Clojure世界：Http Client
使用http client提交表單或者下載網頁也是非常常見的任務，比如使用Java的時候可以用標準庫的HttpURLConnection，也可以選擇 ofollow,noindex" target="_blank"> Apache Http Client 。在clojure裡也有這樣的類庫，這裡我將介紹三個各有特色的http client實現。
首先，我最先推薦使用clj-http這個類庫，它是Apache HttpClient的clojure wrapper，是一個提供同步API的簡單易用的Http Client。
名稱: clj-http
主頁： https://github.com/dakrone/clj-http
依賴：
[clj - http " 0.3.1 " ]
例子：
(require ' [clj-http.client :as client])
(client / get " http://google.com " )
結果：
｛: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
"
:trace
- redirects [ " http://google.com " " http://www.google.com/ " " http://www.google.fr/ " ]}
更多例子：
{: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
})
;; 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
ifredirected 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
ifyou 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相當的簡潔漂亮，使用起來非常便利，強烈推薦。題外，學習clojure的一個好方法就是為現有的java類庫實現一些方便的clojure wrapper。
如果你需要非同步的http client，我會推薦http.async.client這個類庫，它的API是非同步形式的類似 Java的Future模式，對於clojure程式員來說應該更像是agent。
名稱：http.async.client
主頁： https://github.com/neotyk/http.async.client
依賴：
[http.async.client " 0.4.1 " ]
例子：
(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))))輸出：
<! DOCTYPE html
{:code 200 , :msg " OK " , :protocol " HTTP/1.1 " , :major 1 , :minor 1
}
true更多例子：
)
(c
/ DELETE client " http://example.com ")
(c
/ POST client " http://example.com " :body " hello world " :auth {:type :basic :user " admin " :password " admin " })請注意，這些方法都是非同步呼叫的，你需要通過await來等待呼叫完成，或者通過done?來判斷呼叫是否完成。
http.async.client有個比較重要的特性就是對Http Chunked編碼的支援，分別通過LazySeq和callback的方式支援，首先看將Http chunked變成一個lazy seq:
client)] ; Create client
(let [resp (client
/ stream -seq client :get url)]
(doseq [s (s
/string resp)]
(println s))))
這裡非常關鍵的一點是stream-seq返回的chunk序列，每取一個就少一個（通過first函式），也就是說每次呼叫first取到的chunk都不一樣，是順序遞增，不可重複獲取的。
通過callback方式處理：
client)] ; Create client
(let [parts (ref #{})
resp (client
/ request -stream client :get url
(fn [state body]
(dosync (alter parts conj (string body)))
[body :
continue]))]
;;
dosomething to @parts
))
自己傳入一個callback函式接收chunk，比如這裡用一個ref累積。
http.async.client的詳細文件看這裡： http://neotyk.github.com/http.async.client/docs.html
最後，有興趣還可以看下 aleph 這個非同步通訊的框架，它支援Http協議，也提供了http server和client的實現。不過它的API就沒有那麼簡單明瞭，它的模型是類似go語言裡利用channel做非同步通訊的模型，http只是它的一個模組罷了，這是另一個話題了。