Play

Learn by poking at it

Short, hands-on things for the moments between builds. A daily puzzle, a daily quiz, and visual simulators that make Kafka, JPA, threads, the JVM and Spring easier to picture. Everything runs in your browser.

🐞Spot the BugA new Java snippet with one real bug every day. Tap the line, learn the fix, keep a streak. 132 puzzles in the bank.🎯Daily cron quizFive cron expressions a day from Quartz, Spring, Unix and Kubernetes. Can you read them faster than the scheduler?🔀Kafka rebalance simulatorAdd and remove consumers, compare range, round-robin, sticky and cooperative assignment, watch lag, offsets and duplicates.🗃️JPA N+1 visualizerLazy, join fetch, entity graph, batch size and subselect side by side, with the exact SQL and statement counts.🔐Deadlock and starvationStep two threads into a deadlock, fix it with lock ordering or tryLock, then watch a fixed thread pool starve itself.♻️GC and heap visualizerEden, survivor spaces and promotion in motion, with illustrative pause lengths for Parallel, G1 and ZGC.🍃Spring request lifecycleClick through filters, interceptors, validation, the transaction proxy and exception advice for seven scenarios.

Why simulators instead of diagrams

A static diagram can say that a consumer group rebalances. It cannot show that range assignment shuffles half the partitions when one consumer joins, while cooperative sticky moves two. The playgrounds here are small models with the real rules behind them: the Kafka assignors follow the Apache Kafka algorithms, the JPA counts follow Hibernate's documented fetch behaviour, and the logic of each simulator is covered by automated tests. Where a number is only illustrative, such as garbage collection pause lengths, the page says so.

Each playground has an "Explain what I am seeing" panel that rewrites itself as you change the controls, links to the matching concept pages in the Java concepts library, and a Share button that stores the exact setup in the link, never on a server (how sharing works).