springboot是如何启动内置tomcat的

public ConfigurableApplicationContext run(String... args);
this.refreshContext(context);
this.refresh((ApplicationContext)context);
this.refresh((ConfigurableApplicationContext)applicationContext);
applicationContext.refresh();
void refresh() throws BeansException, IllegalStateException;
public final void refresh()
super.refresh();
this.onRefresh();
this.createWebServer();

第一步

private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = this.getServletContext();
        if (webServer == null && servletContext == null) {
            ServletWebServerFactory factory = this.getWebServerFactory();
                                                    //重点
            this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});
            this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
            this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
        } else if (servletContext != null) {
            try {
                this.getSelfInitializer().onStartup(servletContext);
            } catch (ServletException var4) {
                throw new ApplicationContextException("Cannot initialize servlet context", var4);
            }
        }

        this.initPropertySources();
    }

第二步

public WebServer getWebServer(ServletContextInitializer... initializers) {
       if (this.disableMBeanRegistry) {
           Registry.disableRegistry();
       }

       Tomcat tomcat = new Tomcat();
       File baseDir = this.baseDirectory != null ? this.baseDirectory : this.createTempDir("tomcat");
       tomcat.setBaseDir(baseDir.getAbsolutePath());
       Connector connector = new Connector(this.protocol);
       connector.setThrowOnFailure(true);
       tomcat.getService().addConnector(connector);
       this.customizeConnector(connector);
       tomcat.setConnector(connector);
       tomcat.getHost().setAutoDeploy(false);
       this.configureEngine(tomcat.getEngine());
       Iterator var5 = this.additionalTomcatConnectors.iterator();

       while(var5.hasNext()) {
           Connector additionalConnector = (Connector)var5.next();
           tomcat.getService().addConnector(additionalConnector);
       }

       this.prepareContext(tomcat.getHost(), initializers);
       return this.getTomcatWebServer(tomcat);
   }

第三步

private void initialize() throws WebServerException {
        logger.info("Tomcat initialized with port(s): " + this.getPortsDescription(false));
        synchronized(this.monitor) {
            try {
                this.addInstanceIdToEngineName();
                Context context = this.findContext();
                context.addLifecycleListener((event) -> {
                    if (context.equals(event.getSource()) && "start".equals(event.getType())) {
                        this.removeServiceConnectors();
                    }

                });
                this.tomcat.start();
                this.rethrowDeferredStartupExceptions();

                try {
                    ContextBindings.bindClassLoader(context, context.getNamingToken(), this.getClass().getClassLoader());
                } catch (NamingException var5) {
                }

                this.startDaemonAwaitThread();
            } catch (Exception var6) {
                this.stopSilently();
                this.destroySilently();
                throw new WebServerException("Unable to start embedded Tomcat", var6);
            }

        }
    }