基本實現(xiàn)說明

spring-boot-starter-tomcat默認會被spring-boot-starter所依賴，而spring-boot實現(xiàn)的巧妙方式在于，通過依賴判定當前classpath下是否存在Tomcat類來斷定，當前web容器類型。
這種方式也是spring-boot實現(xiàn)通過配置來決定啟用何種服務的根本原理。

代碼分析

spring-boot容器的默認超類是EmbeddedWebApplicationContext，而真正初始化容器的過程是調(diào)用createEmbeddedServletContainer方法。
EmbeddedWebApplicationContext

    private void createEmbeddedServletContainer() {
        EmbeddedServletContainer localContainer = this.embeddedServletContainer;
        ServletContext localServletContext = getServletContext();
        if (localContainer == null && localServletContext == null) {
            EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
            // 從容器中查找實例,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);
        ....
        ....
    }

即查找類型為EmbeddedServletContainerFactory.class的對象，而此對象的BeanDefinition放入的時機則是通過配置來實現(xiàn)的。

其中有一個過程是通過EnableAutoConfigurationImportSelector查找在在spring.factories中配置的所有類型為org.springframework.boot.autoconfigure.EnableAutoConfiguration的配置類，而這些默認實現(xiàn)類在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();
        }
    }
    ...
    ...

}

這個提供了默認三種web容器，Tomcat,Jetty,Undertow，而@ConditionalOnClass注解的配置決定了在什么情況下使用這個類，類似Profile的功能。此條件判斷依據(jù)為當前classpath容器中存在是否存在配置的class，以EmbeddedServletContainerAutoConfiguration$EmbeddedTomcat來說，它存在的依據(jù)在于當前classpath存在Servlet類和Tomcat類，而spring-boot默認依賴Tomcat，所以EmbeddedTomcat將被激活，而EmbeddedServletContainerAutoConfiguration$EmbeddedJetty將不會被采用。
所以，如果你想要使用jetty容器，僅僅需要將Tomcat依賴排除，并添加Jetty依賴即可，其他容器同理。
此時容器中已經(jīng)確定了web容器，初始化的代碼就在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);
    }