express中间件机制补充说明

中间件2种用法

筒子
变胖的筒子

筒子

app.use('/1', function(req, res, next){

})

app.use('/2', function(req, res, next){

})

app.use('/3', function(req, res, next){

})

变胖的筒子

function m1(req, res, next){

}

function m2(req, res, next){

}

app.use('/', m1, m2, function(req, res, next){

})

路由匹配规则

先根据【筒子】栈，匹配到请求
然后看该请求，是不是【变胖的筒子】，如果是，还需要继续解析该路由上的横向的栈

express的next说明

3.x 使用connect作为中间件
4.x 在express/lib/router/里实现router.handle(req, res, next);
5.x 使用独立的router模块

4.0.0 / 2014-04-09
==================

 * remove:
   - node 0.8 support
   - connect and connect's patches except for charset handling
   - express(1) - moved to [express-generator](https://github.com/expressjs/generator)
   - `express.createServer()` - it has been deprecated for a long time. Use `express()`
   - `app.configure` - use logic in your own app code
   - `app.router` - is removed
   - `req.auth` - use `basic-auth` instead
   - `req.accepted*` - use `req.accepts*()` instead
   - `res.location` - relative URL resolution is removed
   - `res.charset` - include the charset in the content type when using `res.set()`
   - all bundled middleware except `static`
 * change:
   - `app.route` -> `app.mountpath` when mounting an express app in another express app
   - `json spaces` no longer enabled by default in development
   - `req.accepts*` -> `req.accepts*s` - i.e. `req.acceptsEncoding` -> `req.acceptsEncodings`
   - `req.params` is now an object instead of an array
   - `res.locals` is no longer a function. It is a plain js object. Treat it as such.
   - `res.headerSent` -> `res.headersSent` to match node.js ServerResponse object
 * refactor:
   - `req.accepts*` with [accepts](https://github.com/expressjs/accepts)
   - `req.is` with [type-is](https://github.com/expressjs/type-is)
   - [path-to-regexp](https://github.com/component/path-to-regexp)
 * add:
   - `app.router()` - returns the app Router instance
   - `app.route()` - Proxy to the app's `Router#route()` method to create a new route
   - Router & Route - public API

大家知道变更即可，过多细节也没啥用

express里的use

主要2个

app.use
还有router.use

给出application里的use实现

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) {
    // non-express app
    if (!fn || !fn.handle || !fn.set) {
      return router.use(path, fn);
    }

    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) {
      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;
};

简单点说就是把中间件变成了路由来处理。

所有的fns中间件都被router.use真正处理了

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

给出router里use实现

/**
 * Use the given middleware function, with optional path, defaulting to "/".
 *
 * Use (like `.all`) will run for any http METHOD, but it will not add
 * handlers for those methods so OPTIONS requests will not consider `.use`
 * functions even if they could respond.
 *
 * The other difference is that _route_ path is stripped and not visible
 * to the handler function. The main effect of this feature is that mounted
 * handlers can operate without any code changes regardless of the "prefix"
 * pathname.
 *
 * @public
 */

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

  // default path to '/'
  // disambiguate router.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 callbacks = flatten(slice.call(arguments, offset));

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

  for (var i = 0; i < callbacks.length; i++) {
    var fn = callbacks[i];

    if (typeof fn !== 'function') {
      throw new TypeError('Router.use() requires middleware function but got a ' + gettype(fn));
    }

    // add the middleware
    debug('use %s %s', path, fn.name || '<anonymous>');

    var layer = new Layer(path, {
      sensitive: this.caseSensitive,
      strict: false,
      end: false
    }, fn);

    layer.route = undefined;

    this.stack.push(layer);
  }

  return this;
};

把参数搞成callbacks

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

遍历callback，转成中间件

    var layer = new Layer(path, {
      sensitive: this.caseSensitive,
      strict: false,
      end: false
    }, fn);

    layer.route = undefined;

