- HELP WANTED
- Introduction
- Installation
- Towards a 3.0.0 release
- Quickstart
- Authentication
- Advanced Usage
- Development
- Tests
- Testimonials
- License
Hi Everyone, I’m in need of help with the 3.0.0 version (the master branch) of
clj-http. In moving to the non-deprecated Apache HTTP client APIs I have broken
a lot of the existing features for clj-http. Since 3.0.0 is basically a rewrite
of core.clj, there are many things that still need implementing before 3.0 can
be released. To see what needs fixing run lein test :all.
If you have interest, please do send PRs for fixing.
clj-http is an HTTP library wrapping the Apache HttpComponents client. This library has taken over from mmcgrana’s clj-http.
The design of clj-http is inspired by the Ring protocol for Clojure HTTP
server applications.
The client in clj-http.core makes HTTP requests according to a given Ring
request map and returns Ring response maps corresponding to the resulting HTTP
response. The function clj-http.client/request uses Ring-style middleware to
layer functionality over the core HTTP request/response implementation. Methods
like clj-http.client/get are sugar over this clj-http.client/request
function.
clj-http is available as a Maven artifact from Clojars.
With Leiningen/Boot:
[clj-http "2.1.0"]If you want to test out the latest version, a 3.x release is also available.
[clj-http "3.0.1"]Previous versions available as:
[clj-http "2.0.1"]
[clj-http "1.1.2"]
[clj-http "1.1.1"]
[clj-http "1.0.1"]clj-http supports clojure 1.6.0 and higher.
So there is a 2.0 branch which is based on the latest stable version of
clj-http. Master, on the other hand, is a rewrite of the core.clj for the new
Apache changes (starting with 4.0 I believe) to use non-deprecated methods.
There are a number of things that need to be accomplished in order for it to be
releasable in a non-snapshot version (3.0.0+ is already on Clojars)
Here are the tests that need to pass:
- [X]
lein test clj-http.test.client-test :all - [X]
lein test clj-http.test.conn-mgr-test :all - [X]
lein test clj-http.test.cookies-test :all - [X]
lein test clj-http.test.core-test :all - [X]
lein test clj-http.test.headers-test :all - [X]
lein test clj-http.test.links-test :all - [X]
lein test clj-http.test.multipart-test :all - [X]
lein test clj-http.test.util-test :all
There are still a few tests that are commented out. Any help or PRs is greatly appreciated.
The main HTTP client functionality is provided by the clj-http.client namespace.
First, require it in the REPL:
(require '[clj-http.client :as client])Or in your application:
(ns my-app.core
(:require [clj-http.client :as client]))The client supports simple get, head, put, post, delete, copy,
move, patch, and options requests. Response are returned as Ring-style
response maps:
(client/head "http://example.com/resource")
(client/head "http://site.com/resource" {:accept :json})
Example requests:
(client/get "http://site.com/resources/id")
(client/get "http://site.com/resources/3" {:accept :json})
;; Specifying headers as either a string or collection:
(client/get "http://example.com"
{:headers {"foo" ["bar" "baz"], "eggplant" "quux"}})
;; Using either string or keyword header names:
(client/get "http://example.com"
{:headers {:foo ["bar" "baz"], :eggplant "quux"}})
;; Set any specific client parameters manually:
(client/post "http://example.com"
{:client-params {"http.protocol.allow-circular-redirects" false
"http.protocol.version" HttpVersion/HTTP_1_0
"http.useragent" "clj-http"}})
;; Set your own cookie policy
(client/post "http://example.com"
{:client-params {:cookie-policy (fn [cookie origin] (your-validation cookie origin))}})
;; Completely ignore cookies:
(client/post "http://example.com"
{:client-params {:cookie-policy (constantly nil)}})
;; 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})
;; Throw an exception if the get takes too long. Timeouts in milliseconds.
