Tomcat在SpringBoot中是如何启动的？

@SpringBootApplicationpublic class TomcatdebugApplication {
public static void main(String[] args) { 
       SpringApplication.run(TomcatdebugApplication.class, args
       );    
}}

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<springbootexceptionreporter> exceptionReporters = new ArrayList&lt;&gt;();
//设置系统属性『java.awt.headless』，为true则启用headless模式支持
    configureHeadlessProperty();
//通过*SpringFactoriesLoader*检索*META-INF/spring.factories*，
//找到声明的所有SpringApplicationRunListener的实现类并将其实例化，
//之后逐个调用其started()方法，广播SpringBoot要开始执行了
    SpringApplicationRunListeners listeners = getRunListeners(args);
//发布应用开始启动事件
    listeners.starting();
try {
//初始化参数
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
//创建并配置当前SpringBoot应用将要使用的Environment（包括配置要使用的PropertySource以及Profile）,
//并遍历调用所有的SpringApplicationRunListener的environmentPrepared()方法，广播Environment准备完毕。
      ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
      configureIgnoreBeanInfo(environment);
//打印banner
      Banner printedBanner = printBanner(environment);
//创建应用上下文
      context = createApplicationContext();
//通过*SpringFactoriesLoader*检索*META-INF/spring.factories*，获取并实例化异常分析器
      exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
//为ApplicationContext加载environment，之后逐个执行ApplicationContextInitializer的initialize()方法来进一步封装ApplicationContext，
//并调用所有的SpringApplicationRunListener的contextPrepared()方法，【EventPublishingRunListener只提供了一个空的contextPrepared()方法】，
//之后初始化IoC容器，并调用SpringApplicationRunListener的contextLoaded()方法，广播ApplicationContext的IoC加载完成，
//这里就包括通过**@EnableAutoConfiguration**导入的各种自动配置类。
      prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新上下文
      refreshContext(context);
//再一次刷新上下文,其实是空方法，可能是为了后续扩展。
      afterRefresh(context, applicationArguments);
      stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
      }
//发布应用已经启动的事件
      listeners.started(context);
//遍历所有注册的ApplicationRunner和CommandLineRunner，并执行其run()方法。
//我们可以实现自己的ApplicationRunner或者CommandLineRunner，来对SpringBoot的启动过程进行扩展。
      callRunners(context, applicationArguments);
    }
catch (Throwable ex) {
      handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
    }

try {
//应用已经启动完成的监听事件
      listeners.running(context);
    }
catch (Throwable ex) {
      handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
    }
return context;
  }

protected ConfigurableApplicationContext createApplicationContext() {
    Class<!--?--> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
          contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
          contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
          contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
        }
      }
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, " + "please specify an ApplicationContextClass",
            ex);
      }
    }
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
  }

 //类：SpringApplication.java

private void refreshContext(ConfigurableApplicationContext context) {
//直接调用刷新方法
    refresh(context);
if (this.registerShutdownHook) {
try {
        context.registerShutdownHook();
      }
catch (AccessControlException ex) {
// Not allowed in some environments.
      }
    }
  }
//类：SpringApplication.java

protected void refresh(ApplicationContext applicationContext) {
    Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
    ((AbstractApplicationContext) applicationContext).refresh();
  }

 // 类：AbstractApplicationContext  
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
      prepareRefresh();

// Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

// Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

try {
// Allows post-processing of the bean factory in context subclasses.
        postProcessBeanFactory(beanFactory);

// Invoke factory processors registered as beans in the context.
        invokeBeanFactoryPostProcessors(beanFactory);

// Register bean processors that intercept bean creation.
        registerBeanPostProcessors(beanFactory);

// Initialize message source for this context.
        initMessageSource();

// Initialize event multicaster for this context.
        initApplicationEventMulticaster();

// Initialize other special beans in specific context subclasses.这里的意思就是调用各个子类的onRefresh()
        onRefresh();

// Check for listener beans and register them.
        registerListeners();

// Instantiate all remaining (non-lazy-init) singletons.
        finishBeanFactoryInitialization(beanFactory);

