SpringBoot是如何创建ServletContext

SpringBoot在run方法启动后，最终还是会调用Spring的refresh方法

入口在这里

public ConfigurableApplicationContext run(String... args) {
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();
   ConfigurableApplicationContext context = null;
   Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
   configureHeadlessProperty();
   SpringApplicationRunListeners listeners = getRunListeners(args);
   listeners.starting();
   try {
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
      ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
      configureIgnoreBeanInfo(environment);
      Banner printedBanner = printBanner(environment);
      context = createApplicationContext();
      exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
            new Class[] { ConfigurableApplicationContext.class }, context);
      prepareContext(context, environment, listeners, applicationArguments, printedBanner);
      refreshContext(context);
      afterRefresh(context, applicationArguments);
      stopWatch.stop();
      if (this.logStartupInfo) {
         new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
      }
      listeners.started(context);
      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;
}

private void refreshContext(ConfigurableApplicationContext context) {
   refresh(context);
   if (this.registerShutdownHook) {
      try {
         context.registerShutdownHook();
      }
      catch (AccessControlException ex) {
         // Not allowed in some environments.
      }
   }
}

protected void refresh(ApplicationContext applicationContext) {
   Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
   ((AbstractApplicationContext) applicationContext).refresh();
}

 AbstractApplicationContext  被ServletWebServerApplicationContext实现，重写了onRefresh方法

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) {
      ServletWebServerFactory 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();
}

 此处servletContext为空，继续往下看getSelfInitializer()方法

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
   return this::selfInitialize;
}

private void selfInitialize(ServletContext servletContext) throws ServletException {
   prepareWebApplicationContext(servletContext);
   ConfigurableListableBeanFactory beanFactory = getBeanFactory();
   ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(
         beanFactory);
   WebApplicationContextUtils.registerWebApplicationScopes(beanFactory,
         getServletContext());
   existingScopes.restore();
   WebApplicationContextUtils.registerEnvironmentBeans(beanFactory,
         getServletContext());
   for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
      beans.onStartup(servletContext);
   }
}

 此处this::selfInitialize返回的是函数式方法实现，类似匿名内部类，而不是直接调用（注：理解了很久，PS:断点一直不进去，很奇怪），此处返回一个ServletContextInitialzer的实现。进入getWebServer方法

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
   File documentRoot = getValidDocumentRoot();
   TomcatEmbeddedContext context = new TomcatEmbeddedContext();
   if (documentRoot != null) {
      context.setResources(new LoaderHidingResourceRoot(context));
   }
   context.setName(getContextPath());
   context.setDisplayName(getDisplayName());
   context.setPath(getContextPath());
   File docBase = (documentRoot != null) ? documentRoot
         : createTempDir("tomcat-docbase");
   context.setDocBase(docBase.getAbsolutePath());
   context.addLifecycleListener(new FixContextListener());
   context.setParentClassLoader(
         (this.resourceLoader != null) ? this.resourceLoader.getClassLoader()
               : ClassUtils.getDefaultClassLoader());
   resetDefaultLocaleMapping(context);
   addLocaleMappings(context);
   context.setUseRelativeRedirects(false);
   configureTldSkipPatterns(context);
   WebappLoader loader = new WebappLoader(context.getParentClassLoader());
   loader.setLoaderClass(TomcatEmbeddedWebappClassLoader.class.getName());
   loader.setDelegate(true);
   context.setLoader(loader);
   if (isRegisterDefaultServlet()) {
      addDefaultServlet(context);
   }
   if (shouldRegisterJspServlet()) {
      addJspServlet(context);
      addJasperInitializer(context);
   }
   context.addLifecycleListener(new StaticResourceConfigurer(context));
   ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
   host.addChild(context);
   configureContext(context, initializersToUse);
   postProcessContext(context);
}

protected void configureContext(Context context,
      ServletContextInitializer[] initializers) {
   TomcatStarter starter = new TomcatStarter(initializers);
   if (context instanceof TomcatEmbeddedContext) {
      // Should be true
      ((TomcatEmbeddedContext) context).setStarter(starter);
   }
   context.addServletContainerInitializer(starter, NO_CLASSES);
   for (LifecycleListener lifecycleListener : this.contextLifecycleListeners) {
      context.addLifecycleListener(lifecycleListener);
   }
   for (Valve valve : this.contextValves) {
      context.getPipeline().addValve(valve);
   }
   for (ErrorPage errorPage : getErrorPages()) {
      new TomcatErrorPage(errorPage).addToContext(context);
   }
   for (MimeMappings.Mapping mapping : getMimeMappings()) {
      context.addMimeMapping(mapping.getExtension(), mapping.getMimeType());
   }
   configureSession(context);
   for (TomcatContextCustomizer customizer : this.tomcatContextCustomizers) {
      customizer.customize(context);
   }
}

 1处ServletContextInitializer封装进了tomcatStarter中，最后starter被添加到了tomcat的context中，在tomcat启动的时候会调用ServletContextInitializer的startUp方法

 最终返回的是ServletContext的门面类。过程太长了，省略了一部分......