1.从应用程序启动入口开始追踪代码：

SpringApplication.run(SpringbootDemoApplication.class, args);

2.进入SpringApplication类的构造方法：

public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {this.resourceLoader = resourceLoader;Assert.notNull(primarySources, "PrimarySources must not be null");this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));this.webApplicationType = WebApplicationType.deduceFromClasspath();this.bootstrapRegistryInitializers = new ArrayList<>(getSpringFactoriesInstances(BootstrapRegistryInitializer.class));setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));this.mainApplicationClass = deduceMainApplicationClass();}

这个步骤做了以下几件事：

a.初始化资源加载器，但是从上下文可以看出是null

b.设置主资源类，即第一步传过来的SpringbootDemoApplication类

c.推断webApplicationType的类型：WebApplicationType.SERVLET

d.设置bootstrapRegistryInitializers数据，通过SpringFactoriesLoader从META-INF/spring.factories目录加载key为BootstrapRegistryInitializer的数据并初始化

e.设置initializers，通过SpringFactoriesLoader从上下文中所以jar根目录META-INF/spring.factories目录加载key为ApplicationContextInitializer的数据并初始化

f.设置listeners，通过SpringFactoriesLoader从上下文中所以jar根目录META-INF/spring.factories目录加载key为ApplicationListener的数据并初始化

j.推断主应用类，即SpringbootDemoApplication类

3.接着继续进入SpringApplication类的run方法中，如下代码：

public ConfigurableApplicationContext run(String... args) {long startTime = System.nanoTime();// a.创建一个springboot上下文对象DefaultBootstrapContext bootstrapContext = createBootstrapContext();ConfigurableApplicationContext context = null;configureHeadlessProperty();// b.获取并启动监听的listenersSpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting(bootstrapContext, this.mainApplicationClass);try {// c.初始化应用环境ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);configureIgnoreBeanInfo(environment);// 打印启动banner数据Banner printedBanner = printBanner(environment);// d.初始化应用上下文context = createApplicationContext();context.setApplicationStartup(this.applicationStartup);// e.刷新应用上下文前的准备工作prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);// f.刷新应用上下文refreshContext(context);// j.刷新应用上下文之后的扩展功能afterRefresh(context, applicationArguments);Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);}listeners.started(context, timeTakenToStartup);// h.调用ApplicationRunner和CommandLineRunner的run方法callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, listeners);throw new IllegalStateException(ex);}try {Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);listeners.ready(context, timeTakenToReady);}catch (Throwable ex) {handleRunFailure(context, ex, null);throw new IllegalStateException(ex);}return context;}

接下来具体解释一下，每一步的细节：

a.创建springboot上下文对象：创建一个类型为DefaultBootstrapContext的上下文对象，并且将第二步获取过来的bootstrapRegistryInitializers执行初始化操作，执行initialize方法

private DefaultBootstrapContext createBootstrapContext() {DefaultBootstrapContext bootstrapContext = new DefaultBootstrapContext();this.bootstrapRegistryInitializers.forEach((initializer) -> initializer.initialize(bootstrapContext));return bootstrapContext;}

BootstrapRegistryInitializer接口类：

@FunctionalInterface
public interface BootstrapRegistryInitializer {/*** Initialize the given {@link BootstrapRegistry} with any required registrations.* @param registry the registry to initialize*/void initialize(BootstrapRegistry registry);}

可以自行实现该接口并做某些初始化作用，这个初始化的时机比较早。

b.获取并启动监听的listeners：获取并实例化所有类型为SpringApplicationRunListener的listener

private SpringApplicationRunListeners getRunListeners(String[] args) {Class[] types = new Class[] { SpringApplication.class, String[].class };return new SpringApplicationRunListeners(logger,getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args),this.applicationStartup);}

启动listener

void starting(ConfigurableBootstrapContext bootstrapContext, Class mainApplicationClass) {doWithListeners("spring.boot.application.starting", (listener) -> listener.starting(bootstrapContext),(step) -> {if (mainApplicationClass != null) {step.tag("mainApplicationClass", mainApplicationClass.getName());}});}

进入org.springframework.boot.context.event.EventPublishingRunListener#starting方法中(说明：EventPublishingRunListener这个runlistener是通过反射初始化的，通过getRunListeners反射调用的是EventPublishingRunListener(SpringApplication application, String[] args) 这个构造方法，有兴趣可以看看，里面初始化了多播器：initialMulticaster和当前应用对象：application)：发布了一个ApplicationStartingEvent事件对象

@Overridepublic void starting(ConfigurableBootstrapContext bootstrapContext) {this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(bootstrapContext, this.application, this.args));}

继续追踪到org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)方法：

public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));Executor executor = getTaskExecutor();for (ApplicationListener listener : getApplicationListeners(event, type)) {if (executor != null) {executor.execute(() -> invokeListener(listener, event));}else {invokeListener(listener, event);}}}

继续追到org.springframework.context.event.SimpleApplicationEventMulticaster#doInvokeListener：

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {try {listener.onApplicationEvent(event);}catch (ClassCastException ex) {// 省略部分代码...}}

最终执行各个listener的onApplicationEvent方法。

springboot中所有的事件发布都是由org.springframework.boot.context.event.EventPublishingRunListener这个类来发布完成的，它会在容器初始化的各个阶段发布各种不同的事件，关注了这个事件的监听器就能接受到，然后执行对应的方法。

c.初始化应用环境，代码如下：

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments, Banner printedBanner) {context.setEnvironment(environment);postProcessApplicationContext(context);// 执行第二步获取的initializers方法applyInitializers(context);// 发布一个ApplicationContextInitializedEvent类型的事件，告诉listener准备监听listeners.contextPrepared(context);bootstrapContext.close(context);if (this.logStartupInfo) {logStartupInfo(context.getParent() == null);logStartupProfileInfo(context);}// Add boot specific singleton beansConfigurableListableBeanFactory beanFactory = context.getBeanFactory();beanFactory.registerSingleton("springApplicationArguments", applicationArguments);if (printedBanner != null) {beanFactory.registerSingleton("springBootBanner", printedBanner);}if (beanFactory instanceof AbstractAutowireCapableBeanFactory) {((AbstractAutowireCapableBeanFactory) beanFactory).setAllowCircularReferences(this.allowCircularReferences);if (beanFactory instanceof DefaultListableBeanFactory) {((DefaultListableBeanFactory) beanFactory).setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);}}if (this.lazyInitialization) {context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());}context.addBeanFactoryPostProcessor(new PropertySourceOrderingBeanFactoryPostProcessor(context));// Load the sourcesSet