入栈

    this.stack.push(layer);

总结一下：核心还是在Layer里

Layer

layer代码非常少，也就100多，方法不过5个

构造函数

function Layer(path, options, fn) {
  if (!(this instanceof Layer)) {
    return new Layer(path, options, fn);
  }

  debug('new %s', path);
  var opts = options || {};

  this.handle = fn;
  this.name = fn.name || '<anonymous>';
  this.params = undefined;
  this.path = undefined;
  this.regexp = pathRegexp(path, this.keys = [], opts);

  if (path === '/' && opts.end === false) {
    this.regexp.fast_slash = true;
  }
}

主要看看this上有那些变量，这是一个请求会产生处理的。比如name我们在遍历中间件的时候可能会出现anonymous，原因就在这里。

通过pathRegexp进行匹配url，然后把fn处理了，很明显这是单个的或者说是final的处理。

var pathRegexp = require('path-to-regexp');

handle_error
handle_request

Layer.prototype.handle_request = function handle(req, res, next) {
  var fn = this.handle;

  if (fn.length > 3) {
    // not a standard request handler
    return next();
  }

  try {
    fn(req, res, next);
  } catch (err) {
    next(err);
  }
};

不知道大家是否好奇

给出express 4.x里的express/lib/router/index.js里的实现

proto.handle = function handle(req, res, out) {
  var self = this;

  debug('dispatching %s %s', req.method, req.url);

  var search = 1 + req.url.indexOf('?');
  var pathlength = search ? search - 1 : req.url.length;
  var fqdn = req.url[0] !== '/' && 1 + req.url.substr(0, pathlength).indexOf('://');
  var protohost = fqdn ? req.url.substr(0, req.url.indexOf('/', 2 + fqdn)) : '';
  var idx = 0;
  var removed = '';
  var slashAdded = false;
  var paramcalled = {};

  // store options for OPTIONS request
  // only used if OPTIONS request
  var options = [];

  // middleware and routes
  var stack = self.stack;

  // manage inter-router variables
  var parentParams = req.params;
  var parentUrl = req.baseUrl || '';
  var done = restore(out, req, 'baseUrl', 'next', 'params');

  // setup next layer
  req.next = next;

  // for options requests, respond with a default if nothing else responds
  if (req.method === 'OPTIONS') {
    done = wrap(done, function(old, err) {
      if (err || options.length === 0) return old(err);
      sendOptionsResponse(res, options, old);
    });
  }

  // setup basic req values
  req.baseUrl = parentUrl;
  req.originalUrl = req.originalUrl || req.url;

  next();

  function next(err) {
    var layerError = err === 'route'
      ? null
      : err;

    // remove added slash
    if (slashAdded) {
      req.url = req.url.substr(1);
      slashAdded = false;
    }

    // restore altered req.url
    if (removed.length !== 0) {
      req.baseUrl = parentUrl;
      req.url = protohost + removed + req.url.substr(protohost.length);
      removed = '';
    }

    // no more matching layers
    if (idx >= stack.length) {
      setImmediate(done, layerError);
      return;
    }

    // get pathname of request
    var path = getPathname(req);

    if (path == null) {
      return done(layerError);
    }

    // find next matching layer
    var layer;
    var match;
    var route;

    while (match !== true && idx < stack.length) {
      layer = stack[idx++];
      match = matchLayer(layer, path);
      route = layer.route;

      if (typeof match !== 'boolean') {
        // hold on to layerError
        layerError = layerError || match;
      }

      if (match !== true) {
        continue;
      }

      if (!route) {
        // process non-route handlers normally
        continue;
      }

      if (layerError) {
        // routes do not match with a pending error
        match = false;
        continue;
      }

      var method = req.method;
      var has_method = route._handles_method(method);

      // build up automatic options response
      if (!has_method && method === 'OPTIONS') {
        appendMethods(options, route._options());
      }

