understand the producer-consumer model, task scheduling, and graceful shutdown — concepts that frequently appear in LLD interviews and real production systems. This video is the first part of a new series on Low Level System Design problems with a focus on concurrency patterns used in real backend systems. If you’re a backend engineer preparing for interviews or trying to master Java concurrency, this series is for you. Subscribe to CodeGranular for deep dives into: Java internals, concurrency patterns, Kafka, distributed systems, and high-performance backend engineering. 🐼⚡ What concurrency interview questions have you faced recently? Let me know in the comments. #Java #JavaConcurrency #ThreadPool #ThreadPoolExecutor #LLDInterview #SystemDesign #LowLevelDesign #BackendEngineering #ConcurrencyInJava #SoftwareEngineering #JavaMultithreading #Programming #TechInterviews