Month 4 - Concurrency & Project Loom (Weeks 13–16)

Java's concurrency story has two eras: pre-Loom (Thread, synchronized, ExecutorService, CompletableFuture, reactive frameworks like RxJava/Reactor) and post-Loom (virtual threads, structured concurrency, scoped values). You must be fluent in both - because every production codebase is somewhere on the transition.

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Weeks

• Week 13 - The Java Memory Model and java.util.concurrent Foundations
• Week 14 - Executors, CompletableFuture, and the Pre-Loom World
• Week 15 - Virtual Threads, Structured Concurrency, and Scoped Values
• Week 16 - Lock-Free Patterns, VarHandle Memory Modes, and jcstress

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Month 4 Exit Criteria

You can: - Reason about visibility and ordering from first principles using happens-before. - Pick the right concurrency primitive ({synchronized, ReentrantLock, VarHandle, virtual thread, structured scope}) for a given problem. - Migrate a CompletableFuture or Reactor chain to virtual threads + structured concurrency, and explain when not to. - Validate a concurrent data structure with jcstress. - Diagnose pinning, contention, and starvation from JFR.

You are now ready for Month 5 - production and distributed systems.

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