基本实现说明

spring-boot-starter-tomcat默认会被spring-boot-starter所依赖，而spring-boot实现的巧妙方式在于，通过依赖判定当前classpath下是否存在Tomcat类来断定，当前web容器类型。
这种方式也是spring-boot实现通过配置来决定启用何种服务的根本原理。

代码分析

spring-boot容器的默认超类是EmbeddedWebApplicationContext，而真正初始化容器的过程是调用createEmbeddedServletContainer方法。
EmbeddedWebApplicationContext

    private void createEmbeddedServletContainer() {
        EmbeddedServletContainer localContainer = this.embeddedServletContainer;
        ServletContext localServletContext = getServletContext();
        if (localContainer == null && localServletContext == null) {
            EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
            // 从容器中查找实例,方法如下
            this.embeddedServletContainer = containerFactory
                    .getEmbeddedServletContainer(getSelfInitializer());
        }
        else if (localServletContext != null) {
            try {
                getSelfInitializer().onStartup(localServletContext);
            }
            catch (ServletException ex) {
                throw new ApplicationContextException("Cannot initialize servlet context",
                        ex);
            }
        }
        initPropertySources();
    }

    protected EmbeddedServletContainerFactory getEmbeddedServletContainerFactory() {
        // Use bean names so that we don't consider the hierarchy
        String[] beanNames = getBeanFactory()
                .getBeanNamesForType(EmbeddedServletContainerFactory.class);
        ....
        ....
    }

即查找类型为EmbeddedServletContainerFactory.class的对象，而此对象的BeanDefinition放入的时机则是通过配置来实现的。

其中有一个过程是通过EnableAutoConfigurationImportSelector查找在在spring.factories中配置的所有类型为org.springframework.boot.autoconfigure.EnableAutoConfiguration的配置类，而这些默认实现类在spring-boot-starter.autoconfigure中，截取部分如下

...
...
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
...
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
\
....
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration
...

查看其中的org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@Import(BeanPostProcessorsRegistrar.class)
public class EmbeddedServletContainerAutoConfiguration {

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

        @Bean
        public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
            return new TomcatEmbeddedServletContainerFactory();
        }

    }

    @Configuration
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
            WebAppContext.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedJetty {

        @Bean
        public JettyEmbeddedServletContainerFactory jettyEmbeddedServletContainerFactory() {
            return new JettyEmbeddedServletContainerFactory();
        }
    }
    ...
    ...

}

这个提供了默认三种web容器，Tomcat,Jetty,Undertow，而@ConditionalOnClass注解的配置决定了在什么情况下使用这个类，类似Profile的功能。此条件判断依据为当前classpath容器中存在是否存在配置的class，以EmbeddedServletContainerAutoConfiguration$EmbeddedTomcat来说，它存在的依据在于当前classpath存在Servlet类和Tomcat类，而spring-boot默认依赖Tomcat，所以EmbeddedTomcat将被激活，而EmbeddedServletContainerAutoConfiguration$EmbeddedJetty将不会被采用。
所以，如果你想要使用jetty容器，仅仅需要将Tomcat依赖排除，并添加Jetty依赖即可，其他容器同理。
此时容器中已经确定了web容器，初始化的代码就在TomcatEmbeddedServletContainerFactory#getEmbeddedServletContainer中

public EmbeddedServletContainer getEmbeddedServletContainer(
            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 getTomcatEmbeddedServletContainer(tomcat);
    }