<!-- server.xml -->
<!-- appBase 属性指定应用程序所在目录 -->
<Host name="localhost"  appBase="webapps"  unpackWARs="true" autoDeploy="true">
    
<!-- context.xml -->
<Context>
    <!-- Default set of monitored resources. If one of these changes, the web application will be reloaded. -->
    <WatchedResource>WEB-INF/web.xml</WatchedResource>
    <WatchedResource>WEB-INF/tomcat-web.xml</WatchedResource>
    <WatchedResource>${catalina.base}/conf/web.xml</WatchedResource>
</Context>


.
├── pom.xml
├── src
    ├── main
    │   ├── java
    │   │   └── cn
    │   │       └── edu
    │   │          └── shopping
    │   │              ├── filter
    │   │              │   └── HelloWorldFilter.java
    │   │              └── servlet
    │   │                  └── HelloWorldServlet.java
    │   └── webapp
    │       └── WEB-INF
    │           └── web.xml
    └── test
        └── java


public class HelloWorldServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/plain");
        PrintWriter writer = response.getWriter();
        writer.println("Hello World");
    }
}


public class HelloWorldFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }


    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("触发 Hello World 过滤器...");
        chain.doFilter(request, response);
    }

    @Override
    public void destroy() {

    }
}



<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

  <servlet>
    <servlet-name>HelloWorldServlet</servlet-name>
    <servlet-class>cn.edu.shopping.servlet.HelloWorldServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>HelloWorldServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
  
  <filter>
    <filter-name>HelloWorldFilter</filter-name>
    <filter-class>cn.edu.shopping.filter.HelloWorldFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>HelloWorldFilter</filter-name>
    <url-pattern>/hello</url-pattern>
  </filter-mapping>

</web-app>


.
├── pom.xml
├── src
    ├── main
    │   ├── java
    │   │   └── cn
    │   │       └── edu
    │   │          └── shopping
    │   │              ├── CustomServletContainerInitializer.java
    │   │              ├── filter
    │   │              │   └── HelloWorldFilter.java
    │   │              └── servlet
    │   │                  └── HelloWorldServlet.java
    │   ├── resources
    │   │   └── META-INF
    │   │       └── services
    │   │           └── javax.servlet.ServletContainerInitializer
    │   └── webapp
    │       └── WEB-INF
    │           └── web.xml
    └── test
        └── java


public class CustomServletContainerInitializer implements ServletContainerInitializer {

    @Override
    public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
        System.out.println("创建 Hello World Servlet...");
        javax.servlet.ServletRegistration.Dynamic servlet = ctx.addServlet(
            HelloWorldServlet.class.getSimpleName(), HelloWorldServlet.class);
        servlet.addMapping("/hello");

        System.out.println("创建 Hello World Filter...");
        javax.servlet.FilterRegistration.Dynamic filter = ctx.addFilter(HelloWorldFilter.class.getSimpleName(), HelloWorldFilter.class);
        EnumSet<DispatcherType> dispatcherTypes = EnumSet.allOf(DispatcherType.class);
        dispatcherTypes.add(DispatcherType.REQUEST);
        dispatcherTypes.add(DispatcherType.FORWARD);
        filter.addMappingForUrlPatterns(dispatcherTypes, true, "/hello");
    }
}


cn.edu.shopping.CustomServletContainerInitializer


org.springframework.web.SpringServletContainerInitializer


/**
 * Servlet 3.0 {@link ServletContainerInitializer} designed to support code-based
 * configuration of the servlet container using Spring's {@link WebApplicationInitializer}
 * SPI as opposed to (or possibly in combination with) the traditional
 * {@code web.xml}-based approach.
 */
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

