了解Express.js中的中间件和路由处理程序

我想了解中间件和路由处理程序如何在Express中工作。 在“ 使用Node和Express进行Web开发”一书中 ,笔者给出了一个有趣的途径和中间件的例子,但并没有给出实际的细节。

有人能帮我理解下面例子中的每一步会发生什么,这样我就可以肯定我正确地思考了吗? 这里是例子(在我的理解和问题的意见):

//get the express module as app var app = require('express')(); //1. when this module is called, this is printed to the console, always. app.use(function(req, res, next){ console.log('\n\nALLWAYS'); next(); }); //2. when a route /a is called, a response 'a' is sent and the app stops. There is no action from here on app.get('/a', function(req, res){ console.log('/a: route terminated'); res.send('a'); }); //3. this never gets called as a consequence from above app.get('/a', function(req, res){ console.log('/a: never called'); }); //4. this will be executed when GET on route /b is called //it prints the message to the console and moves on to the next function //question: does this execute even though route /a (2) terminates abruptly? app.get('/b', function(req, res, next){ console.log('/b: route not terminated'); next(); }); //5. question: this gets called after above (4)? app.use(function(req, res, next){ console.log('SOMETIMES'); next(); }); //6. question: when does this get executed? There is already a function to handle when GET on /b is called (4) app.get('/b', function(req, res, next){ console.log('/b (part 2): error thrown' ); throw new Error('b failed'); }); //7. question: I am not sure when this gets called... ? app.use('/b', function(err, req, res, next){ console.log('/b error detected and passed on'); next(err); }); //8. this is executed when a GET request is made on route /c. It logs to the console; throws an error. app.get('/c', function(err, req){ console.log('/c: error thrown'); throw new Error('c failed'); }); //9. question: this catches the above error and just moves along? app.use('/c', function(err, req, res, next) { console.log('/c: error deteccted but not passed on'); next(); }); //10. question: this follows the above and prints an error based on above? //This also sends a 500 reponse? app.use(function(err, req, res, next){ console.log('unhandled error detected: ' + err.message); res.send('500 - server error'); }); //11. question: this is the catch all for something that falls through and sends a 404? app.use(function(req, res){ console.log('route not handled'); res.send('404 - not found'); }); //12. This app listens on the 3000 port. app.listen(3000, function(){ console.log('listening on 3000'); }); 

在express中,序列中间件注册有很大的不同,当express收到请求时,它只是执行注册的中间件并且匹配请求的url。

expression中间件有以下签名

function(req,res,next){}

 req: request object. res: response object. next: next middleware thunk. 

特殊的error handling中间件

function(err, req,res,next){}

 err: error that was caught req: request object. res: response object. next: next middleware thunk. 

