Bean Lifecycle in Spring Boot

A polite tour through how Spring creates, uses, and destroys your objects while you pretend you are in control.

Spring Boot applications are mostly made of beans.

A bean is just an object that Spring manages for you.
You write the class. Spring handles the birth, setup, wiring, and death of the object. Like a very organized hotel manager for Java classes. Humanity invented dependency injection because manually creating objects became emotionally exhausting.

Let’s understand the Bean Lifecycle using simple questions and answers.

---

First Question: What is a Bean?

Suppose we have this service:

@Service
public class EmailService {

    public void sendEmail() {
        System.out.println("Sending email...");
    }
}

@Service tells Spring:

"Please create this object and manage it for me."

That object becomes a Spring Bean.

---

Second Question: What does “Lifecycle” mean?

Lifecycle means:

1. Bean is created
2. Bean gets dependencies
3. Bean gets initialized
4. Bean is used
5. Bean gets destroyed

Like this:

Create → Inject Dependencies → Initialize → Use → Destroy

Spring does all this automatically.

Because apparently developers once enjoyed writing 200 lines of setup code just to create one object.

---

Third Question: When does Spring create a Bean?

Spring creates beans when the application starts.

Example:

@SpringBootApplication
public class DemoApplication {

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

When run() happens:

• Spring scans classes
• Finds annotations like @Component, @Service
• Creates objects
• Stores them inside the Spring Container

The container is basically Spring’s giant object warehouse.

---

Fourth Question: What happens first in Bean Lifecycle?

Step 1: Bean Instantiation

Spring creates the object.

Example:

@Service
public class UserService {

    public UserService() {
        System.out.println("Constructor called");
    }
}

Output:

Constructor called

This is the very first lifecycle step.

---

Fifth Question: What happens after object creation?

Step 2: Dependency Injection

Suppose this bean needs another bean.

@Service
public class NotificationService {

    public void notifyUser() {
        System.out.println("Notifying user");
    }
}

Now inject it:

@Service
public class UserService {

    private final NotificationService notificationService;

    public UserService(NotificationService notificationService) {
        this.notificationService = notificationService;
    }
}

Spring does this automatically.

It first creates NotificationService, then gives it to UserService.

This is called Dependency Injection.

Fancy name. Simple idea.

---

Sixth Question: Can we run code after bean creation?

Yes.

Step 3: Initialization

Sometimes we want code to run after dependencies are ready.

Example:

@Service
public class DatabaseService {

    @PostConstruct
    public void init() {
        System.out.println("Database connection initialized");
    }
}

Output:

Database connection initialized

@PostConstruct runs after:

• bean creation
• dependency injection

This is useful for:

• opening connections
• loading files
• caching data

---

Seventh Question: What is the exact order until now?

The order is:

1. Constructor
2. Dependency Injection
3. @PostConstruct

Example:

@Service
public class DemoService {

    public DemoService() {
        System.out.println("1. Constructor");
    }

    @PostConstruct
    public void init() {
        System.out.println("2. PostConstruct");
    }
}

Output:

1. Constructor
2. PostConstruct

Simple. Beautiful. Suspiciously efficient.

---

Eighth Question: What happens while the application is running?

Step 4: Bean is Ready to Use

Now the bean works normally.

@RestController
public class HelloController {

    private final UserService userService;

    public HelloController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/hello")
    public String hello() {
        return "Hello";
    }
}

Spring keeps the bean alive inside the container.

By default, beans are Singleton.

Meaning:

Only one object is created for the entire application

---

Ninth Question: What happens when application stops?

Step 5: Bean Destruction

Spring destroys beans gracefully.

We can run cleanup code before destruction.

Example:

@Service
public class FileService {

    @PreDestroy
    public void cleanup() {
        System.out.println("Closing files...");
    }
}

When app shuts down:

Closing files...

Useful for:

• closing DB connections
• stopping threads
• releasing resources

Because memory leaks are the software version of leaving your kitchen gas on.

---

Tenth Question: What is the Full Lifecycle?

Here is the complete flow:

Spring Starts
     ↓
Bean Created
     ↓
Dependencies Injected
     ↓
@PostConstruct Runs
     ↓
Bean Ready to Use
     ↓
Application Stops
     ↓
@PreDestroy Runs
     ↓
Bean Removed

---

Full Example

@Service
public class LifeCycleService {

    public LifeCycleService() {
        System.out.println("1. Constructor called");
    }

    @PostConstruct
    public void init() {
        System.out.println("2. Bean initialized");
    }

    public void doWork() {
        System.out.println("3. Bean is working");
    }

    @PreDestroy
    public void destroy() {
        System.out.println("4. Bean destroyed");
    }
}

Possible Output:

1. Constructor called
2. Bean initialized
3. Bean is working
4. Bean destroyed

---

Final Mental Model

Think of Spring like a factory manager.

You provide class blueprints.

Spring:

• creates objects
• connects objects together
• prepares them
• manages them
• destroys them safely

You focus on business logic.

Spring handles object management.

And somewhere deep inside the framework, thousands of reflection calls whisper through the void while your app boots for 14 seconds because someone added three extra starters.

---

Quick Revision

Step	Description
Instantiation	Bean object created
Dependency Injection	Required beans injected
Initialization	@PostConstruct runs
Ready State	Bean is used
Destruction	@PreDestroy runs