@SpringBootApplication
public 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;CollectionexceptionReporters = 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);//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bannerBanner 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.javaprivate 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.javaprotected void refresh(ApplicationContext applicationContext) {Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);((AbstractApplicationContext) applicationContext).refresh();}

// 类：AbstractApplicationContext
public 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：ServletWebServerApplicationContext
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();}

@Overridepublic 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 setParent() method* should throw IllegalArgumentException.** @author Craig R. McClanahan*/
public interface Engine extends Container {//省略代码
}
/*** * 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. * * 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}/***  * 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. * * The child containers attached to a Context are generally implementations * of Wrapper (representing individual servlet definitions). * * * @author Craig R. McClanahan */public interface Context extends Container, ContextBind {    //省略代码}/** * 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. *上面的注释翻译过来就是，Engine 是最高级别的容器，其子容器是 Host,Host 的子容器是 Context,Wrapper 是 Context 的子容器，所以这 4 个容器的关系就是父子关系，也就是 Engine>Host>Context>Wrapper。我们再看看 Tomcat 类的源码://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servicepublic 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serverpublic Server getServer() {if (server != null) {return server;}System.setProperty("catalina.useNaming", "false");server = new StandardServer();initBaseDir();// Set configuration sourceConfigFileLoader.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 paramsWrapper sw = new ExistingStandardWrapper(servlet);sw.setName(servletName);ctx.addChild(sw);return sw;}}阅读 Tomcat 的 getServer()我们可以知道，Tomcat 的最顶层是 Server,Server 就是 Tomcat 的实例，一个 Tomcat 一个 Server通过 getEngine()我们可以了解到 Server 下面是 Service，而且是多个，一个 Service 代表我们部署的一个应用，而且我们还可以知道，Engine 容器，一个 service 只有一个；根据父子关系，我们看 setHost()源码可以知道，host 容器有多个同理，我们发现 addContext()源码下，Context 也是多个；addServlet()表明 Wrapper 容器也是多个，而且这段代码也暗示了，其实 Wrapper 和 Servlet 是一层意思。另外我们根据 setConnector 源码可以知道，连接器(Connector)是设置在 service 下的，而且是可以设置多个连接器(Connector)。根据上面分析，我们可以小结下：Tomcat 主要包含了 2 个核心组件，连接器(Connector)和容器(Container),用图表示如下：一个 Tomcat 是一个 Server,一个 Server 下有多个 service，也就是我们部署的多个应用，一个应用下有多个连接器(Connector)和一个容器（Container）,容器下有多个子容器，关系用图表示如下：Engine 下有多个 Host 子容器，Host 下有多个 Context 子容器，Context 下有多个 Wrapper 子容器。