# Beans and the Application Context — Spring Boot

Source: https://www.skillbyai.com/en/spring-boot/d-beans

> Inversion of control.

## The container builds the object graph

In Spring, the **application context** creates objects called **beans**, injects their dependencies and manages their lifecycle. Classes become beans through stereotype annotations found by component scanning (`@Component`, `@Service`, `@Repository`, `@RestController`) or through `@Bean` methods in `@Configuration` classes. This **inversion of control** keeps classes focused on their job and makes them easy to test, because dependencies can be replaced with test doubles.

## Wire objects, externalise settings

The Spring container creates and connects your objects and binds configuration into typed classes.

![Three ideas: beans and the container, constructor injection, typed properties.](assets/figures/spring-boot/section-2-map.svg) — Figure 2.1 — Beans, injection and properties.

## A stage manager

Actors (beans) do not hire their own crew; the stage manager (the container) assigns everyone their partners before the show starts.

## Use stereotypes for intent

@Service, @Repository and @RestController all create beans but also document each class's role, and @Repository adds exception translation.

**Quiz:** What is a Spring bean?

- [ ] A Java coffee class
- [x] An object created and managed by the Spring container
- [ ] A database table
- [ ] A configuration file

*Answer:* An object created and managed by the Spring container. The container owns bean lifecycles.