	@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {

		List<WebApplicationInitializer> initializers = new LinkedList<>();

		if (webAppInitializerClasses != null) {
			for (Class<?> waiClass : webAppInitializerClasses) {
				// Be defensive: Some servlet containers provide us with invalid classes,
				// no matter what @HandlesTypes says...
                // <1>
				if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
						WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
					try {
						initializers.add((WebApplicationInitializer)
								ReflectionUtils.accessibleConstructor(waiClass).newInstance());
					}
					catch (Throwable ex) {
						throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
					}
				}
			}
		}

		if (initializers.isEmpty()) {
			servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
			return;
		}

		servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) {
            // <2>
			initializer.onStartup(servletContext);
		}
	}

}


public abstract class AbstractDispatcherServletInitializer extends AbstractContextLoaderInitializer {

	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		// 调用父类启动的逻辑
		super.onStartup(servletContext);
		// 注册 DispacherServlt
		registerDispatcherServlet(servletContext);
	}

	protected void registerDispatcherServlet(ServletContext servletContext) {
		// 获得 Servlet 名
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return null or empty");

		// <1> 创建 WebApplicationContext 对象
		WebApplicationContext servletAppContext = createServletApplicationContext();
		Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");

		// <2> 创建 FrameworkServlet 对象
		FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
		Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
		dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
		if (registration == null) {
			throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
					"Check if there is another servlet registered under the same name.");
		}

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMappings());
		registration.setAsyncSupported(isAsyncSupported());

		// <3> 注册过滤器
		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}
}


// AbstractAnnotationConfigDispatcherServletInitializer.java
@Override
protected WebApplicationContext createServletApplicationContext() {
    AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
    Class<?>[] configClasses = getServletConfigClasses();
    if (!ObjectUtils.isEmpty(configClasses)) {
        context.register(configClasses);
    }
    return context;
}


// AbstractDispatcherServletInitializer.java
protected FrameworkServlet createDispatcherServlet(WebApplicationContext servletAppContext) {
    return new DispatcherServlet(servletAppContext);
}


@WebServlet("/hello")
public class HelloWorldServlet extends HttpServlet{}
@WebFilter("/hello/*")
public class HelloWorldFilter implements Filter {}


@SpringBootApplication
@ServletComponentScan
public class SpringBootServletApplication {
   public static void main(String[] args) {
      SpringApplication.run(SpringBootServletApplication.class, args);
   }
}


@Configuration
public class WebConfig {
    @Bean
    public ServletRegistrationBean<HelloWorldServlet> helloWorldServlet() {
        ServletRegistrationBean<HelloWorldServlet> servlet = new ServletRegistrationBean<>();
        servlet.addUrlMappings("/hello");
        servlet.setServlet(new HelloWorldServlet());
        return servlet;
    }

    @Bean
    public FilterRegistrationBean<HelloWorldFilter> helloWorldFilter() {
        FilterRegistrationBean<HelloWorldFilter> filter = new FilterRegistrationBean<>();
        filter.addUrlPatterns("/hello/*");
        filter.setFilter(new HelloWorldFilter());
        return filter;
    }
}


class TomcatStarter implements ServletContainerInitializer {

	private static final Log logger = LogFactory.getLog(TomcatStarter.class);

	private final ServletContextInitializer[] initializers;

	private volatile Exception startUpException;

	TomcatStarter(ServletContextInitializer[] initializers) {
		this.initializers = initializers;
	}

	@Override
	public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
		try {
			for (ServletContextInitializer initializer : this.initializers) {
				initializer.onStartup(servletContext);
			}
		}
		catch (Exception ex) {
			this.startUpException = ex;
			// Prevent Tomcat from logging and re-throwing when we know we can
			// deal with it in the main thread, but log for information here.
			if (logger.isErrorEnabled()) {
				logger.error("Error starting Tomcat context. Exception: "
						+ ex.getClass().getName() + ". Message: " + ex.getMessage());
			}
		}
	}

	public Exception getStartUpException() {
		return this.startUpException;
	}

}


@Override
protected void onRefresh() {
    super.onRefresh();
    try {
        createWebServer(); //第二层的入口
    }
    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()); // 第三层的入口
    }
    else if (servletContext != null) {
        try {
            getSelfInitializer().onStartup(servletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context",
                    ex);
        }
    }
    initPropertySources();
}


private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
}

private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(
            beanFactory);
    WebApplicationContextUtils.registerWebApplicationScopes(beanFactory,
            getServletContext());
    existingScopes.restore();
    WebApplicationContextUtils.registerEnvironmentBeans(beanFactory,
            getServletContext());
    // 第四层的入口
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}


