Practice Clojure Concurrency with JVM Examples

Concurrency becomes useful when you can recognize the shape of the problem in ordinary application code. This section turns the earlier state tools into concrete examples: counters, coordinated application state, background work, and mixed concurrency designs.

Do the examples in a REPL and deliberately add contention. Java developers often learn the most when they can see which invariants remain stable without locks, which operations retry, and where Java concurrency utilities still belong at the boundary.

In this section

• Build a Thread-Safe Counter with an Atom
  Implement a shared counter with a Clojure atom, compare it with Java AtomicInteger and synchronized counters, and learn the retry-safe rules that keep atom updates correct under contention.
• Coordinate Multithreaded Application State with Refs
  Use refs and Software Transactional Memory to coordinate related state changes in a multithreaded Clojure application without copying Java lock-ordering problems.
• Run Ordered Background Work with Agents
  Use Clojure agents for ordered background state updates, understand when they differ from Java executors and queues, and handle failures without hiding operational risk.
• Combine Clojure Concurrency Primitives Without Mixing Responsibilities
  Design a small concurrent Clojure workflow that uses atoms, refs, agents, futures, and Java queues only where their ownership and coordination responsibilities are clear.