spring-: spring-boot-application-bean-lifecycle-comprehensive-execution-order-with-related-methods

This document details the comprehensive execution order of the Spring Boot application bean lifecycle, encompassing related methods at each stage.

Phase 1: Bootstrapping (JVM & Spring Boot Initialization)

1. JVM Initialization: The Java Virtual Machine (JVM) starts and loads the main application class (containing public static void main(String[] args)).
2. SpringApplication Execution: SpringApplication.run() initiates the application context creation. (Related Method: SpringApplication.run())
3. Environment Configuration: The application loads settings from system properties, environment variables, application.properties/yml files, and command-line arguments. Active and default profiles are determined. (Related Methods: ConfigurableEnvironment#setActiveProfiles(), PropertySourcesPropertyResolver#getProperty())
4. Application Type Determination: Spring identifies the application type (web or non-web). This determines the appropriate application context: AnnotationConfigServletWebServerApplicationContext (web) or AnnotationConfigApplicationContext (non-web). (Related Method: SpringApplication#determineWebApplicationType())
5. Auto-Configuration & SpringFactoriesLoader: Spring automatically registers dependencies found on the classpath (via META-INF/spring.factories). (Related Method: SpringFactoriesLoader#loadFactoryNames())
6. Application Run Listeners: SpringApplicationRunListeners are triggered, firing events like ApplicationStartingEvent and ApplicationEnvironmentPreparedEvent. (Related Methods: SpringApplicationRunListeners#starting(), SpringApplicationRunListeners#environmentPrepared())

Phase 2: Context Initialization & Bean Lifecycle

1. ApplicationContext Creation: The ApplicationContext is created, and beans are scanned using annotations like @ComponentScan and @Configuration. (Related Method: AnnotationConfigApplicationContext#register())
2. Bean Definition Loading: Spring processes bean definitions from configuration classes, XML files, or component scanning. Note: Bean instances are not created yet. (Related Method: BeanDefinitionRegistry#registerBeanDefinition())
3. Bean Instantiation: Beans are instantiated using constructor injection or factory methods. (Related Method: InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation())
4. Lifecycle Aware Bean Processing: Beans implementing lifecycle interfaces (e.g., BeanNameAware, BeanFactoryAware, EnvironmentAware) are processed. (Related Methods: BeanNameAware#setBeanName(), BeanClassLoaderAware#setBeanClassLoader(), BeanFactoryAware#setBeanFactory(), EnvironmentAware#setEnvironment(), EmbeddedValueResolverAware#setEmbeddedValueResolver(), etc.)
5. Conditional Beans & Profiles: Beans marked with @Conditional or @Profile are evaluated and conditionally created based on specified conditions or active profiles. (Related Methods: Condition#matches(), ConfigurableEnvironment#getActiveProfiles())
6. Pre-Initialization Post-Processing: BeanPostProcessor#postProcessBeforeInitialization() methods are executed.
7. Custom Initialization: Custom initialization logic is executed using @PostConstruct, InitializingBean.afterPropertiesSet(), or the init-method attribute in @Bean annotations. (Related Methods: InitializingBean#afterPropertiesSet(), @PostConstruct)
8. Post-Initialization Post-Processing: BeanPostProcessor#postProcessAfterInitialization() methods are executed.

Phase 3: Application Startup Completion

1. ApplicationContext Refresh: The ApplicationContext is refreshed, completing dependency injection. The ContextRefreshedEvent is fired. (Related Method: AbstractApplicationContext#refresh())
2. Embedded Web Server Start (if applicable): If it's a web application, the embedded server (Tomcat, Jetty, Undertow) starts and binds to a port. ServletContextInitializer and WebApplicationInitializer are executed (for servlet-based apps). (Related Method: ConfigurableWebServerApplicationContext#start())
3. CommandLineRunner & ApplicationRunner Execution: Beans implementing CommandLineRunner or ApplicationRunner are executed, performing post-startup tasks. (Related Methods: CommandLineRunner#run(), ApplicationRunner#run())
4. ApplicationReadyEvent: The ApplicationReadyEvent is fired, signaling that the application is fully initialized and ready to handle requests. (Related Method: ApplicationListener#onApplicationEvent(ApplicationReadyEvent))

Phase 4: Bean Destruction & Application Shutdown

1. Graceful Shutdown: The shutdown process begins, controlled by spring.lifecycle.timeout-per-shutdown-phase. (Related Method: SpringApplication#setRegisterShutdownHook(true))
2. Pre-Destruction Processing: DestructionAwareBeanPostProcessor#postProcessBeforeDestruction() methods are executed.
3. Custom Cleanup: Custom cleanup logic is performed using DisposableBean.destroy(), @PreDestroy methods, or the destroy-method attribute in @Bean annotations. (Related Methods: DisposableBean#destroy(), @PreDestroy)
4. ApplicationContext Closure: The ApplicationContext closes, firing the ContextClosedEvent. (Related Method: ConfigurableApplicationContext#close())
5. Custom Exit Codes: SpringApplication.exit() can be used to set custom exit codes (using ExitCodeGenerator). (Related Method: SpringApplication#exit())

Phase 5: Advanced Considerations

• Lazy Initialization (@Lazy): Beans are created only when accessed. (Related Method: DefaultListableBeanFactory#setAllowBeanDefinitionOverriding(false))
• Circular Dependency Handling: Use @Lazy, setter injection, or @DependsOn to manage circular dependencies. (Related Method: AbstractAutowireCapableBeanFactory#doResolveDependency())
• FactoryBean Mechanism: Allows for dynamic bean creation. (Related Method: FactoryBean#getObject())
• Spring Boot Actuator (if enabled): Provides endpoints for monitoring and management (/actuator/health, /actuator/shutdown, /actuator/metrics). (Related Method: HealthIndicator#health())
• Performance Optimizations: Reduce startup time with spring.main.lazy-initialization=true and tune garbage collection.
• Custom Application Listeners (ApplicationListener): Allows hooking into startup/shutdown events. (Related Method: ApplicationListener#onApplicationEvent())

Summary of Execution Order:

1. Bootstrapping: JVM → SpringApplication.run() → Auto-Configuration → Context Creation
2. Context Initialization: Bean Instantiation → Lifecycle Hooks → Dependency Injection
3. Application Startup: Web Server Starts → Runners Execute → Application Ready
4. Shutdown Phase: Pre-Destruction Callbacks → Cleanup → Context Closes

This detailed breakdown provides a comprehensive understanding of the Spring Boot bean lifecycle and its execution order. Understanding this order is crucial for debugging, optimizing, and extending Spring Boot applications.

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