// 基于本地 Tomcat 仓 (9.0.89), org.apache.catalina.connector.CoyoteAdapter
@Override
public void service(org.apache.coyote.Request req, org.apache.coyote.Response res) throws Exception {

    Request request = (Request) req.getNote(ADAPTER_NOTES);    // 参数是 coyote 的 Request, 局部变量是 catalina 的
    Response response = (Response) res.getNote(ADAPTER_NOTES);

    if (request == null) {
        // Create objects
        request = connector.createRequest();        // catalina Request: 实现 HttpServletRequest
        request.setCoyoteRequest(req);              // 只是包一层, 底层数据仍是 coyote 里那份字节
        response = connector.createResponse();
        response.setCoyoteResponse(res);

        // Link objects
        request.setResponse(response);
        response.setRequest(request);

        // Set as notes
        req.setNote(ADAPTER_NOTES, request);        // 挂回 coyote 对象上, 同一连接的下个请求直接复用, 不重复 new
        res.setNote(ADAPTER_NOTES, response);
        // ...
    }
    // ...
    try {
        // Parse and set Catalina and configuration specific
        // request parameters
        postParseSuccess = postParseRequest(req, request, res, response);   // URI 解码/规范化 + Mapper 映射
        if (postParseSuccess) {
            // check valves if we support async
            request.setAsyncSupported(connector.getService().getContainer().getPipeline().isAsyncSupported());
            // Calling the container
            connector.getService().getContainer().getPipeline().getFirst().invoke(request, response);  // 进入 Engine 的 Valve 链
        }
        if (request.isAsync()) {
            async = true;
            // ... Servlet 3.0 异步: 本线程先撤, 请求不结束, 略
        } else {
            request.finishRequest();        // 同步请求: 冲刷输出缓冲, 收尾
            response.finishResponse();
        }

    } catch (IOException e) {
        // Ignore
    } finally {
        // ... access log, 略

        // Recycle the wrapper request and response
        if (!async) {
            updateWrapperErrorCount(request, response);
            request.recycle();      // 对象不丢, 清空状态等下个请求复用, 省 GC
            response.recycle();
        }
    }
}

// —— postParseRequest() 里的关键一步: 把 URL 翻译成容器坐标 ——
    while (mapRequired) {
        // This will map the the latest version by default
        connector.getService().getMapper().map(serverName, decodedURI, version, request.getMappingData());
        // Mapper 按 host名 -> context路径(最长前缀) -> servlet规则 三级查表, 结果填进 MappingData:
        // 之后 request.getHost()/getContext()/getWrapper() 拿到的就是这里定下的三个容器

        // If there is no context at this point, either this is a 404
        // because no ROOT context has been deployed or the URI was invalid
        // so no context could be mapped.
        if (request.getContext() == null) {
            // ...
            return true;    // 不在这抛 404, 放行让 Valve 链去出错误页
        }
        // ... 解析 session id; 若 URL 会话粘在旧版本 webapp 上, 置 mapRequired=true 重新映射(平滑升级), 略
    }

// 基于本地 Tomcat 仓 (9.0.89), org.apache.catalina.core.StandardWrapperValve
// Wrapper 管的 Pipeline 的 basic valve, Valve 链的最后一节
@Override
public void invoke(Request request, Response response) throws IOException, ServletException {
    // ...
    StandardWrapper wrapper = (StandardWrapper) getContainer();     // 一个 Wrapper 对应一个 Servlet 定义
    Servlet servlet = null;
    Context context = (Context) wrapper.getParent();
    // ... 应用/Servlet 不可用检查(503), 略

    // Allocate a servlet instance to process this request
    try {
        if (!unavailable) {
            servlet = wrapper.allocate();   // 常规 Servlet 返回全局单例(首次触发 init); STM Servlet 才走实例池
        }
    } catch (UnavailableException e) {
        // ...
    }
    // ...
    // Create the filter chain for this request
    ApplicationFilterChain filterChain = ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);
    // 每个请求现场组链: 拿 URL 和 servlet 名去匹配 web.xml/注解里的 filter-mapping

    // Call the filter chain for this request
    // NOTE: This also calls the servlet's service() method
    try {
        if ((servlet != null) && (filterChain != null)) {
            // ...
            filterChain.doFilter(request.getRequest(), response.getResponse());
            // getRequest() 给出 RequestFacade: 应用代码从这往上只见 HttpServletRequest, 摸不到容器内部
            // ...
        }
    } catch (IOException e) {
        // ... 各类异常分别记日志、走错误处理, 略
    } finally {
        // Release the filter chain (if any) for this request
        if (filterChain != null) {
            filterChain.release();
        }

        // Deallocate the allocated servlet instance
        try {
            if (servlet != null) {
                wrapper.deallocate(servlet);    // 单例场景只是减计数
            }
        } catch (Throwable e) {
            // ...
        }
        // ...
    }
}

// 基于本地 Tomcat 仓 (9.0.89), org.apache.catalina.core.ApplicationFilterChain
public final class ApplicationFilterChain implements FilterChain {
    // ...
    private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];    // 数组存链, 不是节点链表

    private int pos = 0;    // 游标: 当前执行到第几个 filter
    private int n = 0;      // 链上 filter 总数
    private Servlet servlet = null;     // 链的终点

    private void internalDoFilter(ServletRequest request, ServletResponse response)
            throws IOException, ServletException {

        // Call the next filter if there is one
        if (pos < n) {
            ApplicationFilterConfig filterConfig = filters[pos++];      // 取下一个, 游标前移
            try {
                Filter filter = filterConfig.getFilter();
                // ...
                filter.doFilter(request, response, this);
                // 把链自身(this)传给 Filter; Filter 内部调 chain.doFilter() 又回到本方法, 靠调用栈递归推进。
                // filter.doFilter 返回时, 下游(后续 filter + servlet)已全部执行完 —— 这就是 Filter 后置逻辑的时机
            } catch (IOException | ServletException | RuntimeException e) {
                throw e;
            }
            // ...
            return;
        }

        // We fell off the end of the chain -- call the servlet instance
        try {
            // ...
            servlet.service(request, response);     // 链走完, 请求终于交到 Servlet 手上
        } catch (IOException | ServletException | RuntimeException e) {
            throw e;
        }
        // ...
    }
}