(client/get "http://site.come/redirects-somewhere" {:socket-timeout 1000 :conn-timeout 1000})
;; Query parameters
(client/get "http://site.com/search" {:query-params {"q" "foo, bar"}})
;; "Nested" query parameters
;; (this yields a query string of `a[e][f]=6&a[b][c]=5`)
(client/get "http://site.com/search" {:query-params {:a {:b {:c 5} :e {:f 6})
;; Provide cookies — uses same schema as :cookies returned in responses
;; (see the cookie store option for easy cross-request maintenance of cookies)
(client/get "http://site.com"
{:cookies {"ring-session" {:discard true, :path "/", :value "", :version 0}}})
;; Tell clj-http not to decode cookies from the response header
(client/get "http://example.com" {:decode-cookies false})
;; Support for IPv6!
(client/get "http://[2001:62f5:9006:e472:cabd:c8ff:fee3:8ddf]")
The client will also follow redirects on the appropriate 30* status codes.
The client transparently accepts and decompresses the gzip and deflate
content encodings.
:trace-redirects will contain the chain of the redirections followed.
(client/put "http://example.com/api" {:body "my PUT body"})
;; 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 ;; in milliseconds
:conn-timeout 1000 ;; in milliseconds
:accept :json})
;; Send form params as a urlencoded body (POST or PUT)
(client/post "http://site.com" {:form-params {:foo "bar"}})
;; Send form params as a json encoded body (POST or PUT)
(client/post "http://site.com" {:form-params {:foo "bar"} :content-type :json})
;; Send form params as a json encoded body (POST or PUT) with options
(client/post "http://site.com" {:form-params {:foo "bar"}
:content-type :json
:json-opts {:date-format "yyyy-MM-dd"}})
;; You can also specify the encoding of form parameters
(client/post "http://site.com" {:form-params {:foo "bar"}
:form-param-encoding "ISO-8859-1"})
;; Send form params as a Transit encoded JSON body (POST or PUT) with options
(client/post "http://site.com" {:form-params {:foo "bar"}
:content-type :transit+json
:transit-opts {:handlers {}}})
;; Send form params as a Transit encoded MessagePack body (POST or PUT) with options
(client/post "http://site.com" {:form-params {:foo "bar"}
:content-type :transit+msgpack
:transit-opts {:handlers {}}})
;; Multipart form uploads/posts
;; takes a vector of maps, to preserve the order of entities, :name
;; will be used as the part name unless :part-name is specified
(client/post "http://example.org" {:multipart [{:name "title" :content "My Awesome Picture"}
{:name "Content/type" :content "image/jpeg"}
{:name "foo.txt" :part-name "eggplant" :content "Eggplants"}
{:name "file" :content (clojure.java.io/file "pic.jpg")}]})
;; Multipart :content values can be one of the following:
;; String, InputStream, File, a byte-array, or an instance of org.apache.http.entity.mime.content.ContentBody
;; Some Multipart bodies can also support more keys (like :encoding
;; and :mime-type), check src/clj-http/multipart.clj to see all flags
;; Apache's http client automatically retries on IOExceptions, if you
;; would like to handle these retries yourself, you can specify a
;; :retry-handler. Return true to retry, false to stop trying:
(client/post "http://example.org" {:multipart [["title" "Foo"]
["Content/type" "text/plain"]
["file" (clojure.java.io/file "/tmp/missing-file")]]
:retry-handler (fn [ex try-count http-context]
(println "Got:" ex)
(if (> try-count 4) false true))})
(client/delete "http://example.com/resource")
;; body as a byte-array
(client/post "http://site.com/resources" {:body my-byte-array})
;; body as a string
(client/post "http://site.com/resources" {:body "string"})
;; :body-encoding is optional and defaults to "UTF-8"
(client/post "http://site.com/resources"
{:body "string" :body-encoding "UTF-8"})
;; body as a file
(client/post "http://site.com/resources"
{:body (clojure.java.io/file "/tmp/foo") :body-encoding "UTF-8"})
;; :length is optional for passing in an InputStream; if not
;; supplied it will default to -1 to signal to HttpClient to use
;; chunked encoding
(client/post "http://site.com/resources"
{:body (clojure.java.io/input-stream "/tmp/foo")})
(client/post "http://site.com/resources"
{:body (clojure.java.io/input-stream "/tmp/foo") :length 1000});; The default output is a string body
(client/get "http://site.com/foo.txt")
;; Coerce as a byte-array
(client/get "http://site.com/favicon.ico" {:as :byte-array})
;; Coerce as something other than UTF-8 string
(client/get "http://site.com/string.txt" {:as "UTF-16"})
;; Coerce as json