/**
 * Returns {@link ServletContextInitializer}s that should be used with the embedded web server. 
 * By default this method will first attempt to find {@link ServletContextInitializer}, 
 * {@link Servlet}, {@link Filter} and certain {@link EventListener} beans.
 * @return the servlet initializer beans
 */
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory()); //第五层的入口
}


public ServletContextInitializerBeans(ListableBeanFactory beanFactory) {
    this.initializers = new LinkedMultiValueMap<>();
    addServletContextInitializerBeans(beanFactory); // 第六层的入口
    addAdaptableBeans(beanFactory);
    List<ServletContextInitializer> sortedInitializers = this.initializers.values()
            .stream()
            .flatMap((value) -> value.stream()
                    .sorted(AnnotationAwareOrderComparator.INSTANCE))
            .collect(Collectors.toList());
    this.sortedList = Collections.unmodifiableList(sortedInitializers);
}


// ServletContextInitializerBeans.java
private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
    for (Entry<String, ServletContextInitializer> initializerBean :  getOrderedBeansOfType(
        beanFactory, ServletContextInitializer.class)) {
        addServletContextInitializerBean(initializerBean.getKey(),
                initializerBean.getValue(), beanFactory);
    }
}


@Configuration
public class CustomServletContextInitializer implements ServletContextInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        System.out.println("创建 Hello World Servlet...");
        javax.servlet.ServletRegistration.Dynamic servlet = ctx.addServlet(
            HelloWorldServlet.class.getSimpleName(), HelloWorldServlet.class);
        servlet.addMapping("/hello");

        System.out.println("创建 Hello World Filter...");
        javax.servlet.FilterRegistration.Dynamic filter = ctx.addFilter(HelloWorldFilter.class.getSimpleName(), HelloWorldFilter.class);
        EnumSet<DispatcherType> dispatcherTypes = EnumSet.allOf(DispatcherType.class);
        dispatcherTypes.add(DispatcherType.REQUEST);
        dispatcherTypes.add(DispatcherType.FORWARD);
        filter.addMappingForUrlPatterns(dispatcherTypes, true, "/hello");
    }
}


protected void configureContext(Context context, ServletContextInitializer[] initializers) {
    // <1>
    TomcatStarter starter = new TomcatStarter(initializers);
    // <2>
    if (context instanceof TomcatEmbeddedContext) {
        // Should be true
        ((TomcatEmbeddedContext) context).setStarter(starter);
    }
    // ... 省略相关代码
}


@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
// 这个就是我们 SpringBoot 中 application.yml 配置文件中 server.* 配置类，也就是 Tomcat 相关配置
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
		ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
		ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
		ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

	@Bean
	public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(
			ServerProperties serverProperties) {
		return new ServletWebServerFactoryCustomizer(serverProperties);
	}

	@Bean
    // 保证存在 Tomcat 的 Class 对象
	@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
	public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
			ServerProperties serverProperties) {
		return new TomcatServletWebServerFactoryCustomizer(serverProperties);
	}
    // 省略 WebServerFactoryCustomizerBeanPostProcessor 类
}


class ServletWebServerFactoryConfiguration {

	@Configuration
	@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedTomcat {

		@Bean
		public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
			return new TomcatServletWebServerFactory();
		}
	}
    // 省略 EmbeddedJetty、EmbeddedUndertow
}