一、ApplicationContext在Spring Boot中是如何创建的？

1. SpringApplication

ApplicationContextFactory有三个实现类，分别是AnnotationConfigReactiveWebServerApplicationContext.Factory、AnnotationConfigServletWebServerApplicationContext.Factory、DefaultApplicationContextFactory。

public ConfigurableApplicationContext run(String... args) {...// 创建ApplicationContextcontext = createApplicationContext();...
}// 调用DefaultApplicationContextFactory的create
protected ConfigurableApplicationContext createApplicationContext() {return this.applicationContextFactory.create(this.webApplicationType);
}

2. DefaultApplicationContextFactory

下面有一点代码SpringFactoriesLoader.loadFactories(ApplicationContextFactory.class, getClass().getClassLoader())，是从org.springframework.boot的META-INF/spring.factories寻找ApplicationContextFactory的实现类，也就是AnnotationConfigReactiveWebServerApplicationContext.Factory和AnnotationConfigServletWebServerApplicationContext.Factory。

@Override
public ConfigurableApplicationContext create(WebApplicationType webApplicationType) {try {return getFromSpringFactories(webApplicationType, ApplicationContextFactory::create,AnnotationConfigApplicationContext::new);}catch (Exception ex) {throw new IllegalStateException("Unable create a default ApplicationContext instance, "+ "you may need a custom ApplicationContextFactory", ex);}
}private <T> T getFromSpringFactories(WebApplicationType webApplicationType,BiFunction<ApplicationContextFactory, WebApplicationType, T> action, Supplier<T> defaultResult) {for (ApplicationContextFactory candidate : SpringFactoriesLoader.loadFactories(ApplicationContextFactory.class,getClass().getClassLoader())) {// 判断应用属于三种NONE、SERVLET、REACTIVE类型中的哪种类型，实例化对应类型的ApplicationContenxt。T result = action.apply(candidate, webApplicationType);if (result != null) {return result;}}// 创建AnnotationConfigApplicationContextreturn (defaultResult != null) ? defaultResult.get() : null;
}

二、AnnotationConfigServletWebServerApplicationContext构造函数做了什么事情？

public AnnotationConfigServletWebServerApplicationContext() {this.reader = new AnnotatedBeanDefinitionReader(this);this.scanner = new ClassPathBeanDefinitionScanner(this);
}

1. AnnotatedBeanDefinitionReader

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {Assert.notNull(registry, "BeanDefinitionRegistry must not be null");Assert.notNull(environment, "Environment must not be null");this.registry = registry;this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);注册了以下6个bean。

• org.springframework.context.annotation.internalConfigurationAnnotationProcessor
• org.springframework.context.annotation.internalAutowiredAnnotationProcessor
• org.springframework.context.annotation.internalCommonAnnotationProcessor
• org.springframework.context.annotation.internalPersistenceAnnotationProcessor
• org.springframework.context.event.internalEventListenerProcessor
• org.springframework.context.event.internalEventListenerFactory

2. ClassPathBeanDefinitionScanner

public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,Environment environment, @Nullable ResourceLoader resourceLoader) {Assert.notNull(registry, "BeanDefinitionRegistry must not be null");this.registry = registry;if (useDefaultFilters) {registerDefaultFilters();}setEnvironment(environment);setResourceLoader(resourceLoader);
}

registerDefaultFilters 注册了以下的过滤器

protected void registerDefaultFilters() {this.includeFilters.add(new AnnotationTypeFilter(Component.class));// 这个ClassLoader是干什么的？？？ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();try {this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));logger.trace("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning");}catch (ClassNotFoundException ex) {// JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.}try {this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));logger.trace("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");}catch (ClassNotFoundException ex) {// JSR-330 API not available - simply skip.}
}

• Component.class
• javax.annotation.ManagedBean
• javax.inject.Named