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You bumped spring-boot-starter-parent to 4.1, ran the build, and noticed something in the test output: junit-jupiter:6.x. Nobody asked for it, nothing in your test code changed, and everything still passes. So what did you just get — and what breaks the day it doesn’t? Short version: JUnit 6 is mostly a cleanup release wearing a major version number. For a typical Spring Boot app, the migration is deleting lines from your pom.xml, not adding them. The pain lives in three specific removed APIs and two build plugins. Let’s go through all of it with code. ...

Every fresh VPS (Hetzner, DigitalOcean, your homelab Proxmox box) boots up with the same problem: a root login over password, no firewall, and nothing installed. Before you deploy anything, spend 15 minutes on this checklist. Order matters. Firewall rules and SSH hardening can lock you out permanently if done in the wrong sequence. Do the steps in this order and you won’t have to rescue-boot your way back in. 1. Update everything first Connect as root and patch the box before touching anything else. ...

Rate limiting shows up in two places: your on-call rotation and your system design interview. Both reward the same thing — knowing which algorithm to pick, why, and what breaks at scale. This guide builds three working limiters in Spring Boot, each with tests: Fixed window — the simplest thing that works, hand-rolled. Token bucket — smooth, burst-friendly, via Bucket4j. Distributed sliding window — precise and shared across instances, on Redis with Lua. Then we close with the part interviews actually grade: how to talk through the tradeoffs. ...

Virtual threads can be one of the easiest throughput wins in a blocking Spring Boot application. The switch is one line: spring: threads: virtual: enabled: true But that line changes the concurrency model of the application. It does not make database queries faster, reduce API latency, or create more database connections. It makes waiting cheaper by allowing many request tasks to share a smaller number of carrier threads. ...

JAVA Streams Sum of Elements Given a list of integers, compute the sum of all numbers. // using sum List<Integer> arr = Arrays.asList(1, 4, 5, 6, 22, 3, 90, 89, 2, 1, 3, 4, 55, 6); int sum = arr.stream() .mapToInt(Integer::intValue) .sum(); // or int sum3 = arr.stream() .reduce((a,b) -> a + b) .get(); // or int sum2 = arr.stream() .reduce(0, (total, n) -> total + n, (total1, total2) -> total1 + total2); Average of Elements Given a list of integers, compute the average of all numbers. ...