@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
    return new SpringApplication(primarySource).run(args);
}

public ConfigurableApplicationContext run(String... args) {
    // 1. 創(chuàng)建并啟動計時器
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // 2. 初始化應用上下文和異常報告器
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    // 3. 獲取SpringApplicationRunListeners并啟動
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
        // 4. 準備環(huán)境
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        // 5. 打印Banner
        Banner printedBanner = printBanner(environment);
        // 6. 創(chuàng)建應用上下文
        context = createApplicationContext();
        // 7. 準備應用上下文
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        // 8. 刷新應用上下文(關鍵步驟)
        refreshContext(context);
        // 9. 刷新后處理
        afterRefresh(context, applicationArguments);
        // 10. 停止計時器并發(fā)布啟動完成事件
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                .logStarted(getApplicationLog(), stopWatch);
        }
        listeners.started(context);
        // 11. 執(zhí)行Runner
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }
    try {
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // 1. 準備刷新
        prepareRefresh();
        // 2. 獲取新的BeanFactory
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        // 3. 準備BeanFactory
        prepareBeanFactory(beanFactory);
        try {
            // 4. 后置處理BeanFactory
            postProcessBeanFactory(beanFactory);
            // 5. 調(diào)用BeanFactoryPostProcessor
            invokeBeanFactoryPostProcessors(beanFactory);
            // 6. 注冊BeanPostProcessor
            registerBeanPostProcessors(beanFactory);
            // 7. 初始化MessageSource
            initMessageSource();
            // 8. 初始化事件廣播器
            initApplicationEventMulticaster();
            // 9. 初始化特殊bean(由子類實現(xiàn))
            onRefresh();
            // 10. 注冊監(jiān)聽器
            registerListeners();
            // 11. 初始化所有非懶加載單例
            finishBeanFactoryInitialization(beanFactory);
            // 12. 完成刷新
            finishRefresh();
        }
        catch (BeansException ex) {
            // 處理異常...
        }
    }
}

protected void onRefresh() {
    super.onRefresh();
    try {
        createWebServer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        // 1. 獲取WebServer工廠(Tomcat, Jetty或Undertow)
        ServletWebServerFactory factory = getWebServerFactory();
        // 2. 創(chuàng)建WebServer
        this.webServer = factory.getWebServer(getSelfInitializer());
    }
    else if (servletContext != null) {
        try {
            getSelfInitializer().onStartup(servletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context", ex);
        }
    }
    initPropertySources();
}

public WebServer getWebServer(ServletContextInitializer... initializers) {
    // 1. 創(chuàng)建Tomcat實例
    Tomcat tomcat = new Tomcat();
    // 2. 配置基礎目錄
    File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    // 3. 配置連接器
    Connector connector = new Connector(this.protocol);
    connector.setThrowOnFailure(true);
    tomcat.getService().addConnector(connector);
    customizeConnector(connector);
    tomcat.setConnector(connector);
    // 4. 配置Host
    tomcat.getHost().setAutoDeploy(false);
    configureEngine(tomcat.getEngine());
    // 5. 準備上下文
    prepareContext(tomcat.getHost(), initializers);
    // 6. 創(chuàng)建TomcatWebServer并啟動
    return getTomcatWebServer(tomcat);
}

public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    initialize();
}
private void initialize() throws WebServerException {
    // 啟動Tomcat
    this.tomcat.start();
    // 啟動一個守護線程來等待停止命令
    startDaemonAwaitThread();
}