IoC Container Spring ka HEART hai — objects (beans) ko CREATE, CONFIGURE, aur MANAGE karta hai, poori application lifecycle ke through. Normally, object apni dependencies KHUD create karta hai (control uske paas). IoC mein, ye control INVERT ho jaata hai — container dependencies create karke object ko DETA hai.

Do main container types hain: BeanFactory (basic, lazy initialization) aur ApplicationContext (advanced, ZYAADA features — event handling, internationalization, AOP integration; EAGER initialization by default). Practically, ApplicationContext hi almost hamesha use hota hai — BeanFactory sirf memory-constrained environments ke liye.

// ApplicationContext — Spring ka main container:
ApplicationContext context =
    new AnnotationConfigApplicationContext(AppConfig.class);

UserService userService = context.getBean(UserService.class);
// Container ne UserService, aur uski saari dependencies, khud create ki

// Spring Boot mein, ye automatically hota hai:
@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);   // container internally banta hai
    }
}