🔄 Understanding the Spring Boot Lifecycle

Spring Boot applications go through a well-defined lifecycle, which can be divided into bean-level, application-level, and shutdown phases. Knowing this lifecycle is essential for initialization tasks, resource management, and graceful shutdown in production-grade microservices.

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1️⃣ Application Startup Phases

When a Spring Boot application starts:

1. SpringApplication bootstrap

• Loads ApplicationContext.
• Registers beans, configurations, and property sources.

1. Bean instantiation & dependency injection

• Spring creates all singleton beans.
• Dependencies are injected (constructor, field, or setter injection).

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a. Bean-Level Hooks

Hook	When it Runs	Use Case
@PostConstruct	After bean is created & dependencies injected	Lightweight initialization
InitializingBean.afterPropertiesSet()	After properties are set	Programmatic alternative to @PostConstruct
@Bean(initMethod="...")	After bean creation (Java config)	Legacy or custom init method

Example:

@Component
public class MyBean {
    @PostConstruct
    public void init() {
        System.out.println("Bean initialized");
    }
}

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b. Application-Level Hooks

Hook	When it Runs	Notes
CommandLineRunner	After Spring context is ready	Receives raw String[] args
ApplicationRunner	After Spring context is ready	Receives parsed ApplicationArguments
ApplicationReadyEvent	After the app is fully started and server ready	Ideal for warm-up tasks or metrics

Example with ApplicationReadyEvent:

@Component
public class AppReadyListener {
    @EventListener(ApplicationReadyEvent.class)
    public void onReady() {
        System.out.println("Application is fully started and ready to serve requests");
    }
}

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c. SmartLifecycle Interface

• Used for complex beans that require start/stop order.
• Provides start(), stop(), isRunning(), getPhase() methods.

@Component
public class MyLifecycleBean implements SmartLifecycle {
    @Override
    public void start() { System.out.println("Starting bean"); }
    @Override
    public void stop() { System.out.println("Stopping bean"); }
    @Override
    public boolean isRunning() { return true; }
    @Override
    public int getPhase() { return 0; } 
    @Override
    public boolean isAutoStartup() { return true; }
}

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2️⃣ Application Running

Once started:

• Embedded web server (Tomcat/Jetty/Netty) is up.
• Beans are ready to serve requests.
• Tasks like cache warm-up or external service validation can safely run.

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3️⃣ Application Shutdown

Spring Boot supports graceful shutdown, giving a chance to clean up resources:

a. Bean-Level Shutdown Hooks

Hook	When it Runs	Example
@PreDestroy	Before bean destruction	Close DB connections, release resources
DisposableBean.destroy()	Programmatic cleanup alternative	Same as @PreDestroy

@Component
public class CleanupBean {
    @PreDestroy
    public void cleanup() {
        System.out.println("Cleaning up resources before shutdown");
    }
}

b. Application Context Close Event

• Listen to ContextClosedEvent for global cleanup:

@Component
public class ShutdownListener {
    @EventListener(ContextClosedEvent.class)
    public void onShutdown() {
        System.out.println("Application context is closing");
    }
}

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4️⃣ Summary Timeline

SpringApplication.run()
        │
        ▼
ApplicationContext initialized
        │
        ▼
Bean instantiation & dependency injection
        │
@PostConstruct / InitializingBean / init-method
        │
        ▼
CommandLineRunner / ApplicationRunner
        │
        ▼
ApplicationReadyEvent
        │
        ▼
Application running (serving requests)
        │
Shutdown initiated
        │
@PreDestroy / DisposableBean / ContextClosedEvent
        │
        ▼
Application stops

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5️⃣ Best Practices

1. Separate lightweight vs heavy tasks

• Use @PostConstruct for bean-level setup.
• Use ApplicationReadyEvent or runners for heavy, app-wide initialization.

1. Make startup tasks idempotent

• Especially important for Kubernetes pods that may restart.

1. Graceful shutdown

• Always release resources and deregister services in shutdown hooks.

1. Integrate with observability

• Log startup/shutdown events, expose metrics, and integrate with readiness/liveness probes if running in Kubernetes.

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This lifecycle understanding is crucial for microservices architecture, especially when combining Spring Boot hooks with Kubernetes pod lifecycle hooks like postStart and preStop.

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