Spring Boot 中的 Tomcat 是如何启动的？

@SpringBootApplication
public class TomcatdebugApplication {

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

}

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：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();
			}
		}
	}

 @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 <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
}