June 2021
Intermediate to advanced
290 pages
9h 21m
Chinese
Content preview from Node与Express开发（第2版）
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98
第 10章
中间件
目前为止，我们已经接触了不少中间件。我们已经使用过现有的中间件（
body-parser
、
cookie-parser
、
static
和
express-session
，稍作列举），甚至还写了自己的中间件（把天气
数据加到模板的上下文、配置
flash
消息和
404
处理函数）
。可是，中间件究竟是什么呢？
从概念上讲，
中间件
是封装功能的一种方式，具体来说这种功能就是操作向应用发起的
HTTP
请求
。在实际应用中，中间件只是一个接收
3
个参数的函数，这
3
个参数分别为：
request
对
象、
response
对象和
next()
函数（
next
函数一会儿再解释）。（还有一种用于错误
处理的接收
4
个参数的形式，本章最后再讨论。
）
中间件是在所谓的
管线
中执行的。你可以将其想象成现实中的一根水管。水从管子的一端
泵上来，在流到最终目的地之前会经过水压计和阀门。这个类比的关键点是
顺序很重要
：
如果你把水压计放在阀门前面，那么跟放在阀门后面相比效果完全不同。与此类似，如果
你有一个阀门可以向水里注入某种东西，那么这个阀门“下游”的水都会包含加入的这种
物质。在
Express
应用中，通过调用
app.use
可以把中间件插入管线中。
Express 4.0
之前的版本必须明确地链入
router
1
，这就使管线复杂化了。链入
router
的位置，
可能会导致路由的顺序错乱，当你混用中间件和路由处理函数时，管线的序列就不那么清
楚了。到了
Express 4.0
，中间件和路由处理函数是按照它们链入的顺序调用的
，这个序列
就清楚多了。
对于未匹配到其他任何路由的请求，使用一个兜底函数来处理，并把这个函数作为管线的 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.