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

public ConfigurableApplicationContext run(String... args) { StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<springbootexceptionreporter> exceptionReporters = new ArrayList&lt;&gt;(); //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); //发布应用开始启动事件 listeners.starting(); try { //初始化参数 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); configureIgnoreBeanInfo(environment); //打印banner Banner printedBanner = printBanner(environment); //创建应用上下文 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); //刷新上下文 refreshContext(context); //再一次刷新上下文,其实是空方法，可能是为了后续扩展。 afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch); } //发布应用已经启动的事件 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 { //应用已经启动完成的监听事件 listeners.running(context); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, null); throw new IllegalStateException(ex); } return context; }

protected ConfigurableApplicationContext createApplicationContext() { Class<!--?--> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch (this.webApplicationType) { case SERVLET: contextClass = Class.forName(DEFAULT_SERVLET_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); }

//类：SpringApplication.java
private void refreshContext(ConfigurableApplicationContext context) { //直接调用刷新方法 refresh(context); if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch (AccessControlException ex) { // Not allowed in some environments. } } }//类：SpringApplication.java
protected void refresh(ApplicationContext applicationContext) { Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext); ((AbstractApplicationContext) applicationContext).refresh(); }

// 类：AbstractApplicationContextpublic void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // Prepare this context for refreshing. prepareRefresh();
 // Tell the subclass to refresh the internal bean factory. ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
 // Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory);
 try { // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory(beanFactory);
 // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory);
 // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory);
 // Initialize message source for this context. initMessageSource();
 // Initialize event multicaster for this context. initApplicationEventMulticaster();
 // Initialize other special beans in specific context subclasses.这里的意思就是调用各个子类的onRefresh() onRefresh();
 // Check for listener beans and register them. registerListeners();
 // Instantiate all remaining (non-lazy-init) singletons. finishBeanFactoryInitialization(beanFactory);
 // Last step: publish corresponding event. finishRefresh(); }
 catch (BeansException ex) { if (logger.isWarnEnabled()) { logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex); }
 // Destroy already created singletons to avoid dangling resources. destroyBeans();
 // Reset 'active' flag. cancelRefresh(ex);
 // Propagate exception to caller. throw ex; }
 finally { // Reset common introspection caches in Spring's core, since we // might not ever need metadata for singleton beans anymore... resetCommonCaches(); } } }

//类：ServletWebServerApplicationContextprotected 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(); }

@Override public WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); Connector connector = new Connector(this.protocol); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat); }

public Engine getEngine() { Service service = getServer().findServices()[0]; if (service.getContainer() != null) { return service.getContainer(); } Engine engine = new StandardEngine(); engine.setName( "Tomcat" ); engine.setDefaultHost(hostname); engine.setRealm(createDefaultRealm()); service.setContainer(engine); return engine; }

/** If used, an Engine is always the top level Container in a Catalina * hierarchy. Therefore, the implementation's <code>setParent()</code> method * should throw <code>IllegalArgumentException</code>. * * @author Craig R. McClanahan */public interface Engine extends Container { //省略代码}/** * <p> * The parent Container attached to a Host is generally an Engine, but may * be some other implementation, or may be omitted if it is not necessary. * </p><p> * The child containers attached to a Host are generally implementations * of Context (representing an individual servlet context). * * @author Craig R. McClanahan */public interface Host extends Container {//省略代码
}/*** </p><p> * The parent Container attached to a Context is generally a Host, but may * be some other implementation, or may be omitted if it is not necessary. * </p><p> * The child containers attached to a Context are generally implementations * of Wrapper (representing individual servlet definitions). * </p><p> * * @author Craig R. McClanahan */public interface Context extends Container, ContextBind { //省略代码}/**</p><p> * The parent Container attached to a Wrapper will generally be an * implementation of Context, representing the servlet context (and * therefore the web application) within which this servlet executes. * </p><p> * Child Containers are not allowed on Wrapper implementations, so the * <code>addChild()</code> method should throw an * <code>IllegalArgumentException</code>. * * @author Craig R. McClanahan */public interface Wrapper extends Container {
 //省略代码}

//部分源码，其余部分省略。public class Tomcat {//设置连接器 public void setConnector(Connector connector) { Service service = getService(); boolean found = false; for (Connector serviceConnector : service.findConnectors()) { if (connector == serviceConnector) { found = true; } } if (!found) { service.addConnector(connector); } } //获取service public Service getService() { return getServer().findServices()[0]; } //设置Host容器 public void setHost(Host host) { Engine engine = getEngine(); boolean found = false; for (Container engineHost : engine.findChildren()) { if (engineHost == host) { found = true; } } if (!found) { engine.addChild(host); } } //获取Engine容器 public Engine getEngine() { Service service = getServer().findServices()[0]; if (service.getContainer() != null) { return service.getContainer(); } Engine engine = new StandardEngine(); engine.setName( "Tomcat" ); engine.setDefaultHost(hostname); engine.setRealm(createDefaultRealm()); service.setContainer(engine); return engine; } //获取server public Server getServer() {
 if (server != null) { return server; }
 System.setProperty("catalina.useNaming", "false");
 server = new StandardServer();
 initBaseDir();
 // Set configuration source ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(new File(basedir), null));
 server.setPort( -1 );
 Service service = new StandardService(); service.setName("Tomcat"); server.addService(service); return server; }
 //添加Context容器 public Context addContext(Host host, String contextPath, String contextName, String dir) { silence(host, contextName); Context ctx = createContext(host, contextPath); ctx.setName(contextName); ctx.setPath(contextPath); ctx.setDocBase(dir); ctx.addLifecycleListener(new FixContextListener());
 if (host == null) { getHost().addChild(ctx); } else { host.addChild(ctx); }
 //添加Wrapper容器 public static Wrapper addServlet(Context ctx, String servletName, Servlet servlet) { // will do class for name and set init params Wrapper sw = new ExistingStandardWrapper(servlet); sw.setName(servletName); ctx.addChild(sw);
 return sw; }
}