Springboot如何区分使用的是Tomcat还是Jetty

private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        if (webServer == null && servletContext == null) {
                         //获取到使用的web容器
            ServletWebServerFactory factory = getWebServerFactory();
                        //启动web容器
            this.webServer = factory.getWebServer(getSelfInitializer());

protected ServletWebServerFactory getWebServerFactory() {
        // Use bean names so that we don't consider the hierarchy
        //根据bean的定义，获取到ServletWebServerFactory类型的bean名称
        //这里是获取的bean名字，这一步bean对象可能还没有创建，因为要先判断ServletWebServerFactory类型的bean有几个，如果有0个或者2个及以上，那么或报错，就不用创建bean对象了
        String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
        if (beanNames.length == 0) {
            throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing "
                    + "ServletWebServerFactory bean.");
        }
        if (beanNames.length > 1) {
            throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
                    + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
        }
       //创建web容器的bean对象，并返回
        return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }

//TomcatServletWebServerFactory#getWebServer
    public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
            Registry.disableRegistry();
        }
        //new一个Tomcat并进行各种设置
        Tomcat tomcat = new Tomcat();
        File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
        tomcat.setBaseDir(baseDir.getAbsolutePath());
        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);
    }

   protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
        return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
   }

    //生成TomcatWebServer并启动
    public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
        Assert.notNull(tomcat, "Tomcat Server must not be null");
        this.tomcat = tomcat;
        this.autoStart = autoStart;
        this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;
        initialize();
    }

    private void initialize() throws WebServerException {
             //启动Tomcat
         this.tomcat.start();

内嵌的web容器的配置
在ServletWebServerFactoryConfiguration中，存在几个内部类，这些内部类只有满足ConditionalOnClass的条件，才会生效，比如EmbeddedTomcat，只有pom文件中有Tomcat的依赖，EmbeddedTomcat才会生效。但是如果pom中有超过一个的web容器的配置，那么也会报错。
上面getWebServerFactory中获取bean的name，web容器的bean名称就出自这里。

@Configuration
class ServletWebServerFactoryConfiguration {

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

        @Bean
        public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
            return new TomcatServletWebServerFactory();
        }

    }

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

        @Bean
        public JettyServletWebServerFactory JettyServletWebServerFactory() {
            return new JettyServletWebServerFactory();
        }

    }

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

        @Bean
        public UndertowServletWebServerFactory undertowServletWebServerFactory() {
            return new UndertowServletWebServerFactory();
        }

    }

}