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) {
            StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
            ServletWebServerFactory factory = getWebServerFactory();
            createWebServer.tag("factory", factory.getClass().toString());
            this.webServer = factory.getWebServer(getSelfInitializer());
            createWebServer.end();
            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
        String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
        if (beanNames.length == 0) {
            throw new MissingWebServerFactoryBeanException(getClass(), ServletWebServerFactory.class,
                    WebApplicationType.SERVLET);
        }
        if (beanNames.length > 1) {
            throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
                    + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
        }
        return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }

package org.springframework.boot.autoconfigure.web.servlet;
@Configuration(proxyBeanMethods = false)
class ServletWebServerFactoryConfiguration {

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedTomcat {

        @Bean
        TomcatServletWebServerFactory tomcatServletWebServerFactory(
                ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
                ObjectProvider<TomcatContextCustomizer> contextCustomizers,
                ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
            TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
            factory.getTomcatConnectorCustomizers()
                .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatContextCustomizers()
                .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatProtocolHandlerCustomizers()
                .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

    /**
     * Nested configuration if Jetty is being used.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedJetty {

        @Bean
        JettyServletWebServerFactory jettyServletWebServerFactory(
                ObjectProvider<JettyServerCustomizer> serverCustomizers) {
            JettyServletWebServerFactory factory = new JettyServletWebServerFactory();
            factory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

    /**
     * Nested configuration if Undertow is being used.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedUndertow {

        @Bean
        UndertowServletWebServerFactory undertowServletWebServerFactory(
                ObjectProvider<UndertowDeploymentInfoCustomizer> deploymentInfoCustomizers,
                ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {
            UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();
            factory.getDeploymentInfoCustomizers()
                .addAll(deploymentInfoCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

        @Bean
        UndertowServletWebServerFactoryCustomizer undertowServletWebServerFactoryCustomizer(
                ServerProperties serverProperties) {
            return new UndertowServletWebServerFactoryCustomizer(serverProperties);
        }

    }

}