express中间件实现原理和源码分析

中间件的添加逻辑：如下图

中间件添加实现主要源码：

请求路由绑定逻辑：如下图：

路由绑定相关源码：

接收请求处理逻辑：

接收请求主要源码：

---

中间件的添加逻辑：如下图

这里是app.use()逻辑

其他方式参见从源码分析express中间件添加方式的区别

中间件添加实现主要源码：

app.use = function use(fn) {
  var offset = 0; //这里是指去参数的第几个
  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 a middleware function')
  }

  // 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) {
        setPrototypeOf(req, orig.request)
        setPrototypeOf(res, orig.response)
        next(err);
      });
    });

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

  return this;
};

app.lazyrouter = function lazyrouter() {
  if (!this._router) {
    this._router = new Router({
      caseSensitive: this.enabled('case sensitive routing'),
      strict: this.enabled('strict routing')
    });

    this._router.use(query(this.get('query parser fn')));
    this._router.use(middleware.init(this));
  }
};

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 a middleware function')
  }

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

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

    // add the middleware
    debug('use %o %s', path, fn.name || '')

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

    layer.route = undefined;

    this.stack.push(layer);
  }

  return this;
};

请求路由绑定逻辑：如下图：

路由绑定相关源码：

methods.forEach(function(method){
  app[method] = function(path){
    if (method === 'get' && arguments.length === 1) {
      // app.get(setting)
      return this.set(path);
    }

    this.lazyrouter();

    var route = this._router.route(path);
    route[method].apply(route, slice.call(arguments, 1));
    return this;
  };
});

//Route.js
methods.forEach(function(method){
  Route.prototype[method] = function(){
    var handles = flatten(slice.call(arguments));

    for (var i = 0; i < handles.length; i++) {
      var handle = handles[i];

      if (typeof handle !== 'function') {
        var type = toString.call(handle);
        var msg = 'Route.' + method + '() requires a callback function but got a ' + type
        throw new Error(msg);
      }

      debug('%s %o', method, this.path)

      var layer = Layer('/', {}, handle);
      layer.method = method;

      this.methods[method] = true;
      this.stack.push(layer);
    }

    return this;
  };
});

接收请求处理逻辑：

接收请求主要源码：如下图

app.listen = function listen() {
  //这里的this指的是express()返回的app function(req,res,next){}
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};

function createApplication() {
  var app = function(req, res, next) {
    app.handle(req, res, next);
  };
  //将proto, EventEmitter合并到app中，并复制其属性描述符
  /**
 * Merge the property descriptors of `src` into `dest`
 *
 * @param {object} dest Object to add descriptors to
 * @param {object} src Object to clone descriptors from
 * @param {boolean} [redefine=true] Redefine `dest` properties with `src` properties
 * @returns {object} Reference to dest
 * @public
 */
  mixin(app, EventEmitter.prototype, false);
  mixin(app, proto, false);

  // expose the prototype that will get set on requests
  app.request = Object.create(req, {
    app: { configurable: true, enumerable: true, writable: true, value: app }
  })

  // expose the prototype that will get set on responses
  app.response = Object.create(res, {
    app: { configurable: true, enumerable: true, writable: true, value: app }
  })

  app.init();
  return app;
}

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

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

  var idx = 0;
  var protohost = getProtohost(req.url) || ''
  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  删除添加的斜线
    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 = '';
    }

    // signal to exit router
    if (layerError === 'router') {
      setImmediate(done, null)
      return
    }

    // 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) {
    if (layerPath.length !== 0) {
      // Validate path breaks on a path separator
      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
      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 (!protohost && 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);
    }
  }
};