我试图了解Express中的中间件和路由处理程序是如何工作的。在Web Development with Node and Express中，作者给出了一个有趣的路径和中间件的实例，但没有给出实际的细节。
有人可以帮我理解下面例子中每一步发生的事情，这样我才能确定我是否正确思考它？这是一个例子（我的理解和评论中的问题）：
//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');
});
答案 0 :(得分：10)
在快递中，序列中间件注册有很大的不同，当快递收到请求时，它只执行注册的中间件并匹配请求的URL。
表达中间件有以下签名
function(req,res,next){}
req: request object.
res: response object.
next: next middleware thunk.
特殊错误处理中间件
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 /a. i.e. 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(res, 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');
});
希望这有助于您了解流程。
如果您需要进一步澄清，请与我们联系。
答案 1 :(得分：1)
app.use(function(req,res,next){
由于上述代码未指定任何路由，例如/ a或/ b，因此每次启动API时都会执行此类中间件。所以这个中间件将永远执行。
app.use(function(err,req,res,next){
了解当app.use有4个参数时，Express会将此识别为错误处理中间件。因此，执行时抛出或创建的任何错误都将通过此中间件
因此，＃11 NOT 是一个错误处理中间件。它只是停止了中间件链，因为它没有next（）函数 AND 它是序列中的最后一个中间件。
你现在也应该理解＃7是一个错误处理中间件，它从＃6路由中得到＃6的错误。 ＃7处理错误传递的错误，然后将error参数传递给next（）函数。
next（）
next（）只是沿着链传递控制的函数。如果您觉得某个中间件对于该特定路由（甚至没有路由）是不够的，您可以调用next（）函数，该函数将控制传递给下一个有效的中间件。
您可以使用路径指定有效性，或者使其具有通用性，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（）没有传递错误的参数，因此＃10是一个错误处理中间件，它不会捕获它。 ＃9将进入＃11
答案 2 :(得分：0)
您应该查看Express文档。它充满了你会发现有用的资源，特别是如果你是Express的新手。以下是有关您的问题的文档相关部分的一些链接。
希望这有帮助。