<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)+"===");}
}

<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>

@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)+"===");}@Overrideprotected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {return builder.sources(this.getClass());}
}

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logoBanner 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) {}
}

//创建上下文
protected ConfigurableApplicationContext createApplicationContext() {Class<?> contextClass = this.applicationContextClass;if (contextClass == null) {try {switch(this.webApplicationType) {case SERVLET://创建AnnotationConfigServletWebServerApplicationContextcontextClass = 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);
}

//SpringApplication.java
//刷新上下文
private void refreshContext(ConfigurableApplicationContext context) {this.refresh(context);if (this.registerShutdownHook) {try {context.registerShutdownHook();} catch (AccessControlException var3) {}}
}//这里直接调用最终父类AbstractApplicationContext.refresh()方法
protected void refresh(ApplicationContext applicationContext) {((AbstractApplicationContext)applicationContext).refresh();
}//AbstractApplicationContext.java
public void refresh() throws BeansException, IllegalStateException {synchronized(this.startupShutdownMonitor) {this.prepareRefresh();ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();this.prepareBeanFactory(beanFactory);try {this.postProcessBeanFactory(beanFactory);this.invokeBeanFactoryPostProcessors(beanFactory);this.registerBeanPostProcessors(beanFactory);this.initMessageSource();this.initApplicationEventMulticaster();//调用各个子类的onRefresh()方法，也就说这里要回到子类：ServletWebServerApplicationContext，调用该类的onRefresh()方法this.onRefresh();this.registerListeners();this.finishBeanFactoryInitialization(beanFactory);this.finishRefresh();} catch (BeansException var9) {this.destroyBeans();this.cancelRefresh(var9);throw var9;} finally {this.resetCommonCaches();}}
}//ServletWebServerApplicationContext.java
//在这个方法里看到了熟悉的面孔，this.createWebServer，神秘的面纱就要揭开了。
protected void onRefresh() {super.onRefresh();try {this.createWebServer();} catch (Throwable var2) {}
}//ServletWebServerApplicationContext.java
//这里是创建webServer，但是还没有启动tomcat，这里是通过ServletWebServerFactory创建，那么接着看下ServletWebServerFactory
private void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = this.getServletContext();if (webServer == null && servletContext == null) {ServletWebServerFactory factory = this.getWebServerFactory();this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});} else if (servletContext != null) {try {this.getSelfInitializer().onStartup(servletContext);} catch (ServletException var4) {}}this.initPropertySources();
}//接口
public interface ServletWebServerFactory {WebServer getWebServer(ServletContextInitializer... initializers);
}//实现
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory

//TomcatServletWebServerFactory.java
//这里我们使用的tomcat，所以我们查看TomcatServletWebServerFactory。到这里总算是看到了tomcat的踪迹。
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {Tomcat tomcat = new Tomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());//创建Connector对象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 getTomcatWebServer(tomcat);
}protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {return new TomcatWebServer(tomcat, getPort() >= 0);
}//Tomcat.java
//返回Engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。
public Engine getEngine() {Service service = getServer().findServices()[0];if (service.getContainer() != null) {return service.getContainer();}Engine engine = new StandardEngine();engine.setName( "Tomcat" );engine.setDefaultHost(hostname);engine.setRealm(createDefaultRealm());service.setContainer(engine);return engine;
}
//Engine是最高级别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器

//TomcatWebServer.java
//这里调用构造函数实例化TomcatWebServer
public TomcatWebServer(Tomcat tomcat, boolean autoStart) {Assert.notNull(tomcat, "Tomcat Server must not be null");this.tomcat = tomcat;this.autoStart = autoStart;initialize();
}private void initialize() throws WebServerException {//在控制台会看到这句日志logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));synchronized (this.monitor) {try {addInstanceIdToEngineName();Context context = findContext();context.addLifecycleListener((event) -> {if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {removeServiceConnectors();}});//===启动tomcat服务===this.tomcat.start();rethrowDeferredStartupExceptions();try {ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());}catch (NamingException ex) {}//开启阻塞非守护进程startDaemonAwaitThread();}catch (Exception ex) {stopSilently();destroySilently();throw new WebServerException("Unable to start embedded Tomcat", ex);}}
}//Tomcat.java
public void start() throws LifecycleException {getServer();server.start();
}
//这里server.start又会回到TomcatWebServer的
public void stop() throws LifecycleException {getServer();server.stop();
}//TomcatWebServer.java
//启动tomcat服务
@Override
public void start() throws WebServerException {synchronized (this.monitor) {if (this.started) {return;}try {addPreviouslyRemovedConnectors();Connector connector = this.tomcat.getConnector();if (connector != null && this.autoStart) {performDeferredLoadOnStartup();}checkThatConnectorsHaveStarted();this.started = true;//在控制台打印这句日志，如果在yml设置了上下文，这里会打印logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"+ getContextPath() + "'");}catch (ConnectorStartFailedException ex) {stopSilently();throw ex;}catch (Exception ex) {throw new WebServerException("Unable to start embedded Tomcat server", ex);}finally {Context context = findContext();ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());}}
}//关闭tomcat服务
@Override
public void stop() throws WebServerException {synchronized (this.monitor) {boolean wasStarted = this.started;try {this.started = false;try {stopTomcat();this.tomcat.destroy();}catch (LifecycleException ex) {}}catch (Exception ex) {throw new WebServerException("Unable to stop embedded Tomcat", ex);}finally {if (wasStarted) {containerCounter.decrementAndGet();}}}
}