Fifty Spring Boot 4 interview questions with short answers that fit in a sentence or two aloud, from Boot 4.1 and proxies to Spring Security 7, Kafka, batch and Spring AI. Every answer links a deep dive that reproduced the behaviour by running code.
What a lambda really is, how to pick the right interface from java.util.function by shape, why primitive versions exist, and the five compile errors every beginner meets. Every program was compiled and run on Java 25 and JDK 27.
Spring Boot 3.x is now outside OSS support. Here's how to calculate the risk, choose the right 4.x landing zone, automate the mechanical work with OpenRewrite, and complete the migration without turning it into a six-month rewrite.
AI scanners are flooding the Spring ecosystem with patch PRs faster than any team can review. Build an automated remediation pipeline instead: Dependabot or Renovate tuned for Spring Boot parent POMs and BOMs, a Testcontainers regression gate that decides which bumps are safe, and canary deployments for minor framework upgrades.
A practical Spring Framework 6 to 7 migration guide covering the Jakarta EE 11 baseline, removed and deprecated APIs, AOT and GraalVM changes, BeanRegistrar and proxy lifecycle changes, JSpecify null safety, testing and security implications, an upgrade checklist, and an AI prompt to scan your project.
The Singleton pattern guarantees exactly one instance of a class — but Java's threading model, serialization, and reflection make it harder than it looks. Complete guide: all 6 implementations (Eager, Lazy, synchronized, DCL, Holder, Enum), thread-safety analysis, Spring singletons, pitfalls, and when NOT to use it.
The Prototype pattern lets you clone objects without depending on their concrete classes. Complete Java guide: Cloneable pitfalls, deep vs shallow copy, copy constructors, serialization-based cloning, JDK usage, and when to use each approach.
The Builder pattern separates complex object construction from its representation, eliminating telescoping constructors and invalid partial state. Complete Java guide with runnable examples, Lombok @Builder, JDK usage (StringBuilder, Stream.Builder), pitfalls, and FAQs.
The Abstract Factory pattern lets you create families of related objects without specifying their concrete classes. Complete Java guide with runnable cross-platform UI example, JDK usage, comparison with Factory Method, pitfalls, and FAQs.
The Factory Method pattern lets subclasses decide which object to create, eliminating if/else creation logic and making your codebase open for extension without modification. Complete guide with runnable Java examples, JDK usage, Spring Boot integration, pitfalls, and FAQs.