express的application.js里的路由代码

app.handle = function handle(req, res, callback) {
  var router = this._router;//这里一开始是空的，只有使用app的http verb方法或者use方法时才会去惰性加载

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

  // no routes
  if (!router) {//这里一开始是空的，只有使用app的http verb方法或者use方法时才会去惰性加载
    debug('no routes defined on app');
    done();
    return;
  }

  router.handle(req, res, done);
};

app.use = function use(fn) {
  var offset = 0;
  var path = '/';

  // default path to '/'
  // disambiguate app.use([fn])
  if (typeof fn !== 'function') {
    var arg = fn;

    while (Array.isArray(arg) && arg.length !== 0) {
      arg = arg[0];
    }

    // first arg is the path
    if (typeof arg !== 'function') {
      offset = 1;
      path = fn;
    }
  }

  var fns = flatten(slice.call(arguments, offset));

  if (fns.length === 0) {
    throw new TypeError('app.use() requires middleware functions');
  }

  // setup router
  this.lazyrouter();
  var router = this._router;

  fns.forEach(function (fn) {//每一个fn对应一个Layer,所以app.use(fn)时,无论是同时传入多个参数还是多次使用use，每个函数或中间件都对应一个Layer
    // non-express app
    if (!fn || !fn.handle || !fn.set) {//fn是函数时
      return router.use(path, fn);
    }
    //fn是路由对象route时
    debug('.use app under %s', path);
    fn.mountpath = path;
    fn.parent = this;

    // restore .app property on req and res
    router.use(path, function mounted_app(req, res, next) {//app.use('/',router);
      var orig = req.app;
      fn.handle(req, res, function (err) {
        req.__proto__ = orig.request;
        res.__proto__ = orig.response;
        next(err);
      });
    });

    // mounted an app
    fn.emit('mount', this);
  }, this);

  return this;
};

app.route = function route(path) {
  this.lazyrouter();
  return this._router.route(path);
};

app.all = function all(path) {
  this.lazyrouter();

  var route = this._router.route(path);
  var args = slice.call(arguments, 1);

  for (var i = 0; i < methods.length; i++) {
    route[methods[i]].apply(route, args);
  }

  return this;
};