JVM Stock Pricer — Java vs Kotlin

Side-by-side stock price aggregators for comparing Java 21 virtual threads + Spring Boot vs Kotlin coroutines + Ktor on the same live-data workload.

Both services pull prices from Yahoo Finance and Stooq, apply per-provider:symbol retry + circuit breaker, keep an in-memory history ring buffer, and expose identical REST + WebSocket APIs plus a Chart.js dashboard.

Module	Stack	Default port
kotlin-aggregator/	Ktor + coroutines + kotlinx.serialization	8080
java-aggregator/	Spring Boot 3.4 + virtual threads + Jackson	8081

Design background: stock-aggregator-java-vs-kotlin.md.

Docs: index · dual-run runbook · Kotlin runbook · Java runbook · simultaneous analysis guide

Requirements

• JDK 21+
• Outbound HTTPS to query1.finance.yahoo.com and stooq.com

Run both (for comparison)

# Terminal 1 — Kotlin
./gradlew :kotlin-aggregator:run

# Terminal 2 — Java
./gradlew :java-aggregator:bootRun

Then open:

• Kotlin dashboard: http://localhost:8080
• Java dashboard: http://localhost:8081

curl http://localhost:8080/prices
curl http://localhost:8081/prices

Full dual-run steps (identical env, warm-up, clean stop): docs/runbooks/dual-run.md.
How to benchmark both at once: docs/ANALYSIS.md.

Shared API contract

Endpoint	Purpose
GET /	Chart.js dashboard
GET /symbols	Configured tickers
GET /prices	All latest aggregated prices
GET /prices/{symbol}	One symbol (404 if none yet)
GET /prices/{symbol}/history	In-memory history ring buffer
WS /ws/prices	Push every update as JSON (seeds cache on connect)

Shared configuration (environment variables)

Variable	Default	Meaning
SYMBOLS	AAPL,MSFT,GOOG	Comma-separated tickers
POLL_INTERVAL_MS	5000	Per-symbol poll interval
PROVIDER_TIMEOUT_MS	3000	Per-attempt provider timeout
MAX_RETRIES	2	Extra attempts with exponential backoff
RETRY_BASE_DELAY_MS	200	Backoff base (200, 400, 800, ...)
HISTORY_SIZE	200	Max history points per symbol
CB_FAILURE_THRESHOLD	3	Failures before circuit opens
CB_RESET_TIMEOUT_MS	30000	Open-circuit cool-down
PORT	8080 (Kotlin) / 8081 (Java)	HTTP/WebSocket port

Example:

SYMBOLS=AAPL,TSLA,NVDA POLL_INTERVAL_MS=10000 ./gradlew :kotlin-aggregator:run
PORT=8081 SYMBOLS=AAPL,TSLA,NVDA POLL_INTERVAL_MS=10000 ./gradlew :java-aggregator:bootRun

What differs (the point of the comparison)

Concern	Kotlin	Java
Concurrency	supervisorScope + coroutines + withTimeoutOrNull	Virtual-thread executor + blocking HTTP + futures/timeouts
HTTP server	Ktor Netty	Spring MVC (+ VT-enabled request threads)
Serialization	kotlinx.serialization	Jackson
Fan-out	SharedFlow → WebSocket	Listener/ConcurrentHashMap of sessions → WebSocket

Build / test

./gradlew :kotlin-aggregator:test :java-aggregator:test
./gradlew build

Tracking

Progress is tracked with beads (bd). Epic: Java vs Kotlin stock price aggregators.