Spring Bean

What Is a Spring Bean?

An independent object managed by the Spring Framework.
It is an ordinary Java object, except that Spring manages it.
Simply put, it is an object that performs a task and can be used for DI at any time while Spring manages it.

A container generally manages the life cycle of instances and provides additional features to created instances.
- In other words, it references application code and controls object creation and destruction.

Manages Spring Bean objects.
Creates and uses Bean objects based on developer-defined Bean classes.
- The container, rather than the developer, creates objects. This enables DI and lets the Spring container control them.

It is called inversion of control because the container controls the program flow previously controlled by the developer.
Spring IoC manages object creation and life cycles. Transferring control to the Spring Framework enables DI and AOP.

singleton
- Only one object exists in the Spring IoC container for a Bean definition.
- This is the default scope when none is specified.
prototype
- Multiple objects can exist for one Bean definition.
- A new object is created for every request.
  - A new prototype Bean is created when injected into a dependent Bean.
  - It is removed normally by garbage collection.
request
- One object exists for one HTTP request life cycle.
- Each HTTP request has its own object.
- Valid only in a web-aware Spring ApplicationContext.
session
- One object exists for one HTTP session life cycle.
- Valid only in a web-aware Spring ApplicationContext.
global session
- One object exists for one global HTTP session life cycle.
- Generally valid in a portlet context.
- Valid only in a web-aware Spring ApplicationContext.

Use it to register a class from an external library as a Spring Bean.
- Use @Bean for external libraries that you cannot directly control.
- Declare it inside a class annotated with @Configuration.
- The object returned by a developer-written method becomes a Bean.

Use it to register a developer-written class as a Spring Bean.
- A developer-written class becomes a Bean.

Use the following annotations when the class has a specific role:

@Controller
- Fundamentally the same as @Component.
- Emphasizes a presentation-layer component.
@Service
- Fundamentally the same as @Component.
- Emphasizes a business-layer component that implements business logic.
@Repository
- Fundamentally the same as @Component.
- Emphasizes a persistence-layer component, such as database access.
- Catches platform-specific exceptions and throws Spring unchecked exceptions.