tomcat实现：
核心类org.apache.catalina.startup.ContextConfig

//支持注解 see:org.apache.catalina.deploy.WebXml
protected void processClass(WebXml fragment, JavaClass clazz) {
        AnnotationEntry[] annotationsEntries = clazz.getAnnotationEntries();
        if (annotationsEntries != null) {
            String className = clazz.getClassName();
            for (AnnotationEntry ae : annotationsEntries) {
                String type = ae.getAnnotationType();
                if ("Ljavax/servlet/annotation/WebServlet;".equals(type)) {
                    processAnnotationWebServlet(className, ae, fragment);
                }else if ("Ljavax/servlet/annotation/WebFilter;".equals(type)) {
                    processAnnotationWebFilter(className, ae, fragment);
                }else if ("Ljavax/servlet/annotation/WebListener;".equals(type)) {
                    fragment.addListener(className);
                } else {
                    // Unknown annotation - ignore
                }
            }
        }
    }

//支持web.xml
protected InputSource getContextWebXmlSource() {
        InputStream stream = null;
        InputSource source = null;
        URL url = null;

        String altDDName = null;

        // Open the application web.xml file, if it exists
        ServletContext servletContext = context.getServletContext();
        try {
            if (servletContext != null) {
                altDDName = (String)servletContext.getAttribute(Globals.ALT_DD_ATTR);
                if (altDDName != null) {
                    try {
                        stream = new FileInputStream(altDDName);
                        url = new File(altDDName).toURI().toURL();
                    } catch (FileNotFoundException e) {
                        log.error(sm.getString("contextConfig.altDDNotFound",
                                               altDDName));
                    } catch (MalformedURLException e) {
                        log.error(sm.getString("contextConfig.applicationUrl"));
                    }
                }
                else {
                    stream = servletContext.getResourceAsStream
                        (Constants.ApplicationWebXml);//Constants.ApplicationWebXml = "/WEB-INF/web.xml"
                    try {
                        url = servletContext.getResource(
                                Constants.ApplicationWebXml);
                    } catch (MalformedURLException e) {
                        log.error(sm.getString("contextConfig.applicationUrl"));
                    }
                }
            }
            if (stream == null || url == null) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("contextConfig.applicationMissing") + " " + context);
                }
            } else {
                source = new InputSource(url.toExternalForm());
                source.setByteStream(stream);
            }
        } finally {
            if (source == null && stream != null) {
                try {
                    stream.close();
                } catch (IOException e) {
                    // Ignore
                }
            }
        }

        return source;
    }

springboot中做了增强:
核心类：org.springframework.boot.web.servlet.ServletRegistrationBean org.springframework.boot.web.servlet.ServletListenerRegistrationBean均实现ServletContextInitializer接口，spring启动tomcat时被调用

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) {
                                //因为ServletListenerRegistrationBean FilterRegistrationBean ServletListenerRegistrationBean均实现此类，故启动时会被注册到tomcat容器中
				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;
	}

}

https://mp.weixin.qq.com/s/Z3ZemxPHBDAUWE-YEHyyDg