Two databases in one Spring Boot 4.1 application: a DataSource, Flyway, EntityManagerFactory and transaction manager per database, package-based repository scanning, and every way it fails, each run and quoted from a committed transcript.
Spring events are blocking method calls, and that one fact explains the surprises: a listener that rolls the publisher back, an AFTER_COMMIT listener that silently never runs, and an async listener that sees a different world. Every claim is reproduced on Spring Boot 4.1 with a committed transcript, including a rollback that proves AFTER_COMMIT semantics.
Build a real Spring Boot 4.1 starter from an empty directory: the two-jar layout, AutoConfiguration.imports, @ConditionalOnMissingBean back-off, configuration metadata, an optional Jackson 3 integration, and the ApplicationContextRunner test that passes while the starter is broken. Every claim comes from a committed transcript.
Spring Boot 4 refuses to start with "The dependencies of some of the beans in the application context form a cycle". This article reproduces the failure, reads the report, and applies four fixes (redesign, events, @Lazy, ObjectProvider), plus what allow-circular-references really does on Spring Framework 7.0.9. Every claim comes from a committed test transcript.
Every Spring bean lifecycle callback printed in order from a real run, @PostConstruct running before the proxy exists, the non-static BeanPostProcessor warning, SmartLifecycle phases measured, and a request in flight during graceful shutdown with virtual threads. Tested on Spring Boot 4.1.1, Spring Framework 7.0.9 and Java 25.
Instance counts for singleton, prototype, request, session and application scope taken from real runs, the prototype-in-singleton trap and four ways to fix it, a singleton leaking one caller's data into another, and request scope without a proxy failing at start-up. Tested on Spring Boot 4.1.1, Spring Framework 7.0.9 and Java 25.
How Spring chooses between several beans of the same type: the real NoUniqueBeanDefinitionException, @Qualifier, @Primary, @Priority, @Fallback and parameter-name matching in their measured order, then Optional, ObjectProvider, and List and Map injection. Tested on Spring Boot 4.1.1, Spring Framework 7.0.9 and Java 25.
The same small order service written with constructor, setter and field injection, then built without Spring, given two constructors and closed into a circular loop, with the real output of each run and the Spring Boot failure report. Tested on Spring Boot 4.1.1, Spring Framework 7.0.9 and Java 25.
JEP 506's ScopedValue, final in Java 25: the API, the one inheritance trap that breaks ThreadLocal-based code silently, and a measured answer to the memory claim -- including a first measurement too noisy to trust and the jcmd object census that replaced it.
Every built-in Stream Gatherer (JEP 485, final in JDK 24) with real output, a custom Gatherer built from scratch, and mapConcurrent with virtual threads -- including the one failure-handling surprise that isn't in the JEP's prose, and when a Collector is still the better tool.