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Concurrency in Java: From Threads to Async Pipelines (What Actually Breaks in Production)
Opening Hook
At 2:14 a.m., a payment service slowed to a crawl. CPU usage was low. Memory looked fine. No errors in logs. Yet requests piled up like cars at a broken toll gate. After hours of digging, the root cause was painfully simple: a shared in-memory cache updated by multiple threads without proper synchronization.
Nothing crashed. Nothing screamed. The system just quietly lied.
Concurrency bugs are like that. They don’t announce themselves loudly. They hide in timing, in assumptions, in code that “worked fine in testing.” This post walks from concurrency fundamentals to modern async design, not as theory, but as survival skills for real production systems.
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Core Concepts (Part 1): Thread Lifecycle and Shared State
Thread Lifecycle in Practice
A Java thread moves through these real, observable phases:
- NEW: Created, not started
- RUNNABLE: Eligible to run (not “running”)
- BLOCKED: Waiting for a monitor lock
- WAITING / TIMED_WAITING: Waiting for a signal or timeout
- TERMINATED: Finished execution