(client/get "http://site.com/foo.json" {:as :json})
(client/get "http://site.com/foo.json" {:as :json-strict})
(client/get "http://site.com/foo.json" {:as :json-string-keys})
(client/get "http://site.com/foo.json" {:as :json-strict-string-keys})
;; Coerce as Transit encoded JSON or MessagePack
(client/get "http://site.com/foo" {:as :transit+json})
(client/get "http://site.com/foo" {:as :transit+msgpack})
;; Coerce as a clojure datastructure
(client/get "http://site.com/foo.clj" {:as :clojure})
;; Coerce as x-www-form-urlencoded
(client/post "http://site.com/foo" {:as :x-www-form-urlencoded})
;; Try to automatically coerce the output based on the content-type
;; header (this is currently a BETA feature!). Currently supports
;; text, json and clojure (with automatic charset detection)
;; clojure coercion requires "application/clojure" or
;; "application/edn" in the content-type header
(client/get "http://site.com/foo.json" {:as :auto})
;; Return the body as a stream
(client/get "http://site.com/bigrequest.html" {:as :stream})
;; Note that the connection to the server will NOT be closed until the
;; stream has been readJSON coercion defaults to only an “unexceptional” statuses, meaning status codes
in the #{200 201 202 203 204 205 206 207 300 301 302 303 307} range. If you
would like to change this, you can send the :coerce option, which can be set
to:
:always ;; always json decode the body
:unexceptional ;; only json decode when not an HTTP error response
:exceptional ;; only json decode when it IS an HTTP error responseThe :coerce setting defaults to :unexceptional.
clj-http’s treatment of headers is a little more permissive than the ring spec specifies.
Rather than forcing all request headers to be lowercase strings, clj-http allows strings or keywords of any case. Keywords will be transformed into their canonical representation, so the :content-md5 header will be sent to the server as “Content-MD5”, for instance. String keys in request headers, however, will be sent to the server with their casing unchanged.
Response headers can be read as keywords or strings of any case. If the server responds with a “Date” header, you could access the value of that header as :date, “date”, “Date”, etc.
If for some reason you require access to the original header name that the server specified, it is available by invoking (keys …) on the header map.
This special treatment of headers is implemented in the wrap-header-map middleware, which (like any middleware) can be disabled by using with-middleware to specify different behavior.
HTML 4.01 allows using the tag <meta http-equiv="..." /> and HTML 5 allows
using the tag <meta charset="..." /> to specify a header that should be
treated as an HTTP response header. By default, clj-http will ignore the body of
the response (other than the regular output coercion), but if you need clj-http
to parse the headers out of the body, you can use the :decode-body-headers
option:
;; without decoding body headers (defaults to off):
(:headers (client/get "http://www.yomiuri.co.jp/"))
=> {"server" "Apache",
"content-encoding" "gzip",
"content-type" "text/html",
"date" "Tue, 09 Oct 2012 18:02:41 GMT",
"cache-control" "max-age=0, no-cache",
"expires" "Tue, 09 Oct 2012 18:02:41 GMT",
"etag" "\"1dfb-2686-4cba2686fb8b1\"",
"pragma" "no-cache",
"connection" "close"}
;; with decoding body headers, notice the content-type,
;; content-style-type and content-script-type headers:
(:headers (client/get "http://www.yomiuri.co.jp/" {:decode-body-headers true}))
=> {"server" "Apache",
"content-encoding" "gzip",
"content-script-type" "text/javascript",
"content-style-type" "text/css",
"content-type" "text/html; charset=Shift_JIS",
"date" "Tue, 09 Oct 2012 18:02:59 GMT",
"cache-control" "max-age=0, no-cache",
"expires" "Tue, 09 Oct 2012 18:02:59 GMT",
"etag" "\"1dfb-2686-4cba2686fb8b1\"",
"pragma" "no-cache",
"connection" "close"}This can be used to have clj-http correctly interpret the body’s charset by using:
(client/get "http://www.yomiuri.co.jp/" {:decode-body-headers true :as :auto})
=> ;; correctly formatted :body (Shift_JIS charset instead of UTF-8)Note that this feature is currently beta and uses Crouton to parse the body of the request. If you do not want to use this feature, you can include Crouton in addition to clj-http as a dependency like so:
(defproject foo "0.1.0-SNAPSHOT"
:dependencies [[org.clojure/clojure "1.3.0"]
[clj-http "0.6.0"]
[crouton "1.0.0"]])Note also that HEAD requests will not return a body, in which case this setting will have no effect.
