Blogged by Ujihisa. Standard methods of programming and thoughts including Clojure, Vim, LLVM, Haskell, Ruby and Mathematics written by a Japanese programmer. github/ujihisa

Sunday, December 18, 2011

Read Raw POST Request Body in Compojure

Brief introduction to Compojure

Compojure gives you a shortcut to make a monofunctional web application. For example a Lingr bot is a web application that only need to be responsible with one single endpoint that handles POST request. The below is a web application that only shows "hello" in / endpoint with GET request.

(defroutes hello
           (GET "/" [] "hello"))
(run-jetty hello {:port 80})

Note that it requires you to be the root of the system if you are going to run a web app on port 80.

The main part of the app is just 3 lines of code. That reminds me of code examples for Sinatra, a Ruby web library.

get '/' do
  'hello'
end

Anyways the Compojure example code doesn't work only with the main logic. You are supposed to make a Leiningen project usually to manage the app and its dependent libraries.

$ lein new hello
$ cd hello

project.clj

(defproject hello "1.0.0-SNAPSHOT"
            :main hello.core
            :description "FIXME: write description"
            :dependencies [[org.clojure/clojure "1.2.1"]
                           [compojure "1.0.0-RC2"]
                           [ring "1.0.1"]])

src/hello/core.clj

(ns hello.core
  (:use
    compojure.core
    ring.adapter.jetty))

(defroutes hello
           (GET "/" [] "hello"))
(run-jetty hello {:port 80})

then

$ lein deps
$ lein run

it should work.

Parameters

(defroutes hello
           (GET "/" [] "hello"))
(run-jetty hello {:port 80})

The 2nd argument of GET, [] in this case, is parameter list for the expression you give in 3rd argument, which mostly for referring GET parameters. That's actually a hashmap that contains :params key which value is also a hashmap of GET parameters. Ditto in POST.

How can we get the raw post parameter?

(POST "/" {params :params} (...))

In that way you cannot get raw data because it's after the process. You can reconstruct the raw data only when the given parameter is like proper a=1\nb=2 form. These days some web apis are required to handle raw POST data, which is mostly in JSON, like a Lingr Bot API.

The answer is in :body of the parameter, but it's not a String but a mysterious HttpParser.Input object, assuming you are using ring as the middleware.

http://jetty.codehaus.org/jetty/jetty-6/apidocs/org/mortbay/jetty/HttpParser.Input.html

This class looks weird because even though this has read() method the return value type isn't String but int. The other read() looks like you are supposed to pass a mutable data and refer the changed data.

Fortunately we can use slurp Clojure function to hide this complicated behaviour.

(defroutes hello
           (POST "/" {body :body} (slurp body)))

This shows the given raw POST parameter!

5 comments:

  1. Great, thanks for posting!

    ReplyDelete
  2. In the defroutes above, where you write "{body :body}", is body an instance of HttpParser.Input?

    ReplyDelete
  3. yes

    It was org.mortbay.jetty.HttpParser$Input

    ReplyDelete
  4. Thanks for posting, this saved me a lot of time, thanks!

    ReplyDelete

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