      // don't even bother matching route
      if (!has_method && method !== 'HEAD') {
        match = false;
        continue;
      }
    }

    // no match
    if (match !== true) {
      return done(layerError);
    }

    // store route for dispatch on change
    if (route) {
      req.route = route;
    }

    // Capture one-time layer values
    req.params = self.mergeParams
      ? mergeParams(layer.params, parentParams)
      : layer.params;
    var layerPath = layer.path;

    // this should be done for the layer
    self.process_params(layer, paramcalled, req, res, function (err) {
      if (err) {
        return next(layerError || err);
      }

      if (route) {
        return layer.handle_request(req, res, next);
      }

      trim_prefix(layer, layerError, layerPath, path);
    });
  }

  function trim_prefix(layer, layerError, layerPath, path) {
    var c = path[layerPath.length];
    if (c && '/' !== c && '.' !== c) return next(layerError);

     // Trim off the part of the url that matches the route
     // middleware (.use stuff) needs to have the path stripped
    if (layerPath.length !== 0) {
      debug('trim prefix (%s) from url %s', layerPath, req.url);
      removed = layerPath;
      req.url = protohost + req.url.substr(protohost.length + removed.length);

      // Ensure leading slash
      if (!fqdn && req.url[0] !== '/') {
        req.url = '/' + req.url;
        slashAdded = true;
      }

      // Setup base URL (no trailing slash)
      req.baseUrl = parentUrl + (removed[removed.length - 1] === '/'
        ? removed.substring(0, removed.length - 1)
        : removed);
    }

    debug('%s %s : %s', layer.name, layerPath, req.originalUrl);

    if (layerError) {
      layer.handle_error(layerError, req, res, next);
    } else {
      layer.handle_request(req, res, next);
    }
  }
};

核心在next();方法

    while (match !== true && idx < stack.length) {
      layer = stack[idx++];
      match = matchLayer(layer, path);
      route = layer.route;

      if (typeof match !== 'boolean') {
        // hold on to layerError
        layerError = layerError || match;
      }

      if (match !== true) {
        continue;
      }

      if (!route) {
        // process non-route handlers normally
        continue;
      }

      if (layerError) {
        // routes do not match with a pending error
        match = false;
        continue;
      }

      var method = req.method;
      var has_method = route._handles_method(method);

      // build up automatic options response
      if (!has_method && method === 'OPTIONS') {
        appendMethods(options, route._options());
      }

      // don't even bother matching route
      if (!has_method && method !== 'HEAD') {
        match = false;
        continue;
      }
    }

看到这个

layer = stack[idx++];

你会想到什么？遍历栈要做什么呢？

一个路由，其实就是stack，可以1个也可以多个中间件，想想上面说的“先根据【筒子】栈，匹配到请求”

然后，根据遍历结果，match和route来处理。只有匹配了路由，才会走到具体的处理

    // no match
    if (match !== true) {
      return done(layerError);
    }

    // store route for dispatch on change
    if (route) {
      req.route = route;
    }

    // Capture one-time layer values
    req.params = self.mergeParams
      ? mergeParams(layer.params, parentParams)
      : layer.params;
    var layerPath = layer.path;

    // this should be done for the layer
    self.process_params(layer, paramcalled, req, res, function (err) {
      if (err) {
        return next(layerError || err);
      }

      if (route) {
        return layer.handle_request(req, res, next);
      }

      trim_prefix(layer, layerError, layerPath, path);
    });

整体来说，设计的职责清晰，还算是比较精巧的。代码的可读性能做到这样，还是真的很难得的，推荐学习。

i5ting 1楼•7 年前作者

最近事儿较多，先写点，有问题大家大家评论即可

DuanPengfei 2楼•7 年前

👍 前一段也去阅读 Express 的源码了，看到 next 这里理解起来还是需要些时间的，狼叔要是稍微详细点解释一下更好了 😄，方便以后其他人阅读

hellozhangran 3楼•7 年前