Spring Boot Tricky Interview Questions & Answers
1. Microservices & Communication
Q1. How do you handle inter-service communication in a microservices architecture using Spring Boot?
Choose based on coupling and synchronization requirements: (1) Synchronous Non-blocking: Use
WebClient(Spring WebFlux) for real-time HTTP calls (avoid deprecatedRestTemplate). (2) Declarative HTTP Clients: UseSpring Cloud OpenFeignfor clean REST client interfaces. (3) Asynchronous Event-Driven: Use Apache Kafka or RabbitMQ to decouple services for eventual consistency. Mitigate synchronous cascading failures using Resilience4j Circuit Breaker and fallback methods.
@CircuitBreaker(name = "userService", fallbackMethod = "getUserFallback")
public User getUser(Long id) {
return webClient.get().uri("/users/{id}", id).retrieve().bodyToMono(User.class).block();
}
private User getUserFallback(Long id, Throwable t) {
return User.defaultUser(); // Return cached or safe fallback payload
}
2. Caching & Performance Optimization
Q2. How does Spring Boot's caching abstraction work and what are the common self-invocation gotchas?
Spring's cache abstraction (
@Cacheable,@CachePut,@CacheEvict) provides a transparent caching layer backed by providers like Caffeine or Redis. Spring uses AOP Proxies to intercept method calls. If a method invokes another@Cacheablemethod inside the same class (this.findById()), the call bypasses the Spring AOP proxy, causing cache annotations to be silently ignored.
Q3. How do you diagnose and resolve HikariCP database connection pool exhaustion under high load?
Monitor
hikaricp.connections.activeandhikaricp.connections.pending. If pending requests time out (connectionTimeout=30000ms), size the pool according to the HikariCP formula: Enable SQL query profiling to fix N+1 lazy loading bugs using@EntityGraphorJOIN FETCH.
3. Core Framework & Proxy Mechanics
Q4. How does Spring Boot choose between JDK Dynamic Proxies and CGLIB Proxies for AOP and @Transactional?
If a target bean implements at least one interface, Spring Framework historically defaults to JDK Dynamic Proxies (
java.lang.reflect.Proxy). If the bean implements no interfaces, Spring uses CGLIB to generate a subclass at runtime. Since Spring Boot 2.x, Spring Boot defaults to CGLIB Proxies (spring.aop.proxy-target-class=true) for all beans. Note thatfinalclasses orfinalmethods cannot be subclassed by CGLIB, causing AOP advice (like@Transactional) to fail silently.
Q5. What is the difference between @EnableAutoConfiguration in Spring Boot 2.x vs Spring Boot 3.x?
In Spring Boot 2.x, candidate auto-configuration class names were loaded from
META-INF/spring.factories. In Spring Boot 3.x (Spring Framework 6),spring.factoriesfor auto-configuration is deprecated. Auto-configuration class names must be registered insideMETA-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports.