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.
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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.
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