初始化SpringBoot

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();
}

获取初始化应用程序的主要来源配置类
判断应用程序的类型是否是 web 类型
设置所有的ApplicationContextInitializer，用于在应用程序上下文（ApplicationContext）被创建但尚未刷新时对其进行初始化和定制，例如添加属性源、注册自定义的 Bean 定义、激活 Spring 的 Profile、配置环境属性
设置所有的ApplicationListener，监听应用程序中的事件并做出相应的响应

run方法

public ConfigurableApplicationContext run(String... args) {
    Startup startup = Startup.create();
    if (this.registerShutdownHook) {
       SpringApplication.shutdownHook.enableShutdownHookAddition();
    }
    DefaultBootstrapContext bootstrapContext = createBootstrapContext();
    ConfigurableApplicationContext context = null;
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting(bootstrapContext, this.mainApplicationClass);
    try {
       ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
       ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
       Banner printedBanner = printBanner(environment);
       context = createApplicationContext();
       context.setApplicationStartup(this.applicationStartup);
       prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
       refreshContext(context);
       afterRefresh(context, applicationArguments);
       startup.started();
       if (this.logStartupInfo) {
          new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), startup);
       }
       listeners.started(context, startup.timeTakenToStarted());
       callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
       throw handleRunFailure(context, ex, listeners);
    }
    try {
       if (context.isRunning()) {
          listeners.ready(context, startup.ready());
       }
    }
    catch (Throwable ex) {
       throw handleRunFailure(context, ex, null);
    }
    return context;
}

获取定义的 RunListeners，并且调用所有的RunListener的启动方法，启动监听器模块
DefaultApplicationArguments准备所有参数，包括我们传入的参数 arguments
prepareEnvironment准备环境对象，也会调用我们自定义的 listener 中的环境准备方法，配置环境模块
打印图标Banner，配置彩蛋
prepareContext创建上下文对象 context，也就是 ioc 容器，应用上下文模块
refreshContext刷新 ioc 容器，加载更多的Bean，更新应用上下文
然后就启动成功了，调用一些回调方法，最后返回 ioc 的容器

六、SpringBoot 启动流程分析

初始化SpringBoot

run方法