<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
</dependency>

 <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-tomcat</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jetty</artifactId>
        </dependency>

@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			// 啟動(dòng)Tomcat
			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 一個(gè)接口  實(shí)現(xiàn)類有TomcatServletWebServerFactory 等等容器工廠
			//通過getWebServerFactory() 獲取到對(duì)應(yīng)容器的創(chuàng)建工廠
			ServletWebServerFactory factory = getWebServerFactory();
			//通過實(shí)現(xiàn)類的getWebServer來啟動(dòng)對(duì)應(yīng)的容器
			this.webServer = factory.getWebServer(getSelfInitializer());
			getBeanFactory().registerSingleton("webServerGracefulShutdown",
					new WebServerGracefulShutdownLifecycle(this.webServer));
			getBeanFactory().registerSingleton("webServerStartStop",
					new WebServerStartStopLifecycle(this, this.webServer));
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}

protected ServletWebServerFactory getWebServerFactory() {
		// Use bean names so that we don't consider the hierarchy
		//獲取到ServletWebServerFactory 所有實(shí)現(xiàn)類的bean的名稱 也就是springboot中配置了哪些容器 如tomcat jetty等
		String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
		//如果沒有bean 表示沒有配置容器
		if (beanNames.length == 0) {
			throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing "
					+ "ServletWebServerFactory bean.");
		}
		//如果bean大于1 表示配置了多個(gè)容器
		if (beanNames.length > 1) {
			throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
					+ "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
		}
		//最終返回唯一的容器工廠 比如配置了tomcat 那么就返回了TomcatServletWebServerFactory
		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
	}

public WebServer getWebServer(ServletContextInitializer... initializers) {
		if (this.disableMBeanRegistry) {
			Registry.disableRegistry();
		}
		Tomcat tomcat = new Tomcat();
		File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
		tomcat.setBaseDir(baseDir.getAbsolutePath());
		for (LifecycleListener listener : this.serverLifecycleListeners) {
			tomcat.getServer().addLifecycleListener(listener);
		}
		Connector connector = new Connector(this.protocol);
		connector.setThrowOnFailure(true);
		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);
	}