Henry Brown's Notes on the JVM

Make Invalid Domain States Hard to Represent with Kotlin and Arrow
Using value classes, context parameters and typed errors to build a small functional domain model
Aug 24, 20267 min read7
Stop External Services from Breaking Your Build with JUnit Tags
Keep everyday tests fast and dependable while running strict live integration tests only when you choose
Aug 20, 202611 min read13
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Henry Brown's Notes on the JVM
9 posts
Practical notes on software development for the JVM, from Cape Town. Mostly Kotlin and Spring Boot: coroutines and structured concurrency, virtual threads, reactive programming, and the occasional Git trick or experiment with local AI models. Every post starts with something I actually needed to build, debug, or measure — so expect working code, real numbers, and honest trade-offs rather than theory. I also cover some of this on YouTube. All views are my own.
PgBouncer with Spring Boot 4: Setup, Gotchas, and the Proof
Part 2: Add transaction pooling to the benchmark app, avoid prepared-statement traps, and see where connection pooling earns its keep
Aug 17, 20266 min read60
I Built a Spring Boot 4 App to Put PgBouncer to the Test
Part 1: Build a production-style Kotlin testbed with hexagonal architecture, PostgreSQL, Testcontainers, and a repeatable concurrency benchmark
Aug 17, 20269 min read18
Guardrails for AI-Generated Kotlin with detekt and ktlint
Turn your coding standards into checks that both developers and AI agents must pass
Aug 10, 202611 min read36
Fan-in with Kotlin Coroutines and Flow
Sometimes you need to start several pieces of work at the same time and handle each result as soon as it is available. The fan-in pattern is a good fit for this: many concurrent producers send their r
Jul 22, 20266 min read12
Accessing Apple's on-device AI from Kotlin using apfel
If you own a modern Mac with an Apple Silicon chip and Apple Intelligence enabled, then you already have access to a local foundation model. There is no separate model to download, no API key to creat
Jul 9, 20266 min read
How many threads can you create with Spring Boot and Java virtual threads?
In looking at enabling virtual threads for a Spring Boot application, I began to wonder how many virtual threads I could spawn compared to platform threads. I knew the answer would be "many more", but
May 22, 20269 min read51
Configure a background coroutine for your Spring Boot application
Sometimes you need to run work in the background of a Spring Boot application. This may be work that should not block the HTTP request that triggered it, or work that is started by a scheduled process
May 1, 20266 min read8