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Spring Boot in Depth
A complete Spring Boot course in simple English: from your first project to internals, REST APIs, databases, messaging, microservices, security, testing and deployment. Every topic has its own page with detailed explanations, internal workflow diagrams, working code, common mistakes and interview questions. The examples use Java 17 or higher and follow Spring Boot 3.x and 4.x; where the two versions differ, the page tells you.
Getting Started
Understand what Spring Boot is, create a project and learn how auto-configuration and starters work.
What is Spring Boot?Spring Boot is a framework built on top of the Spring Framework.Create Your First Spring Boot ProjectThis page shows how to create a project, what every folder means, how to run it and how to package it.Auto-Configuration InternalsAuto-configuration is the heart of Spring Boot.Starters and Dependency ManagementA starter is a single dependency that pulls in everything you need for one feature, with versions that are tested together.
Core Concepts
The Spring core that every Spring Boot application depends on: beans, injection, configuration and AOP.
IoC Container and Dependency InjectionInversion of Control (IoC) means you do not create your objects with new.Bean Lifecycle and ScopesEvery bean goes through a fixed life: it is created, filled with dependencies, initialized, used and finally destroyed.Configuration, Properties and ProfilesSpring Boot lets you keep configuration outside the code, so the same jar runs in development, test and production.Aspect-Oriented Programming (AOP)AOP lets you write cross-cutting code (logging, timing, security, transactions) once, and apply it to many methods without changing them.
Spring Boot Internals
How startup, the ApplicationContext, bean creation, design patterns and the embedded server really work.
How Spring Boot Starts: SpringApplication.run() Step by StepEvery Spring Boot application starts with one line: SpringApplication.run().Inside the ApplicationContext: refresh() and Bean CreationThe ApplicationContext.refresh() method is the heart of Spring.Design Patterns Used Inside SpringSpring is a very good example of design patterns in real code.Embedded Tomcat Internals and TuningSpring Boot starts Tomcat inside your application.
Web and REST
Build REST APIs, validate input, handle errors, work with JSON and files, document and call APIs, and go reactive.
REST ControllersA REST controller receives HTTP requests, calls your service layer and returns data (usually JSON).Validation and Exception HandlingNever trust input.Spring MVC InternalsWhen a request reaches your controller, many components work behind the scenes.JSON with Jackson: Customization and PitfallsSpring Boot converts objects to JSON and back with Jackson.File Upload and DownloadUploading files looks easy but has serious security and memory issues.Calling Other APIs: RestClient and WebClientMost applications call other services.API Documentation with OpenAPI and Swagger UIA REST API without documentation is hard to use.Reactive Programming with Spring WebFluxWebFlux is the non-blocking web stack of Spring.
Data and Transactions
Spring Data JPA, Hibernate internals, relationships, advanced JPA, transactions, other data stores, pooling and migrations.
Spring Data JPASpring Data JPA removes almost all database boilerplate.Hibernate Internals: Persistence Context, Dirty Checking and FlushMost JPA bugs come from not understanding the persistence context.JPA Relationships and the N+1 ProblemRelationships connect entities.Advanced JPA: Specifications, Auditing, Locking and MappingThese are the features that separate demo projects from real systems: dynamic filters, automatic created and updated timestamps, protection against lost updates, soft delete and richer mappings.Transactions with @TransactionalA transaction groups database operations so that they all succeed or all fail together.JdbcTemplate, MongoDB and Redis with Spring BootNot every problem needs JPA.Connection Pool (HikariCP) and Flyway MigrationsTwo things every real project needs: a tuned connection pool, and versioned database migrations instead of letting Hibernate change your schema.Complete CRUD REST API (All Layers)This page puts everything together: entity, repository, DTOs, service, controller, validation and error handling, in a clean layered structure.
Messaging and Microservices
Kafka, RabbitMQ, resilience patterns, Spring Cloud and batch processing.
Apache Kafka with Spring BootKafka is the standard choice for event streaming between services.RabbitMQ with Spring Boot (AMQP)RabbitMQ is a message broker that routes messages to queues using exchanges.Resilience4j: Circuit Breaker, Retry, Rate Limiter and BulkheadIn a distributed system, one slow service can bring down all others because threads pile up waiting for it.Microservices with Spring CloudMicroservices split one big application into small services.Spring Batch: Processing Large Data ReliablySpring Batch is for jobs that process large amounts of data in the background: importing files, nightly reports, migrations.
Security
Spring Security basics, internals, JWT and refresh tokens.
Spring Security BasicsSpring Security protects your application.Spring Security Internals: Authentication Flow, Custom Filters, Multiple ChainsTo debug 401 and 403 errors and write custom security you must know what happens inside the filter chain.JWT and OAuth2 Resource ServerFor REST APIs the common approach is stateless authentication with JSON Web Tokens (JWT).JWT Access and Refresh Token Flow (Complete Design)Short-lived access tokens are safe but annoying, long-lived tokens are convenient but dangerous.
Production and Testing
Logging, monitoring, tracing, testing, performance, tooling, deployment, upgrades and interview preparation.
LoggingGood logs are your best tool when something fails in production.Actuator and MonitoringActuator gives your application production features: health checks, metrics, environment information and more, all as HTTP endpoints.Observability: Logs, Metrics and Distributed TracingWhen a request passes through five services and is slow, logs of one service are not enough.Testing Spring Boot ApplicationsSpring Boot gives you fast unit tests, focused slice tests and full integration tests.Caching, Scheduling and AsyncThree features that improve performance and automate work: cache repeated results, run tasks on a timer, and run slow work on another thread.Performance Tuning: A Practical MethodGuessing is the enemy of performance work.Lombok and MapStruct: Less BoilerplateLombok generates getters, constructors and builders at compile time.Packaging, Docker and DeploymentSpring Boot builds a single executable jar.Upgrading Spring Boot: 2.x to 3.x to 4.xUpgrades are a normal part of a Spring Boot project because each minor version has limited support.Spring Boot Interview Questions and AnswersA collection of the questions asked most often in Spring Boot interviews, with short and clear answers.