Spring Boot - Bean LifeCycle

 Here is a clear, step-by-step lifecycle of a Spring Boot application, explained in a simple + interview-ready way.

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🔄 Spring Boot Application Lifecycle

1️⃣ Application Start (Bootstrap Phase)

public static void main(String[] args) {
    SpringApplication.run(MyApp.class, args);
}

What happens internally:

1. SpringApplication object created

2. Application type detected

   • Servlet (Tomcat)

   • Reactive (Netty)

3. Environment prepared

   • application.properties / yml

   • Profiles (dev, test, prod)

4. Banner printed

5. ApplicationContext created

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2️⃣ ApplicationContext Initialization

Spring creates the IoC container.

Key steps:

• Classpath scanning

• Load @Configuration classes

• Process @EnableAutoConfiguration

• Apply @ComponentScan

📌 Beans are defined, not created yet.

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3️⃣ Bean Creation Phase (Core Lifecycle)

For each bean, Spring follows this order:

1. Instantiate Bean
2. Populate Properties (@Autowired)
3. Aware Interfaces
4. BeanPostProcessor (Before Init)
5. @PostConstruct / init-method
6. BeanPostProcessor (After Init)

Detailed Order:

🔹 1. Bean Instantiation

• Object is created using constructor

🔹 2. Dependency Injection

• @Autowired

• @Value

🔹 3. Aware Interfaces (Optional)

BeanNameAware
ApplicationContextAware
EnvironmentAware

🔹 4. BeanPostProcessor (Before Init)

postProcessBeforeInitialization()

🔹 5. Initialization

• @PostConstruct

• InitializingBean.afterPropertiesSet()

• init-method

🔹 6. BeanPostProcessor (After Init)

postProcessAfterInitialization()

📌 Bean is now ready to use

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4️⃣ Embedded Server Startup

• Embedded Tomcat / Jetty / Netty starts

• DispatcherServlet initialized

• Application is ready to serve requests

Log example:

Started MyApp in 4.5 seconds

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5️⃣ Application Ready Events

Spring fires lifecycle events:

Event	Purpose
ApplicationStartingEvent	App starting
ApplicationEnvironmentPreparedEvent	Env ready
ApplicationContextInitializedEvent	Context created
ApplicationPreparedEvent	Beans loaded
ApplicationReadyEvent	App ready

Example:

@EventListener(ApplicationReadyEvent.class)
public void onReady() {
    System.out.println("App is ready");
}

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6️⃣ Request Processing Lifecycle (Runtime)

Client Request
   ↓
Filter
   ↓
Interceptor (preHandle)
   ↓
Controller
   ↓
Service
   ↓
Repository
   ↓
Response

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7️⃣ Graceful Shutdown (End of Lifecycle)

Triggered by:

• Ctrl+C

• SIGTERM

• Kubernetes pod termination

Steps:

1. ContextClosedEvent

2. @PreDestroy

3. DisposableBean.destroy()

4. destroy-method

5. Embedded server stops

Example:

@PreDestroy
public void cleanup() {
    System.out.println("App shutting down");
}

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🧠 Complete Lifecycle Diagram (Text)

main()
 ↓
SpringApplication.run()
 ↓
Prepare Environment
 ↓
Create ApplicationContext
 ↓
Bean Definition Load
 ↓
Bean Creation & Initialization
 ↓
Embedded Server Start
 ↓
ApplicationReadyEvent
 ↓
Application Running
 ↓
Shutdown Trigger
 ↓
Bean Destruction
 ↓
Context Closed

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🎯 Interview One-Liner Answer

“Spring Boot lifecycle starts with SpringApplication.run, prepares the environment, initializes the ApplicationContext, creates and initializes beans, starts the embedded server, handles requests, and finally performs graceful shutdown by destroying beans.”

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⚠️ Common Interview Follow-Ups

Q. Difference between @PostConstruct and @Bean init-method?
👉 @PostConstruct is JSR-250, init-method is Spring-specific.

Q. When are BeanPostProcessors executed?
👉 Before and after bean initialization.

Q. When does embedded Tomcat start?
👉 After ApplicationContext is refreshed.

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If you want:

• 📌 Lifecycle diagram image

• 📌 Lifecycle vs Spring Framework comparison

• 📌 Real production troubleshooting examples

• 📌 Bean lifecycle coding question

Just tell me 👍