Spring
Spring Core, Spring Boot, Data JPA and Security.
Spring Boot
Spring Boot annotations and internals: stereotypes, Spring MVC request handling, request/response binding, exception handling, validation, dependency injection, bean lifecycle, AOP, security, auto-configuration, Actuator, microservices, performance and configuration properties.
Spring Data JPA & Hibernate
Repositories and save() internals, derived query keywords, pagination and keyset scrolling, @Query and JPQL, native SQL, entity mapping and ID strategies, the persistence context and dirty checking, relationships and cascading, fetching and the N+1 problem, transactions, propagation and locking, projections, Specifications and auditing, inheritance and composite keys, second-level caching, HikariCP, Flyway migrations, testing with Testcontainers and advanced Hibernate features.
Spring Security & JWT
Securing Spring Boot REST APIs: password hashing with PasswordEncoder and BCrypt, JWT structure and signing algorithms, a complete JWT authentication filter and Spring Security 6 configuration, JJWT 0.12, refresh tokens, logout and the security pitfalls interviewers ask about.
Logging (SLF4J & Log4j2)
Production logging for Java and Spring Boot: SLF4J vs Logback vs Log4j2, log levels, log4j2.xml, rolling files and retention, PatternLayout, MDC request IDs, async and JSON logging, Actuator level changes and Log4Shell.