SpringBoot有啥高科技？是怎么做到XML零配置的？

    <!-- 配置监听器 -->      
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
  

    <!-- 配置DispatcherServlet -->  
    <servlet>  
    　　<servlet-name>dispatcherServlet</servlet-name>  
    　　<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>  
　　</servlet>

    <!-- ServletContext参数 -->  
   <context-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>/WEB-INF/spring-config/*.xml</param-value>
   </context-param>

  protected void configureAndRefreshWebApplicationContext
        (ConfigurableWebApplicationContext wac, ServletContext sc) {
    //添加ServletContext
	wac.setServletContext(sc);
	String configLocationParam = sc.getInitParameter("contextConfigLocation");
   //添加Spring*.xml
	wac.setConfigLocation(configLocationParam);
    customizeContext(sc, wac);
	//读取配置加载，刷新Spring上下文
    wac.refresh();
  }

<!--扫描包注解   不扫描@controller-->
<context:component-scan base-package="com.wangnian">
        <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
</context:component-scan> 

<!--扫描包注解  只扫描@Controller-->
<context:component-scan base-package="com.wangnian.controller" >
	   <context:include-filter type="annotation"  expression="org.springframework.stereotype.Controller" />
</context:component-scan>  

public class MyWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        //SpringWeb注解配置应用程序上下本
        AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
        //注册带有注解的类
        ac.register(AppConfig.class);
        //spring上下文刷新
        ac.refresh();

        //创建DispatcherServlet
        DispatcherServlet servlet = new DispatcherServlet(ac);

        //servlet中注册DispatcherServlet
        ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}

@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {
        //一个装WebApplicationInitializer实现的集合
		List<WebApplicationInitializer> initializers = new LinkedList<>();
		for (Class<?> waiClass : webAppInitializerClasses) {
            //通过反射拿到HandlesTypes注解指定的class的实现类
			initializers.add(ReflectionUtils.accessibleConstructor(waiClass).newInstance());
		}
        //进行排序
		AnnotationAwareOrderComparator.sort(initializers);
        //循环调用所有集合里的onStartup方法
		for (WebApplicationInitializer initializer : initializers) {
			initializer.onStartup(servletContext);
		}
	}

protected static class DispatcherServletConfiguration {

		@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {
			DispatcherServlet dispatcherServlet = new DispatcherServlet();
			dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
			dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
			dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
			dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
			dispatcherServlet.setEnableLoggingRequestDetails(httpProperties.isLogRequestDetails());
			return dispatcherServlet;
		}
}

if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
      && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
   return WebApplicationType.REACTIVE;
}
for (String className : SERVLET_INDICATOR_CLASSES) {
   if (!ClassUtils.isPresent(className, null)) {
      return WebApplicationType.NONE;
   }
}
return WebApplicationType.SERVLET;

	@Override
	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();
      //根据工厂模式调用对应的servlet实现的容器，如果是tomcat就调用TomcatServletWebServerFactory的getWebServer()
      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) {
		if (this.disableMBeanRegistry) {
			Registry.disableRegistry();
		}
		Tomcat tomcat = new Tomcat();
		File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
		tomcat.setBaseDir(baseDir.getAbsolutePath());
		Connector connector = new Connector(this.protocol);
		connector.setThrowOnFailure(true);
		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);
		}
        //准备上下文，主要会提前放入他自己的ServletContextInitializer(Servlet上下文是初始化器)
        //实现类，供SpringBoot在ServletContainerInitializer的onStartup()里遍历调用自己的onStartup()
        //为了注册用户自定义的Filter和Servlet到ServletContext中
		prepareContext(tomcat.getHost(), initializers);
        //启动tomcat
		return getTomcatWebServer(tomcat);
	}

private void initialize() throws WebServerException {
	// Start the server to trigger initialization listeners
	this.tomcat.start();
}

@Override
public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
   
   for (ServletContextInitializer initializer : this.initializers) {
         initializer.onStartup(servletContext);
   }
   
}

@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
   String description = getDescription();
   if (!isEnabled()) {
      logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
      return;
   }
   //拿到Tomcat传过来的ServletContext进行注册
   register(description, servletContext);
}

@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
   String name = getServletName();
   return servletContext.addServlet(name, this.servlet);
}

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter</artifactId>
  <version>2.1.6.RELEASE</version>
  <scope>compile</scope>
</dependency>

<dependency>
  <groupId>org.apache.tomcat.embed</groupId>
  <artifactId>tomcat-embed-core</artifactId>
  <version>9.0.21</version>
  <scope>compile</scope>
  <exclusions>
    <exclusion>
      <artifactId>tomcat-annotations-api</artifactId>
      <groupId>org.apache.tomcat</groupId>
    </exclusion>
  </exclusions>
</dependency>

package com.example;

import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.context.web.SpringBootServletInitializer;  
  
public class SpringBootServletStart extends SpringBootServletInitializer {  
  
    @Override  
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
        //这里是@SpringBootApplication类
        return application.sources(DemoApplication.class);
    }  
  
}  

 <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
  </plugin>

Manifest-Version: 1.0
Created-By: Maven Archiver 3.4.0
Build-Jdk-Spec: 13
Implementation-Title: demo
Implementation-Version: 0.0.1-SNAPSHOT
Main-Class: org.springframework.boot.loader.JarLauncher
Start-Class: com.example.demo.DemoApplication
Spring-Boot-Version: 2.2.2.RELEASE
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/