我正在更新代码本身的评论

  //get the express module as app var app = require('express')(); //1. when this *middleware* is called, this is printed to the console, always. //As this is first registered middleware, it gets executed no matter what because no url match were provided. This middleware does not stop the middleware chain execution as it calls next() and executes next middleware in chain. app.use(function(req, res, next){ console.log('\n\nALLWAYS'); next(); }); //2. when a route /a is called, a response 'a' is sent and //the app stops. There is no action from here on //chain stops because this middleware does not call next() app.get('/a', function(req, res, next){ console.log('/a: route terminated'); res.send('a'); }); //3. this never gets called as a consequence from above //because (2) never calls next. app.get('/a', function(req, res){ console.log('/a: never called'); }); //4. this will be executed when GET on route /b is called //it prints the message to the console and moves on to the next function //question: does this execute even though route /a (2) terminates abruptly? //app.get('/b' ... does not depend in any way on (2). as url match criteria are different in both middleware, even if /a throws an exception, /b will stay intact. but this middleware will get executed only at /b not /aie if /a calls next(), this middleware will not get executed. app.get('/b', function(req, res, next){ console.log('/b: route not terminated'); next(); }); //5. question: this gets called after above (4)? //Yes, as (4) calls next(), this middleware gets executed as this middleware does not have a url filter pattern. app.use(function(req, res, next){ console.log('SOMETIMES'); next(); }); //6. question: when does this get executed? There is already a function to handle when GET on /b is called (4) //As (4) calls next(), (5) gets called, again (5) calls next() hence this is called. if this was something other '/b' like '/bbx' this wouldn't get called --- hope this makes sense, url part should match. app.get('/b', function(req, res, next){ console.log('/b (part 2): error thrown' ); throw new Error('b failed'); }); //7. question: I am not sure when this gets called... ? // This happens (4) calls next() -> (5) calls next() -> (6) throws exception, hence this special error handling middleware that catches error from (6) and gets executed. If (6) does not throw exception, this middleware won't get called. //Notice next(err) this will call (10). -- as we are passing an error app.use('/b', function(err, req, res, next){ console.log('/b error detected and passed on'); next(err); }); //8. this is executed when a GET request is made on route /c. It logs to the console; throws an error. app.get('/c', function(err, req){ console.log('/c: error thrown'); throw new Error('c failed'); }); //9. question: this catches the above error and just moves along? //Yes, as this middleware calls next(), it will move to next matching middleware. so it will call (11) and not (10) because (10) is error handling middleware and needs to be called like next(err) app.use('/c', function(err, req, res, next) { console.log('/c: error deteccted but not passed on'); next(); }); //10. question: this follows the above and prints an error based on above? //Which ever middleware from above calls next(err) will end up calling this one. ie. (7) does so. //This also sends a 500 response? //This just sends text as '500 - server error' //in order to set status code you'll need to do res.status(500).send ... app.use(function(err, req, res, next){ console.log('unhandled error detected: ' + err.message); res.send('500 - server error'); //Also set status code //res.status(500).send('500 - server error'); }); //11. question: this is the catch all for something that falls through and sends a 404? //No, this does not catch error, as in (7). This route will get elected when non of the above middleware were able to respond and terminate the chain. So this is not an error handling route, this route just sends 404 message if non of the above routes returned a response and stopped chain of execution app.use(function(req, res){ console.log('route not handled'); res.send('404 - not found'); //Also set status code //res.status(400).send('404 - not found'); }); //12. This app listens on the 3000 port. app.listen(3000, function(){ console.log('listening on 3000'); }); 

希望这可以帮助你理解stream程。

让我知道如果你需要更多的澄清。

中间件可以很好用,起初有点难理解。 在中间件中要记住的最重要的事情是

  • 序列
  • next()函数
  • 序列
    正如前面的答案中所提到的,sorting在中间件中非常重要。 由于中间件是一个接一个地执行的,所以试着严格了解代码

     app.use(function(req,res,next){ 

    由于上面的代码没有指定任何路由,比如/ a或/ b,所以这个types的中间件将会在你的API每次被命中时执行。 所以这个中间件将永远被执行

     app.use(function(err,req,res,next){ 

    了解,当app.use有4个参数时,Express将此标识为error handling中间件。 所以在执行时抛出或创build的任何错误都会通过这个中间件。
    因此,#11 不是error handling中间件。 它只是停止中间件的链,因为它没有next()函数它是序列中的最后一个中间件。
    你现在也应该明白#7是一个error handling中间件,并且从/#6路由器出现错误。 #7处理在err中传递的错误,然后将errorparameter passing给next()函数。

    下一个()
    next()就是沿链传递控制的函数。 如果你觉得一个中间件对于那个特定的路由来说是不够的(甚至是没有路由的话),你可以调用next()函数,这个函数会把控制权交给下一个有效的中间件。
    您可以使用路由指定有效性,或使其具有通用性,例如#9和#10。 来自#9的错误不会传递给#10。 这是因为#9中的next()没有传递一个err参数,因此#10是一个error handling中间件,它不会捕获它。 #9将达到#11

    你应该看一下Express文档。 它充满了您会发现有用的资源,特别是如果您是Express的新手。 以下是有关您的问题的相关文档的几个链接。

    • 路由
    • 编写中间件
    • 使用中间件

    希望这可以帮助。