The Spring Framework manages objects using IoC (Inversion of Control).
To implement IoC, Spring provides two types of containers:

1. BeanFactory
2. ApplicationContext

Both containers create, configure, and manage beans — but they differ in features, performance, and use cases.

Understanding their differences is essential for grasping how Spring Boot works internally.

---

1. What Is BeanFactory?

BeanFactory is the simplest Spring IoC container.

It provides the core capabilities of managing beans:

• Creating beans
• Injecting dependencies
• Managing bean lifecycle
• Lazy initialization (beans created only when needed)

BeanFactory Example:

BeanFactory factory = new XmlBeanFactory(new ClassPathResource("beans.xml"));
Car car = factory.getBean("car", Car.class);

BeanFactory is minimal and lightweight, but lacks many convenience features.

---

2. What Is ApplicationContext?

ApplicationContext is the advanced IoC container built on top of BeanFactory.

It includes everything BeanFactory offers plus:

• Eager initialization (faster startup)
• Event publishing
• Internationalization support
• Environment abstraction
• Resource loading
• Automatic BeanPostProcessors
• Automatic BeanFactoryPostProcessors

It is the default container in Spring Boot.

ApplicationContext Example:

ApplicationContext context = 
        new ClassPathXmlApplicationContext("beans.xml");

Car car = context.getBean(Car.class);

---

3. BeanFactory vs ApplicationContext (Feature Comparison)

Feature	BeanFactory	ApplicationContext
Bean creation	Lazy	Eager
Event handling	No	Yes
Annotation processing	Manual	Automatic
BeanPostProcessor support	Manual	Automatic
Internationalization	No	Yes
Environment abstraction	No	Yes
Used in Spring Boot?	Rarely	Always
Primary use case	Memory-constrained apps	Modern enterprise apps

---

4. Lazy vs Eager Initialization

BeanFactory → Lazy Initialization

Beans are created only when accessed.

ApplicationContext → Eager Initialization

Beans are created at startup.

This makes ApplicationContext:

• Faster during runtime
• Better for detecting errors early

Spring Boot follows eager initialization, making applications reliable.

---

5. Why Spring Boot Uses ApplicationContext

Spring Boot uses ApplicationContext by default because it offers:

✔ Auto-configuration

Required for Spring Boot's magic.

✔ Component scanning

Automatically detects beans in your project.

✔ Profiles

Load different configs for dev, test, prod.

✔ Environment abstraction

Access to system/env properties.

✔ Event publishing

Used for lifecycle events.

✔ Integration with Spring Boot starters

Ensures everything works smoothly.

Without ApplicationContext, Spring Boot features would not function properly.

---

6. Real-World Analogy

Think of IoC containers like smartphones:

BeanFactory = Basic Phone

• Calls only
• No internet
• No apps
• Very lightweight

ApplicationContext = Smartphone

• Apps
• Bluetooth
• Wi-Fi
• Camera
• Notifications

You can do much more with ApplicationContext.

---

7. When Should You Use BeanFactory?

Use BeanFactory only when:

• You're working on low-memory devices (rare)
• You need lazy initialization only
• You're maintaining legacy XML-based applications

Today, almost nobody uses BeanFactory in real projects.

---

8. When Should You Use ApplicationContext?

Always use ApplicationContext when:

• Building web applications
• Using Spring Boot
• Using annotations
• Using Spring MVC, Data JPA, Security
• Working with microservices

In Spring Boot, ApplicationContext is used automatically — you never configure it manually.

---

9. Summary

• BeanFactory → Basic IoC container with lazy initialization.
• ApplicationContext → Full-featured IoC container used in modern Spring apps.
• Spring Boot always uses ApplicationContext.
• ApplicationContext provides auto-configuration, event handling, internationalization, and many other features.

Understanding this difference helps you appreciate how Spring Boot manages beans and application startup.

---

What's Next?

Next post:

09 - Spring Bean Lifecycle

We dive into how Spring creates, initializes, and destroys beans behind the scenes.