clj-http will automatically disable the :decode-body-headers option.
clj-http parses any link headers returned in the response, and adds them to the
:links key on the response map. This is particularly useful for paging RESTful
APIs:
(:links (client/get "https://api.github.com/gists"))
=> {:next {:href "https://api.github.com/gists?page=2"}
:last {:href "https://api.github.com/gists?page=22884"}}clj-http conforms its behaviour regarding automatic redirects to the RFC.
It means that redirects on status 301, 302 and 307 are not redirected on
methods other than GET and HEAD. If you want a behaviour closer to what most
browser have, you can set :force-redirects to true in your request to have
automatic redirection work on all methods by transforming the method of the
request to GET.
clj-http can simplify the maintenance of cookies across requests if it is provided with a cookie store.
(binding [clj-http.core/*cookie-store* (clj-http.cookies/cookie-store)]
(client/post "http://site.com/login" {:form-params {:username "..."
:password "..."}})
(client/get "http://site.com/secured-page")
...)(The clj-http.cookies/cookie-store function returns a new empty instance of a
default implementation of org.apache.http.client.CookieStore.)
This will allow cookies to only be written to the cookie store. Cookies from the cookie-store will not automatically be sent with future requests.
If you would like cookies from the cookie-store to automatically be sent with
each request, specify the cookie-store with the :cookie-store option:
(let [my-cs (clj-http.cookies/cookie-store)]
(client/post "http://site.com/login" {:form-params {:username "..."
:password "..."}
:cookie-store my-cs})
(client/post "http://site.com/update" {:body my-data
:cookie-store my-cs}))You can also us the get-cookies function to retrieve the cookies
from a cookie store:
(def cs (clj-http.cookies/cookie-store))
(client/get "http://google.com" {:cookie-store cs})
(clojure.pprint/pprint (clj-http.cookies/get-cookies cs))
{"NID"
{:domain ".google.com",
:expires #<Date Tue Oct 02 10:12:06 MDT 2012>,
:path "/",
:value
"58=c387....",
:version 0},
"PREF"
{:domain ".google.com",
:expires #<Date Wed Apr 02 10:12:06 MDT 2014>,
:path "/",
:value
"ID=3ba...:FF=0:TM=133...:LM=133...:S=_iRM...",
:version 0}}You can also specify your own keystore/trust-store to be used:
(client/get "https://example.com" {:keystore "/path/to/keystore.ks"
:keystore-type "jks" ; default: jks
:keystore-pass "secretpass"
:trust-store "/path/to/trust-store.ks"
:trust-store-type "jks" ; default jks
:trust-store-pass "trustpass"})The :keystore/:trust-store values may be either paths to keystore
files or KeyStore instances.
The client will throw exceptions on, well, exceptional status codes, meaning all
HTTP responses other than #{200 201 202 203 204 205 206 207 300 301 302 303
307}. clj-http will throw a Slingshot Stone that can be caught by a regular
(catch Exception e ...) or in Slingshot’s try+ block:
(client/get "http://site.com/broken")
=> ExceptionInfo clj-http: status 404 clj-http.client/wrap-exceptions/fn--583 (client.clj:41)
;; Or, if you would like the Exception message to contain the entire response:
(client/get "http://site.com/broken" {:throw-entire-message? true})
=> ExceptionInfo clj-http: status 404 {:status 404,
:headers {"server" "nginx/1.0.4",
"x-runtime" "12ms",
"content-encoding" "gzip",
"content-type" "text/html; charset=utf-8",
"date" "Mon, 17 Oct 2011 23:15 :36 GMT",
"cache-control" "no-cache",
"status" "404 Not Found",
"transfer-encoding" "chunked",
"connection" "close"},
:body "...body here..."}
clj-http.client/wrap-exceptions/fn--584 (client.clj:42
;; You can also ignore HTTP-status-code exceptions and handle them yourself:
(client/get "http://site.com/broken" {:throw-exceptions false})
;; Or ignore an unknown host (methods return 'nil' if this is set to
;; true and the host does not exist:
(client/get "http://aoeuntahuf89o.com" {:ignore-unknown-host? true})(spacing added by me to be human readable)
How to use with Slingshot:
; Response map is thrown as exception obj.