// Last step: publish corresponding event.
        finishRefresh();
      }

catch (BeansException ex) {
if (logger.isWarnEnabled()) {
          logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
        }

// Destroy already created singletons to avoid dangling resources.
        destroyBeans();

// Reset 'active' flag.
        cancelRefresh(ex);

// Propagate exception to caller.
throw ex;
      }

finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
        resetCommonCaches();
      }
    }
  }

 //类：ServletWebServerApplicationContext
protected void onRefresh() {
super.onRefresh();
try {
      createWebServer();
    }
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
    }
  }

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
      ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
    }
else if (servletContext != null) {
try {
        getSelfInitializer().onStartup(servletContext);
      }
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context", ex);
      }
    }
    initPropertySources();
  }

@Override
public WebServer getWebServer(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 getTomcatWebServer(tomcat);
  } 

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;
    }

/**
 If used, an Engine is always the top level Container in a Catalina
 * hierarchy. Therefore, the implementation's <code>setParent()</code> method
 * should throw <code>IllegalArgumentException</code>.
 *
 * @author Craig R. McClanahan
 */
public interface Engine extends Container {
//省略代码
}
/**
 * <p>
 * The parent Container attached to a Host is generally an Engine, but may
 * be some other implementation, or may be omitted if it is not necessary.
 * </p><p>
 * The child containers attached to a Host are generally implementations
 * of Context (representing an individual servlet context).
 *
 * @author Craig R. McClanahan
 */
public interface Host extends Container {
//省略代码

}
/*** </p><p>
 * The parent Container attached to a Context is generally a Host, but may
 * be some other implementation, or may be omitted if it is not necessary.
 * </p><p>
 * The child containers attached to a Context are generally implementations
 * of Wrapper (representing individual servlet definitions).
 * </p><p>
 *
 * @author Craig R. McClanahan
 */
public interface Context extends Container, ContextBind {
//省略代码
}
/**</p><p>
 * The parent Container attached to a Wrapper will generally be an
 * implementation of Context, representing the servlet context (and
 * therefore the web application) within which this servlet executes.
 * </p><p>
 * Child Containers are not allowed on Wrapper implementations, so the
 * <code>addChild()</code> method should throw an
 * <code>IllegalArgumentException</code>.
 *
 * @author Craig R. McClanahan
 */
public interface Wrapper extends Container {

//省略代码
}

//部分源码，其余部分省略。
public class Tomcat {
//设置连接器
public void setConnector(Connector connector) {
        Service service = getService();
boolean found = false;
for (Connector serviceConnector : service.findConnectors()) {
if (connector == serviceConnector) {
                found = true;
            }
        }
if (!found) {
            service.addConnector(connector);
        }
    }
//获取service
public Service getService() {
return getServer().findServices()[0];
    }
//设置Host容器
public void setHost(Host host) {
        Engine engine = getEngine();
boolean found = false;
for (Container engineHost : engine.findChildren()) {
if (engineHost == host) {
                found = true;
            }
        }
if (!found) {
            engine.addChild(host);
        }
    }
//获取Engine容器
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;
    }
//获取server
public Server getServer() {

if (server != null) {
return server;
        }

        System.setProperty("catalina.useNaming", "false");

        server = new StandardServer();

        initBaseDir();

// Set configuration source
        ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(new File(basedir), null));

        server.setPort( -1 );

        Service service = new StandardService();
        service.setName("Tomcat");
        server.addService(service);
return server;
    }

//添加Context容器
public Context addContext(Host host, String contextPath, String contextName,
            String dir) {
        silence(host, contextName);
        Context ctx = createContext(host, contextPath);
        ctx.setName(contextName);
        ctx.setPath(contextPath);
        ctx.setDocBase(dir);
        ctx.addLifecycleListener(new FixContextListener());

if (host == null) {
            getHost().addChild(ctx);
        } else {
            host.addChild(ctx);
        }

//添加Wrapper容器
public static Wrapper addServlet(Context ctx,
                                      String servletName,
                                      Servlet servlet) {
// will do class for name and set init params
        Wrapper sw = new ExistingStandardWrapper(servlet);
        sw.setName(servletName);
        ctx.addChild(sw);

return sw;
    }

}