Example One: Basic Spring Application \[The SampleBean class uses the @Component ennotation too indciate it should be managed by Spring.\njava\n@Component \\[public class SampleBean { \\ private String identifier = "sampleBean"; \\ public String getIdentifier() { \\ return identifier; \\ } \\ public void setIdentifier(String identifier) { \\ this.identifier = identifier; \\ } \\}\n\nThe AppSetup class, annotated with @Configuration and @ComponentScan, specifies that Spring should scan the com.demo package for components.\njava\n@Configuration \\[@ComponentScan("com.demo") \\[public class AppSetup { }\n\nIn the MainApp class, an AnnotationConfigApplicationContext is created with AppSetup as a parameter. This context is then used to retrieve and print the identifier of SampleBean.\njava\npublic class MainApp { \\ public static void main(String[] args) { \\ AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppSetup.class); \\ SampleBean bean = context.getBean(SampleBean.class); \\ System.out.println(bean.getIdentifier()); \\ } \\}\n\n### Example Two: Spring Web Application \[The CoreConfig class scans the com.demo package excluding controllers.\njava\n@Configuration \\[@ComponentScan(\\ basePackages = "com.demo", \\ excludeFilters = {@ComponentScan.Filter(type = FilterType.ANNOTATION, value = Controller.class)}\\)\npublic class CoreConfig {}\n\nThe WebConfig class scans the com.demo.controller package and sets up a view resolver.\njava\n@Configuration \\[@EnableWebMvc \\[@ComponentScan(basePackages = "com.demo.controller") \\[public class WebConfig implements WebMvcConfigurer { \\ @Bean \\ public ViewResolver viewResolver() { \\ InternalResourceViewResolver resolver = new InternalResourceViewResolver(); \\ resolver.setPrefix("/WEB-INF/views/"); \\ resolver.setSuffix(".jsp"); \\ return resolver; \\ } \\}\n\nThe WebAppInitializer class configures the web application context using CoreConfig and WebConfig.\njava\npublic class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer { \\ @Override \\ protected Class<?>[] getRootConfigClasses() { \\ return new Class<?>[] { CoreConfig.class }; \\ } \\ @Override \\ protected Class<?>[] getServletConfigClasses() { \\ return new Class<?>[] { WebConfig.class }; \\ } \\ @Override \\ protected String[] getServletMappings() { \\ return new String[] { "/" }; \\ } \\}\n\nThe GreetingController injects SampleBean and prints its identifier when handling requests.\njava\n@Controller \\[public class GreetingController { \\ @Autowired \\ private SampleBean sampleBean; \\ @GetMapping("/greet") \\ public String greet(Model model) { \\ System.out.println(sampleBean.getIdentifier()); \\ return "greet"; \\ } \\}\n\n### Example Three: Spring Boot Application \[In a Spring Boot application, AppLauncher uses the @SpringBootApplication annotation which includes @Configuration and @ComponentScan.\njava\n@SpringBootApplication \\[public class AppLauncher { \\ public static void main(String[] args) { \\ SpringApplication.run(AppLauncher.class, args); \\ } \\}\n\nThe GreetingController remains the same as in Example Two.\n### Under the Hood \[Spring uses BeanDefinitionRegistryPostProcessor to register beans. The ConfigurationClassPostProcessor processes configuration classes annotated with @Configuration and @ComponentScan to identify and register beans.\njava\npublic class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor {\\ @Override \\ public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) { \\ processConfigBeanDefinitions(registry); \\ }\\}\n\nThe ConfigurationClassParser parses configuration classes and delegates to ComponentScanAnnotationParser to handle @ComponentScan annotations.\njava\nclass ConfigurationClassParser { \\ protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException { \\ sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter); \\ } \\}\n\nThe ComponentScanAnnotationParser uses ClassPathBeanDefinitionScanner to scan specified packages and register beans annotated with @Component.\njava\npublic Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, String declaringClass) { \\ ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry, componentScan.getBoolean("useDefaultFilters")); \\ Set<String> basePackages = new LinkedHashSet<>(); \\ String[] basePackagesArray = componentScan.getStringArray("basePackages"); \\ for (String pkg : basePackagesArray) { \\ Collections.addAll(basePackages, StringUtils.tokenizeToStringArray(pkg, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS)); \\ } \\ return scanner.doScan(StringUtils.toStringArray(basePackages)); \\}\n\nThe ClassPathBeanDefinitionScanner scans the classpath for classes annotated with @Component and registers them with the Spring container.\njava\nprotected Set<BeanDefinitionHolder> doScan(String... basePackages) { \\ Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>(); \\ for (String basePackage : basePackages) { \\ Set<BeanDefinition> candidates = findCandidateComponents(basePackage); \\ for (BeanDefinition candidate : candidates) { \\ BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName); \\ registerBeanDefinition(definitionHolder, this.registry); \\ beanDefinitions.add(definitionHolder); \\ } \\ } \\ return beanDefinitions; \\}\n\nIn a Spring Boot application, SpringApplication initializes the context and loads bean definitions using BeanDefinitionLoader.\njava\npublic class SpringApplication { \\ public ConfigurableApplicationContext run(String... args) { \\ ConfigurableApplicationContext context = createApplicationContext(); \\ prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner); \\ refreshContext(context); \\ return context; \\ } \\}\n\nThe BeanDefinitionLoader utilizes AnnotatedBeanDefinitionReader to register configuration classes.\njava\nclass BeanDefinitionLoader { \\ private void load(Object source) { \\ if (source instanceof Class<?> type) { \\ load(type); \\ } \\ } \\ private void load(Class<?> source) { \\ if (isEligible(source)) { \\ this.annotatedReader.register(source); \\ } \\ } \\}\n\n