; We filter out by status codes
(try+
(client/get "http://some-site.com/broken")
(catch [:status 403] {:keys [request-time headers body]}
(log/warn "403" request-time headers))
(catch [:status 404] {:keys [request-time headers body]}
(log/warn "NOT Found 404" request-time headers body))
(catch Object _
(log/error (:throwable &throw-context) "unexpected error")
(throw+)))
By default, clj-http will add the {"Accept-Encoding" "gzip, deflate"} header
to requests, and automatically decompress the resulting gzip or deflate stream
if the Content-Encoding header is found on the response. If this is undesired,
the {:decompress-body false} option can be specified:
;; Auto-decompression used: (google requires a user-agent to send gzip data)
(def h {"User-Agent" "Mozilla/5.0 (Windows NT 6.1;) Gecko/20100101 Firefox/13.0.1"})
(def resp (client/get "http://google.com" {:headers h}))
(:orig-content-encoding resp)
=> "gzip" ;; <= google sent response gzipped
;; and without decompression:
(def resp2 (client/get "http://google.com" {:headers h :decompress-body false})
(:orig-content-encoding resp2)
=> nilIf clj-http decompresses something, the “content-encoding” header is removed
from the headers (because the encoding is no longer true). This allows clj-http
to be used as a pass-through proxy with ring. The original content-encoding is
available as :orig-content-encoding in the response map if auto-decompression
is enabled.
There are four debugging methods you can use:
;; print request info to *out*:
(client/get "http://example.org" {:debug true})
;; print request info to *out*, including request body:
(client/post "http://example.org" {:debug true :debug-body true :body "..."})
;; save the request that was sent in a :request key in the response:
(client/get "http://example.org" {:save-request? true})
;; save the request that was sent in a :request key in the response,
;; including the body content:
(client/get "http://example.org" {:save-request? true :debug-body true})
;; add an HttpResponseInterceptor to the request. This callback
;; is called for each redirects with the following args:
;; ^HttpResponse resp, HttpContext^ ctx
;; this allows low level debugging + access to socket.
;; see http://hc.apache.org/httpcomponents-core-ga/httpcore/apidocs/org/apache/http/HttpResponseInterceptor.html
(client/get "http://example.org" {:response-interceptor (fn [resp ctx] (println ctx))})(client/get "http://site.com/protected" {:basic-auth ["user" "pass"]})
(client/get "http://site.com/protected" {:basic-auth "user:pass"})
(client/get "http://site.com/protected" {:digest-auth ["user" "pass"]})
(client/get "http://site.com/protected" {:oauth-token "secret-token"})
A more general request function is also available, which is useful as a
primitive for building higher-level interfaces:
(defn api-action [method path & [opts]]
(client/request
(merge {:method method :url (str "http://site.com/" path)} opts)))Since 0.9.0, all boolean options can be expressed as either {:debug true} or
{:debug? true}, with or without the question mark.
clj-http can use persistent connections to speed up connections if multiple connections are being used:
(with-connection-pool {:timeout 5 :threads 4 :insecure? false :default-per-route 10}
(get "http://aoeu.com/1")
(post "http://aoeu.com/2")
(get "http://aoeu.com/3")
...
(get "http://aoeu.com/999"))This is MUCH faster than sequentially performing all requests, because a persistent connection can be used instead creating a new connection for each request.
If you would prefer to handle managing the connection manager yourself, you can create a connection manager yourself and specify it for each request:
(def cm (clj-http.conn-mgr/make-reusable-conn-manager {:timeout 2 :threads 3}))
(def cm2 (clj-http.conn-mgr/make-reusable-conn-manager {:timeout 10 :threads 1}))
(get "http://aoeu.com/1" {:connection-manager cm2})
(post "http://aoeu.com/2" {:connection-manager cm})
(get "http://aoeu.com/3" {:connection-manager cm2})
;; Don't forget to shut it down when you're done!
