public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {this.resourceLoader = resourceLoader;Assert.notNull(primarySources, "PrimarySources must not be null");this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));//判断当前web应用程序类型this.webApplicationType = deduceWebApplicationType();//找到*META-INF/spring.factories*中声明的所有ApplicationContextInitializer的实现类并将其实例化
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));//找到*META-INF/spring.factories*中声明的所有ApplicationListener的实现类并将其实例化setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//获得当前执行main方法的类对象this.mainApplicationClass = deduceMainApplicationClass();
}

private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet","org.springframework.web.context.ConfigurableWebApplicationContext" };private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."+ "web.reactive.DispatcherHandler";private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework."+ "web.servlet.DispatcherServlet";private WebApplicationType deduceWebApplicationType() {if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)&& !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) {return WebApplicationType.REACTIVE;}for (String className : WEB_ENVIRONMENT_CLASSES) {if (!ClassUtils.isPresent(className, null)) {return WebApplicationType.NONE;}}return WebApplicationType.SERVLET;
}

public ConfigurableApplicationContext run(String... args) {//开启任务执行时间监听器StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();//设置系统属性『java.awt.headless』，为true则启用headless模式支持
    configureHeadlessProperty();//通过*SpringFactoriesLoader*检索*META-INF/spring.factories*，//找到声明的所有SpringApplicationRunListener的实现类并将其实例化，//之后逐个调用其started()方法，广播SpringBoot要开始执行了。SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);//创建并配置当前SpringBoot应用将要使用的Environment（包括配置要使用的PropertySource以及Profile）,//并遍历调用所有的SpringApplicationRunListener的environmentPrepared()方法，广播Environment准备完毕。                ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);//是否搜索BeanInfo类
        configureIgnoreBeanInfo(environment);//Banner打印Banner printedBanner = printBanner(environment);//根据WebApplicationType的值来决定创建何种类型的ApplicationContext对象context = createApplicationContext();//通过*SpringFactoriesLoader*检索*META-INF/spring.factories*，获取并实例化异常分析器exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,new Class[] { ConfigurableApplicationContext.class }, context);//为ApplicationContext加载environment，之后逐个执行ApplicationContextInitializer的initialize()方法来进一步封装ApplicationContext，//并调用所有的SpringApplicationRunListener的contextPrepared()方法，【EventPublishingRunListener只提供了一个空的contextPrepared()方法】，//之后初始化IoC容器，并调用SpringApplicationRunListener的contextLoaded()方法，广播ApplicationContext的IoC加载完成，//这里就包括通过**@EnableAutoConfiguration**导入的各种自动配置类。
        prepareContext(context, environment, listeners, applicationArguments,printedBanner);//初始化所有自动配置类，调用ApplicationContext的refresh()方法
        refreshContext(context);//空方法
        afterRefresh(context, applicationArguments);/关闭任务执行时间监听器stopWatch.stop();//如果开启日志，则打印执行是时间if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);}//调用所有的SpringApplicationRunListener的started()方法，广播SpringBoot已经完成了ApplicationContext初始化的全部过程。
        listeners.started(context);//遍历所有注册的ApplicationRunner和CommandLineRunner，并执行其run()方法。//我们可以实现自己的ApplicationRunner或者CommandLineRunner，来对SpringBoot的启动过程进行扩展。
        callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, listeners);throw new IllegalStateException(ex);}try {//调用所有的SpringApplicationRunListener的running()方法，广播SpringBoot已经可以处理服务请求了。
        listeners.running(context);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, null);throw new IllegalStateException(ex);}return context;
}

//应用程序非web环境
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."+ "annotation.AnnotationConfigApplicationContext";//应用程序web环境
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";//应用程序响应式web环境
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";public enum WebApplicationType {//应用程序不需要任何应用服务器
    NONE,//应用程序内嵌web服务器
    SERVLET,//应用程序内嵌响应式web服务器
    REACTIVE}protected ConfigurableApplicationContext createApplicationContext() {Class<?> contextClass = this.applicationContextClass;if (contextClass == null) {try {switch (this.webApplicationType) {case SERVLET:contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);break;case REACTIVE:contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);break;default:contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);}}catch (ClassNotFoundException ex) {throw new IllegalStateException("Unable create a default ApplicationContext, "+ "please specify an ApplicationContextClass",ex);}}return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

@Override
protected void onRefresh() {super.onRefresh();try {//创建web应用服务
        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) {//获取ServletWebServerFactory类型的web服务器工厂类，比如TomcatServletWebServerFactory，JettyServletWebServerFactory，UndertowServletWebServerFactoryServletWebServerFactory factory = getWebServerFactory();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 WebApplicationContext initWebApplicationContext(ServletContext servletContext)；

上下文创建好之后调用

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

其中 servletContext的实例是 org.apache.catalina.core.ApplicationContext；

WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = org.springframework.web.context.WebApplicationContext.ROOT）

这样rootcontext就创建好了，并且放入了servletContext中保存。rootcontext获取方式如下。

WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext());

Springmvc项目中的DispatcherServlet是在xml中按照servlet格式配置的，这种方式创建的servlet实例没有被spring容器管理。

/** WebApplicationContext for this servlet */
private WebApplicationContext webApplicationContext;

这种情况下webApplicationContext变量是无法注入的【DispatcherServlet实例没有被spring容器管理】。看一下DispatcherServlet的父类FrameworkServlet是如何初始化上下文的，如下。

protected WebApplicationContext initWebApplicationContext() {WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());WebApplicationContext wac = null;
    //springmvc项目这块的判断为false；springboot项目为ture。if (this.webApplicationContext != null) {......

DispatcherServlet所属上下文的存储

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {...request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());...
}

DispatcherServlet所属上下文获取org.springframework.web.servlet.support.RequestContextUtils。

Springboot项目上下文

//ServletWebServerApplicationContextprotected void prepareWebApplicationContext(ServletContext servletContext) {...servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this);...
}