Spring Foundations
Why Spring Boot exists, how it builds on the Spring Framework, and the three concepts everything else depends on: inversion of control, dependency injection, and auto-configuration.
IoC and Dependency Injection: Who Builds Your Objects
Spring's container answers one question: who calls new? Inversion of control, the three injection styles and why only one is safe, and what bean scopes change.
Auto-configuration: How a JAR on the Classpath Becomes a Bean
Take apart @SpringBootApplication, follow the AutoConfiguration.imports mechanism, and learn why @ConditionalOnMissingBean means your own bean always wins.
Project Setup: Initializr, Starters and Package Structure
What a starter actually contains, why dependency management means you stop writing versions, and the package layout component scanning rewards.
Configuration and Profiles: Precedence, Binding and Secrets
Why @ConfigurationProperties beats @Value, how profiles separate environments, and the precedence order that explains why your value isn't the one that won.