使用http client提交表单或者下载网页也是非常常见的任务，比如使用Java的时候可以用标准库的HttpURLConnection，也可以选择Apache Http Client。在clojure里也有这样的类库，这里我将介绍三个各有特色的http client实现。
首先，我最先推荐使用clj-http这个类库，它是Apache HttpClient的clojure wrapper，是一个提供同步API的简单易用的Http Client。
名称: clj-http
主页：https://github.com/dakrone/clj-http
依赖：
[clj-http "0.3.1"]
例子：
(require '[clj-http.client :as client])
(client/get "http://google.com")
结果：
=> ｛:cookies {"NID" {:domain ".google.com.hk", :expires #<Date Tue Aug 14 18:20:38 CST 2012>, :path "/", :value "56=qn2OWtODE2D3fUKi_vbi44jZepOeLI9xC4Ta1JQLEicqUvIZAqr7TCmft_hq8i_FRwnFXdTK1jV2S5IrSZFyYhlAN2KcQEXgWX1iK36gM2iYPaKPihuUZDCqgiAamDOl", :version 0}, "PREF" {:domain ".google.com.hk", :expires #<Date Wed Feb 12 18:20:38 CST 2014>, :path "/", :value "ID=8b73a654ff0a2783:FF=0:NW=1:TM=1329128438:LM=1329128438:S=uEM4SsFuHlkqtVhp", :version 0}},
:status 200
:headers {"date" "Sun, 01 Aug 2010 07:03:49 GMT"
"cache-control" "private, max-age=0"
"content-type" "text/html; charset=ISO-8859-1"
}
:body "<!doctype html>"
:trace-redirects ["http://google.com" "http://www.google.com/" "http://www.google.fr/"]}
更多例子：
(client/get "http://site.com/resources/3" {:accept :json})
;; Various options:
(client/post "http://site.com/api"
{:basic-auth ["user" "pass"]
:body "{\"json\": \"input\"}"
:headers {"X-Api-Version" "2"}
:content-type :json
:socket-timeout 1000
:conn-timeout 1000
:accept :json})
;; Need to contact a server with an untrusted SSL cert?
(client/get "https://alioth.debian.org" {:insecure? true})
;; If you don't want to follow-redirects automatically:
(client/get "http://site.come/redirects-somewhere" {:follow-redirects false})
;; Only follow a certain number of redirects:
(client/get "http://site.come/redirects-somewhere" {:max-redirects 5})
;; Throw an exception if redirected too many times:
(client/get "http://site.come/redirects-somewhere" {:max-redirects 5 :throw-exceptions true})
;; Send form params as a urlencoded body
(client/post "http//site.com" {:form-params {:foo "bar"}})
;; Multipart form uploads/posts
;; a map or vector works as the multipart object. Use a vector of
;; vectors if you need to preserve order, a map otherwise.
(client/post "http//example.org" {:multipart [["title" "My Awesome Picture"]
["Content/type" "image/jpeg"]
["file" (clojure.java.io/file "pic.jpg")]]})
;; Multipart values can be one of the following:
;; String, InputStream, File, or a byte-array
;; Basic authentication
(client/get "http://site.com/protected" {:basic-auth ["user" "pass"]})
(client/get "http://site.com/protected" {:basic-auth "user:pass"})
;; Query parameters
(client/get "http://site.com/search" {:query-params {"q" "foo, bar"}})
clj-http的API相当的简洁漂亮，使用起来非常便利，强烈推荐。题外，学习clojure的一个好方法就是为现有的java类库实现一些方便的clojure wrapper。
如果你需要异步的http client，我会推荐http.async.client这个类库，它的API是异步形式的类似 Java的Future模式，对于clojure程序员来说应该更像是agent。
名称：http.async.client
主页：https://github.com/neotyk/http.async.client
依赖：
[http.async.client "0.4.1"]
例子：
(require '[http.async.client :as c])
(with-open [client (c/create-client)]
(let [response (c/GET client "http://neotyk.github.com/http.async.client/")]
(prn (c/done? response))
(c/await response)
(prn (c/string response))
(prn (c/status response))
(prn (c/done? response))))
输出：
false
<!DOCTYPE html
{:code 200, :msg "OK", :protocol "HTTP/1.1", :major 1, :minor 1}
true
更多例子：
(c/POST client "http://example.com" :body "hello world" :timeout 3000)
(c/DELETE client "http://example.com")
(c/POST client "http://example.com" :body "hello world" :auth {:type :basic :user "admin" :password "admin"})
请注意，这些方法都是异步调用的，你需要通过await来等待调用完成，或者通过done?来判断调用是否完成。
http.async.client有个比较重要的特性就是对Http Chunked编码的支持，分别通过LazySeq和callback的方式支持，首先看将Http chunked变成一个lazy seq:
(with-open [client (client/create-client)] ; Create client
(let [resp (client/stream-seq client :get url)]
(doseq [s (s/string resp)]
(println s))))
这里非常关键的一点是stream-seq返回的chunk序列，每取一个就少一个（通过first函数），也就是说每次调用first取到的chunk都不一样，是顺序递增，不可重复获取的。
通过callback方式处理：
(with-open [client (client/create-client)] ; Create client
(let [parts (ref #{})
resp (client/request-stream client :get url
(fn [state body]
(dosync (alter parts conj (string body)))
[body :continue]))]
;; do something to @parts
))
自己传入一个callback函数接收chunk，比如这里用一个ref累积。
http.async.client的详细文档看这里：http://neotyk.github.com/http.async.client/docs.html
最后，有兴趣还可以看下aleph这个异步通讯的框架，它支持Http协议，也提供了http server和client的实现。不过它的API就没有那么简单明了，它的模型是类似go语言里利用channel做异步通讯的模型，http只是它的一个模块罢了，这是另一个话题了。
文章转自庄周梦蝶 ，原文发布时间2012-02-13