java-spring

Upgrade to the newest 3.5.x first and clear all deprecation warnings, then rename starters (spring-boot-starter-web becomes spring-boot-starter-webmvc), add spring-boot-starter-flyway explicitly, replace @MockBean with @MockitoBean, and rewrite security config as a SecurityFilterChain bean with the lambda DSL. Run spring-boot-properties-migrator temporarily to catch renamed properties.

@Transactional is proxy-based, so an internal this.method() call, a private method, or a final method silently bypasses the proxy and no transaction starts. Move the inner method to its own injected bean rather than changing the annotation or propagation value.

Fix N+1 with a projection selecting only needed columns first, an @EntityGraph second, and a JPQL join fetch third. Never join fetch two collections in one query, since the cartesian product forces in-memory pagination.

No. Spring Boot 4 removed @MockBean and @SpyBean in favor of @MockitoBean and @MockitoSpyBean, and TestRestTemplate is replaced by RestTestClient with @AutoConfigureRestTestClient. @SpringBootTest also no longer auto-configures MockMvc, so add @AutoConfigureMockMvc explicitly.

Choose Spring MVC unless the entire request path is non-blocking. One JDBC call or block() on a Reactor event loop costs more than MVC, and with virtual threads enabled on Java 21+, MVC handles I/O-bound concurrency without the reactive model.

Without flush() and clear() on TestEntityManager before a read assertion, the assertion is served from Hibernate's first-level cache and the query never executes. Also run repository tests against the real database with Testcontainers instead of H2, which has different SQL and constraint behavior.