Spring Boot lifecycle

Spring Boot lifecycle = bootstrap → context creation → bean definition → dependency injection → initialization → ready → shutdown.

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The Spring Boot lifecycle describes how a Spring Boot application starts, runs, and shuts down. It builds on the Spring Framework’s lifecycle, but with extra automation and conventions. Here’s a clear breakdown:

1. Startup Phase

Launch with SpringApplication.run()

• The entry point (your main method) calls SpringApplication.run(...).
• Bootstraps logging, sets up environment variables, and creates the ApplicationContext (the IoC container).

Environment Preparation

• Reads properties from application.properties or application.yml.
• Profiles (spring.profiles.active) and externalized configuration are applied.

ApplicationContext Creation

• Instantiates the IoC container (e.g., AnnotationConfigApplicationContext).
• Registers beans, components, and configurations (@Component, @Configuration, @Bean).

Bean Definition Loading

• Classpath scanning finds candidate beans.
• Beans are defined and metadata is loaded.

2. Dependency Injection (DI) & Bean Lifecycle

Bean Instantiation

• Creates bean objects (default scope: singleton).

Dependency Injection

• Injects dependencies using:

Constructor injection (recommended for immutability)

• Setter injection
• Field injection

Post-Processing

• BeanPostProcessors modify beans (e.g., proxy wrapping for @Transactional or @Async).

Initialization

• @PostConstruct, InitializingBean.afterPropertiesSet(), or custom init methods run.

Ready-to-use Beans

• Beans are placed in the context and can be used by other components.

3. Application Ready Phase

Context Refresh Complete

• Spring publishes lifecycle events (ContextRefreshedEvent, ApplicationReadyEvent).
• CommandLineRunner and ApplicationRunner beans are executed.

Application is Running

• Your REST controllers, services, repositories, etc. are available.
• Auto-configurations (like embedded Tomcat/Jetty) start listening for requests.

4. Shutdown Phase

Context Close

• Triggered when the JVM shuts down or you call context.close().
• Spring publishes a ContextClosedEvent.

Bean Destruction

• Executes cleanup callbacks:
• @PreDestroy
• DisposableBean.destroy()
• Custom destroy methods in @Bean(destroyMethod="...").

Resources Released

• Database connections, thread pools, caches, etc. are cleaned up.