Spring Boot Cheat Sheet

What is Spring Boot?

Opinionated wrapper around the Spring Framework
Auto-configuration based on classpath + beans present
Embedded server (Tomcat/Jetty/Undertow) — no WAR deployment needed
Production-ready via Actuator metrics & health endpoints
Convention over configuration — zero XML required

Spring Application Lifecycle

IoC Container & Bean Scopes

Key Spring Layers

Maven pom.xml — Essential Dependencies

Standard Folder Structure

Main Application Class

Stereotype & Bean Annotations

Web / REST Annotations

JPA / Entity Annotations

Misc Important Annotations

Validation Annotations (jakarta.validation)

🔥 Annotation Combinations to Know

Constructor Injection (Preferred ⭐)

Field & Setter Injection (Avoid in prod)

@Configuration Bean Factory

@Value & @ConfigurationProperties

Server & App Properties

DataSource / JPA Properties

Logging & Actuator

Profiles — Multi-env Config

Full CRUD Controller

ResponseEntity & HTTP Status Codes

HTTP Status Code Summary

Entity Class

JpaRepository — Built-in Methods

Relationships

Pagination & Sorting

@Transactional — Key Rules 🔥

SecurityFilterChain (Spring Boot 3.x / Spring 6)

UserDetailsService Implementation

JWT Filter (Skeleton)

Method-Level Security

Global Exception Handler

Custom Exception Classes

Common Endpoints

Custom Health Indicator & Config

Unit Test — Service Layer (Mockito)

Integration Test — Controller (@WebMvcTest)

Test Annotations Cheat Table

DTO Pattern (Records — Java 16+)

@Scheduled Tasks

@Async Background Processing

CommandLineRunner & ApplicationRunner

🔥 Top Interview Questions

What does @SpringBootApplication do? — Combines @Configuration + @EnableAutoConfiguration + @ComponentScan
Difference: @Component vs @Service vs @Repository? — All are @Component; @Repository adds exception translation, @Service is semantic
What is Spring IoC? — Inversion of Control: Spring manages object lifecycle & dependencies, not your code
Constructor vs Field injection? — Constructor is preferred: immutable, testable, required deps explicit
How does auto-configuration work? — Reads spring.factories (now AutoConfiguration.imports), conditionally registers beans
What is @Transactional propagation? — REQUIRED (default), REQUIRES_NEW, NESTED, SUPPORTS, MANDATORY, NOT_SUPPORTED, NEVER
EAGER vs LAZY fetching? — EAGER loads related entities immediately; LAZY loads on access. LAZY is default for @OneToMany
What is N+1 problem? — Loading 1 entity causes N extra queries for associations. Fix: JOIN FETCH in JPQL or @EntityGraph
How does Spring Security work? — FilterChain processes each request; filters authenticate/authorize before hitting controller

⚡ Common Gotchas & Pitfalls

@Transactional only works on public methods — private methods are not proxied
Self-invocation bypasses proxy — calling @Transactional method from same class skips transaction
@Async has same proxy limitation — cannot call async methods from same bean
Circular dependencies in constructor injection → BeanCurrentlyInCreationException. Use setter or @Lazy
@OneToMany without mappedBy creates an unwanted join table
Using EAGER fetch on @OneToMany = N+1 problem by default
Bidirectional relationships need helper methods to sync both sides
@MockBean resets application context — slows tests. Use judiciously
spring.jpa.hibernate.ddl-auto=create-drop in prod = data loss 💀
@Value doesn't work in static fields — inject into non-static and use manually

Bean Lifecycle & @PostConstruct vs @PreDestroy

Quick Comparison Table

Transactional Propagation Levels