不得不说SpringBoot的开发者是在为大众程序猿谋福利，把大家都惯成了懒汉，xml不配置了，连tomcat也懒的配置了，典型的一键启动系统，那么tomcat在springboot是怎么启动的呢？
内置tomcat开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> <version>2.1.6.RELEASE</version> </dependency> @SpringBootApplication public class MySpringbootTomcatStarter{ public static void main(String[] args) { Long time=System.currentTimeMillis(); SpringApplication.run(MySpringbootTomcatStarter.class); System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); } }这里是main函数入口，两句代码最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。
发布生产发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包（war）然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> <!-- 移除嵌入式tomcat插件 --> <exclusions> <exclusion> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-tomcat</artifactId> </exclusion> </exclusions> </dependency> <!--添加servlet-api依赖---> <dependency> <groupId>javax.servlet</groupId> <artifactId>javax.servlet-api</artifactId> <version>3.1.0</version> <scope>provided</scope> </dependency>更新main函数，主要是继承SpringBootServletInitializer，并重写configure()方法。
@SpringBootApplication public class MySpringbootTomcatStarter extends SpringBootServletInitializer { public static void main(String[] args) { Long time=System.currentTimeMillis(); SpringApplication.run(MySpringbootTomcatStarter.class); System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); } @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) { return builder.sources(this.getClass()); } } 从main函数说起 public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) { return run(new Class[]{primarySource}, args); } --这里run方法返回的是ConfigurableApplicationContext public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { return (new SpringApplication(primarySources)).run(args); } public ConfigurableApplicationContext run(String... args) { ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList(); this.configureHeadlessProperty(); SpringApplicationRunListeners listeners = this.getRunListeners(args); listeners.starting(); Collection exceptionReporters; try { ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments); this.configureIgnoreBeanInfo(environment); //打印banner，这里你可以自己涂鸦一下，换成自己项目的logo Banner printedBanner = this.printBanner(environment); //创建应用上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); //预处理上下文 this.prepareContext(context, environment, listeners, applicationArguments, printedBanner); //刷新上下文 this.refreshContext(context); //再刷新上下文 this.afterRefresh(context, applicationArguments); listeners.started(context); this.callRunners(context, applicationArguments); } catch (Throwable var10) { } try { listeners.running(context); return context; } catch (Throwable var9) { } }既然我们想知道tomcat在SpringBoot中是怎么启动的，那么run方法中，重点关注创建应用上下文（createApplicationContext）和刷新上下文（refreshContext）。
创建上下文 //创建上下文 protected ConfigurableApplicationContext createApplicationContext() { Class<?> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch(this.webApplicationType) { case SERVLET: //创建AnnotationConfigServletWebServerApplicationContext contextClass = Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext"); break; case REACTIVE: contextClass = Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext"); break; default: contextClass = Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext"); } } catch (ClassNotFoundException var3) { throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3); } } return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass); }