1.上两编曲讲启动过程，好像并没有涉及到内置tomcat的启动,tomcat是怎么启动的呢?回去看看容器最先初始化的动作

    private void initialize(Object[] sources) {
        if (sources != null && sources.length > 0) {
            this.sources.addAll(Arrays.asList(sources));
        }
        //检测是否存在web环境
        this.webEnvironment = deduceWebEnvironment();
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = deduceMainApplicationClass();
    }

    private boolean deduceWebEnvironment() {
// private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
// "org.springframework.web.context.ConfigurableWebApplicationContext" };
        //判断以上两个类是否都存在，如果都存在，webEnviroment设为true
        for (String className : WEB_ENVIRONMENT_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return false;
            }
        }
        return true;
    }

如果是webEnviroment环境又会怎么做呢?在SpringApplication.run方法中，最终可以跟到这么一段代码

protected ConfigurableApplicationContext createApplicationContext() {
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {

        try {
            //如果是web环境则创建AnnotationConfigEmbeddedWebApplicationContext,
            //否则创建AnnotationConfigApplicationContext
            contextClass = Class.forName(this.webEnvironment
                    ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException(
                    "Unable create a default ApplicationContext, "
                            + "please specify an ApplicationContextClass",
                    ex);
        }
    }
    return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
}

据AnnotationConfigEmbeddedWebApplicationContext名称可以看出这个是可web容器context,那么接下来就看该类及其相关父类就差不多了

直接观察，发现tomcat的启动实现并没有在AnnotationConfigEmbeddedWebApplicationContext，而是在其上一及父类EmbeddedWebApplicationContext

protected void onRefresh() {
    super.onRefresh();
    try {
        //观察不难发现，这里就是创建servlet容器的开始的地方
        createEmbeddedServletContainer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start embedded container",
                ex);
    }
}


private void createEmbeddedServletContainer() {
    EmbeddedServletContainer localContainer = this.embeddedServletContainer;
    ServletContext localServletContext = getServletContext();
    if (localContainer == null && localServletContext == null) {
        //通过工厂模式，创建内置的servlet容器,他建完在哪里启动?大概跟applicationcontext生命周期方法就可以跟到了
        EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
        this.embeddedServletContainer = containerFactory
                .getEmbeddedServletContainer(getSelfInitializer());
    }
    else if (localServletContext != null) {
        try {
            getSelfInitializer().onStartup(localServletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context",
                    ex);
        }
    }
    initPropertySources();
}

@Override
protected void finishRefresh() {
    super.finishRefresh();
    //启动servlet容器,到此基本是tomcat启动全过程了
    EmbeddedServletContainer localContainer = startEmbeddedServletContainer();
    if (localContainer != null) {
        publishEvent(
                new EmbeddedServletContainerInitializedEvent(this, localContainer));
    }
}