path = ('^' + path + (strict ? '' : path[path.length - 1] === '/' ? '?' : '/?'))
// .replace...
.replace(/(\\\/)?(\\\.)?:(\w+)(\(.*?\))?(\*)?(\?)?/g, function (match, slash, format, key, capture, star, optional, offset) {// ...
    keys.push({name: key,optional: !!optional,offset: offset + extraOffset});// ...
});

/*** @example path = /users/params* @example router.get('/users/:id')*/
Layer.prototype.match = function match(path) {var matchif (path != null) {// ...快速匹配
match = this.regexp.exec(path)}if (!match) { /*...*/ }// 缓存paramsthis.params = {};this.path = match[0]// [{ name: prarms,... }]var keys = this.keys;var params = this.params;for (var i = 1; i < match.length; i++) {var key = keys[i - 1];var prop = key.name;// decodeURIComponent(val)var val = decode_param(match[i]);// layer.params.id = paramsif (val !== undefined || !(hasOwnProperty.call(params, prop))) {params[prop] = val;}}return true;
};

proto.process_params = function process_params(layer, called, req, res, done) {var params = this.params;// 获取keys数组var keys = layer.keys;if (!keys || keys.length === 0) return done();var i = 0;var name;var paramIndex = 0;var key;var paramVal;var paramCallbacks;var paramCalled;function param(err) {if (err) return done(err);if (i >= keys.length) return done();paramIndex = 0;key = keys[i++];name = key.name;// req.params = layer.paramsparamVal = req.params[name];// 后面讨论paramCallbacks = params[name];// 初始为空对象paramCalled = called[name];if (paramVal === undefined || !paramCallbacks) return param();// param previously called with same value or error occurredif (paramCalled && (paramCalled.match === paramVal || (paramCalled.error && paramCalled.error !== 'route'))) {// error...
        }// 设置值called[name] = paramCalled = {error: null,match: paramVal,value: paramVal};paramCallback();}// single param callbacksfunction paramCallback(err) {//...
    }param();
};

function paramCallback(err) {// 依次取出callback数组的fnvar fn = paramCallbacks[paramIndex++];// 标记valparamCalled.value = req.params[key.name];if (err) {// store errorparamCalled.error = err;param(err);return;}if (!fn) return param();// 调用回调函数try {fn(req, res, paramCallback, paramVal, key.name);} catch (e) {paramCallback(e);}
}

router.param('user', function(req, res, next, id) {// ...do something
    next();
})

// app.js
app.use('/user',userRouter);
// userRouter.js
router.get('/abcd',()=>{...});
// client的get请求
path => '/users/abcd'

/*** * @param   layer       匹配到的layer* @param   layerError  error* @param   layerPath   layer.path => '/users'* @param   path        req.url.pathname => '/users/abcd'*/
function trim_prefix(layer, layerError, layerPath, path) {if (layerPath.length !== 0) {// 保证路径后面的字符串合法var c = path[layerPath.length]if (c && c !== '/' && c !== '.') return next(layerError)debug('trim prefix (%s) from url %s', layerPath, req.url);// 缓存被移除的pathremoved = layerPath;req.url = protohost + req.url.substr(protohost.length + removed.length);// 保证移除后的路径以/开头if (!protohost && req.url[0] !== '/') {req.url = '/' + req.url;slashAdded = true;}// 基本路径拼接req.baseUrl = parentUrl + (removed[removed.length - 1] === '/' ?removed.substring(0, removed.length - 1) :removed);}debug('%s %s : %s', layer.name, layerPath, req.originalUrl);// 将新的req.url传进去处理if (layerError) {layer.handle_error(layerError, req, res, next);} else {layer.handle_request(req, res, next);}
}

var done = restore(out, req, 'baseUrl', 'next', 'params');

function restore(fn, obj) {var props = new Array(arguments.length - 2);var vals = new Array(arguments.length - 2);// 在请求到来的时候先缓存原始信息/*** props = ['baseUrl', 'next', 'params']* vals = ['url','next方法','动态路由的params']*/for (var i = 0; i < props.length; i++) {props[i] = arguments[i + 2];vals[i] = obj[props[i]];}return function () {// 在请求处理完后对值进行回滚for (var i = 0; i < props.length; i++) {obj[props[i]] = vals[i];}return fn.apply(this, arguments);};
}

var done = callback || finalhandler(req, res, {env: this.get('env'),onerror: logerror.bind(this)
});

function finalhandler(req, res, options) {// 获取配置参数var opts = options || {}var env = opts.env || process.env.NODE_ENV || 'development'var onerror = opts.onerrorreturn function (err) {// ...
  }
}

// 遇到router标记直接调用done
if (layerError === 'router') {setImmediate(done, null)return
}// 走完了layer匹配
if (idx >= stack.length) {setImmediate(done, layerError);return;
}// path为null
var path = getPathname(req);if (path == null) {return done(layerError);
}

function done(err) {var headersvar msgvar status// 请求已发送的情况if (!err && headersSent(res)) {debug('cannot 404 after headers sent')return}// unhandled errorif (err) {// ...} else {// not foundstatus = 404msg = 'Cannot ' + req.method + ' ' + encodeUrl(getResourceName(req))}debug('default %s', status)// 处理错误if (err && onerror) {defer(onerror, err, req, res)}// 请求已发送销毁req的socket实例if (headersSent(res)) {debug('cannot %d after headers sent', status)req.socket.destroy()return}// 发送请求
  send(req, res, status, headers, msg)
}

function write () {// response bodyvar body = createHtmlDocument(message)// response statusres.statusCode = statusres.statusMessage = statuses[status]// response headers
  setHeaders(res, headers)// security headersres.setHeader('Content-Security-Policy', "default-src 'self'")res.setHeader('X-Content-Type-Options', 'nosniff')// standard headersres.setHeader('Content-Type', 'text/html; charset=utf-8')res.setHeader('Content-Length', Buffer.byteLength(body, 'utf8'))// 只请求页面的首部if (req.method === 'HEAD') {res.end()return}res.end(body, 'utf8')
}