(clj-http.conn-mgr/shutdown-manager cm)
(clj-http.conn-mgr/shutdown-manager cm2)See the docstring on make-reusable-conn-manager for options and default
values.
A proxy can be specified by setting the Java properties: <scheme>.proxyHost
and <scheme>.proxyPort where <scheme> is the client scheme used (normally
‘http’ or ‘https’). http.nonProxyHosts allows you to specify a pattern for
hostnames which do not require proxy routing - this is shared for all schemes.
Additionally, per-request proxies can be specified with the proxy-host and
proxy-port options (this overrides http.nonProxyHosts too):
(client/get "http://foo.com" {:proxy-host "127.0.0.1" :proxy-port 8118})You can also specify the proxy-ignore-hosts parameter with a list of
hosts where the proxy should be ignored. By default this list is
#{"localhost" "127.0.0.1"}.
A SOCKS proxy can be used by creating a proxied connection manager with
clj-http.conn-mgr/make-socks-proxied-conn-manager. Then using that connection
manager in the request.
For example if you wanted to connect to a local socks proxy on port 8081 you
would:
(ns foo.bar
(:require [clj-http.client :as client]
[clj-http.conn-mgr :as conn-mgr]))
(client/get "https://google.com"
{:connection-manager
(conn-mgr/make-socks-proxied-conn-manager "localhost" 8081)})You can also store the proxied connection manager and reuse it later.
Sometime it is desirable to run a request with some middleware enabled and some
left out, the with-middleware method provides this functionality:
(with-middleware [#'clj-http.client/wrap-method
#'clj-http.client/wrap-url
#'clj-http.client/wrap-exceptions]
(get "http://example.com")
(post "http://example.com/foo" {:body (.getBytes "foo")}))To see available middleware, check the clj-http.client/default-middleware var,
which is a vector of the default middleware that clj-http uses.
clj-http.client/*current-middleware* is bound to the current list of
middleware during request time.
Please send a pull request or open an issue if you have any problems.
If you need to fake clj-http responses (for things like testing and such), check out the clj-http-fake library.
In 2.0.0+ clj-http’s optional dependencies at excluded by default, in order to
use the features you will need to add them to your project.clj file.
clj-http currently has four optional dependencies, cheshire, crouton,
tools.reader and ring/ring-codec. Any number of them may be included by
adding them with the clj-http dependency in your project.clj:
;; optional dependencies
[cheshire] ;; for :as :json
[crouton] ;; for :decode-body-headers
[org.clojure/tools.reader] ;; for :as :clojure
[ring/ring-codec] ;; for :as :x-www-form-urlencodedPrior to 2.0.0, you can exclude the dependencies and clj-http will work without them.
Like clj-http but need something more lightweight without as many external dependencies? Check out clj-http-lite for a project that can be used as a drop-in replacement for clj-http.
This is actually caused by your project attempting to use clj-json and cheshire in the same classloader. You can fix the issue by either not using clj-json (and thus choosing cheshire), or specifying an exclusion for clj-http in your project like this:
(defproject foo "0.1.0-SNAPSHOT"
:dependencies [[org.clojure/clojure "1.3.0"]
[clj-http "0.3.4" :exclusions [cheshire]]])Note that if you exclude cheshire, json decoding of response bodies and json encoding of form-params cannot happen, you are responsible for your own encoding/decoding.
As of clj-http 0.3.5, you should no longer see this, as Cheshire 3.1.0 and clj-json can now live together without causing problems.
Persistent connections kept alive by the connection manager become stale: the target server shuts down the connection on its end without HttpClient being able to react to that event, while the connection is being idle, thus rendering the connection half-closed or ‘stale’.
This can be solved by using (with-connection-pool) as described in the ‘Using Persistent Connection’ section above.
To run the tests:
$ lein deps $ lein test Run all tests (including integration): $ lein test :all Run tests against 1.2.1, 1.3 and 1.4 $ lein all test $ lein all test :all
With close to a million downloads, clj-http is a widely used, battle-tested clojure library. It is also included in other libraries (like database clients) as a low-level http wrapper.
Libraries using clj-http:
Libraries inspired by clj-http:
Released under the MIT License: http://www.opensource.org/licenses/mit-license.php