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What is it?

clj-apache-http is a Clojure wrapper library for the Apache HTTP Client (version 4.0).

http://hc.apache.org/

It defines functions to perform HTTP requests, extensibly returning Clojure data structures for most outputs.

Some knowledge of the underlying Apache HttpClient API is necessary for more advanced usage, such as specifying cookie jars.

For real-world code using this library, see the clj-mql project:

https://github.com/rnewman/clj-mql/tree/master

Building with Ant

Invoke ant, optionally passing -Dclojure.jar="..." and -Dclojure.contrib.jar="...".

Put clj-apache-http.jar on your classpath.

Building with Leiningen

If you use Leiningen, run lein uberjar. This will download the necessary dependencies and build a single .jar named clj-apache-http-standalone.jar.

You can also refer to com.twinql.clojure/clj-apache-http "2.2.0" in Leiningen or Maven to have the dependency automatically satisfied.

Loading

This library defines a function named get, so it's best to require it like this:

(require ['com.twinql.clojure.http :as 'http])

or in your (ns) form:

(:require [com.twinql.clojure.http :as http])

Functions

The exported interface consists of the functions get, post, put, head, and delete, and the utility function http-host (to return a value suitable for the host parameters, such as :default-proxy / DEFAULT_PROXY).

All functions take a "uri part" as input -- a URI, a string parsed as a URI, or a map like

{:host "foo.com" :path "/bar/" :port 9000 :query {:x 5} :scheme "https" :fragment "hah"}

and the following keyword arguments:

  • :query — a query parameter map.
  • :headers — a map of HTTP headers.
  • :parameters — a map of values to be passed to HttpParams.setParameter.

post and put additionally have a :body argument, which must be an HttpEntity.

The result of calling these functions is a map as follows:

  • The status code: :code
  • The reason phrase: :reason
  • The content (subject to transformation): :content
  • The Apache Entity associated with the request/response pair: :entity
  • The Apache HttpClient used for the request (which allows access to the cookie jar): :client
  • The response headers (also subject to transformation): :headers.

Typically most of these can be ignored; :code and :content are the most important fields.

Note that minimal processing is applied to these results (you don't pay for what you don't use).

You can specify the format in which you wish to receive body content and headers using the :as and :headers-as keyword arguments. See below for details.

Query parameters *

Query parameters (as supplied to the :query argument) should be associative: either a map or a sequence of pairs. Parameters will be processed with as-str. Non-sequential values will also be processed with as-str; sequential values (such as vectors) will be turned into multiple query parameters, as expected by most HTTP servers. For example:

(encode-query {:foo "bar" :baz ["noo" 5 true] :top {:x 5 :y 7}})
=>
"foo=bar&baz=noo&baz=5&baz=true&top=%7B%3Ax+5%2C+%3Ay+7%7D"

Examples

(:content (http/get (java.net.URI. "http://example.com") :as :string))
=>
"<HTML>\r\n<HEAD>\r\n  <TITLE>Example Web Page</TITLE>\r\n</HEAD> \r\n<body>…"

(select-keys
  (http/get "http://clojure.org/api" :as :stream) [:code :reason :content])
=> 
{:content #<EofSensorInputStream org.apache.http.conn.EofSensorInputStream@4ba57633>,
 :reason "OK",
 :code 200}

(:reason (http/post "http://google.com/search" :query {:q "frobnosticate"}))
=>
"Method Not Allowed"

(:code (http/get "http://google.com/search" :query {:q "frobnosticate"}))
=>
200

(:headers (http/get "http://google.com/search" :query {:q "frobnosticate"} :headers-as :map))
=>
{"Transfer-Encoding" ["chunked"],
 "Server" ["gws"],
 "Set-Cookie"
 ["SS=Q0=ZnJvYm5vc3RpY2F0ZQ; path=/search"
  ["PREF=ID=9174bb4f419f1279:TM=1248028028:LM=1248028028:S=-Nxn6QHqif9SORnQ; expires=Tue, 19-Jul-2011 18:27:08 GMT; path=/; domain=.google.com"]
  ["NID=24=E9wDEpOrfSFh2bt36RkK7ZIkjH78DeKeA1mulw1A5562byJ2ngJjDPClEHUceb-6ewf7ANSrA-6CrXjUfeHszmv3OM7giddIDfX-RBvtZGYIWI0FbUNbYvoKQXtQRu9S; expires=Mon, 18-Jan-2010 18:27:08 GMT; path=/; domain=.google.com; HttpOnly"]],
 "Content-Type" ["text/html; charset=ISO-8859-1"],
 "Expires" ["-1"],
 "Date" ["Sun, 19 Jul 2009 18:27:08 GMT"],
 "Cache-Control" ["private, max-age=0"]}

Keyword parameters

You can use :query, :headers, :parameters, :as, and :headers-as.

The first three are associative. :as can be:

  • :identity (or nil), returning the Apache HC entity object,
  • :stream, returning a stream,
  • :reader, returning a Reader,
  • :json, which will parse a JSON response body and return the keys of maps as keywords,
  • :json-string-keys, which leaves maps with string keys,
  • or :string.

:headers-as can be

  • :identity, returning a HeaderIterator,
  • :seq (or nil), returning a sequence of [header, value] pairs,
  • :element-seq, returning a sequence of [header, Element[]] pairs,
  • :map, returning a map from header name to vector of values, or
  • :element-map, returning a map from header name to vector of Element[].

Define your own extensions by defining a method on 'entity-as' that turns an HttpEntity into the appropriate format, or 'headers-as' that turns a HeaderIterator into the format of your choice.

Each entity transformer receives the entity, the format, and the status code as arguments: you can thus avoid trying to process certain failure responses. For example:

(defmethod http/entity-as :success-json [#^HttpEntity entity as status]
  (http/entity-as entity
    (if (<= 200 status 299)
     :json
     :string)
    status))

Apache connection parameters

You can pass a parameter map to the HTTP functions. This is used to set various options on the HTTP client.

The keys are long-winded Java constants, but the capability is very useful (e.g., for proxying). See

http://hc.apache.org/httpcomponents-client-4.0.1/httpclient/apidocs/org/apache/http/client/params/AllClientPNames.html

To avoid verbosity, a function map->params is provided. This will rename the keys in your input to the appropriate constants.

For example, to issue a HEAD request via a proxy:

(http/head "http://github.com/"
  :parameters (http/map->params
                {:default-proxy (http/http-host :host "localhost" :port 8080)}))

Optimization

If you're planning intensive use of HTTP operations, you might want more control over the connection manager (the component which maintains a pool of connections).

clj-apache-http exposes one utility, thread-safe-connection-manager. It can be called with no arguments, or with a SchemeRegistry.

Simply hold on to one of these, then pass it as the value of :connection-manager, as in this example:

(let [ccm (http/thread-safe-connection-manager)]
  (println "Result:"
    (:code (http/get "http://github.com" :as :string
                     :connection-manager ccm)))
  (http/shutdown-connection-manager ccm))

Don't forget to call shutdown-connection-manager afterwards. The with-connection-manager macro does this for you, if your uses will all be within the same lexical scope:

(http/with-connection-manager [ccm :thread-safe]
  (println "Result:"
    (:code (http/get "http://github.com" :as :string
                     :connection-manager ccm))))