File uploads in a Clojure web application using Compojure

If you happen to build a Clojure web application, you will most probably use Compojure somewhere along the stack. In this article I will show how easy it is to implement a file upload endpoint using that library.

Let’s start by generating a basic Compojure application from a template

lein new compojure fupload

Next, we need to add POST route inside src/fupload/handler.clj

(POST "/upload"
    {{{tempfile :tempfile filename :filename} :file} :params :as params}
    (io/copy tempfile (io/file "resources" "public" filename))
    "Success")

A couple of things happen here, first we specify that a file should be passed using a field named file. Once the request is issued, a ring handler transforms its parameters to the following map:

{ :size 1234,
  :tempfile #<File /var/folders/hq/az8xf5d9zv7g4dvhli9nmr_c1000gn/T/ring-multipart-4366831661746413539.tmp>,
  :content-type application/text,
  :filename project.clj}

In route definition we immediately destruct that map by specifying we are only interested in fields tempfile and filename.

Finally, we copy uploaded file to resources/public on the server using clojure.java.io and we return “Success”. You have to add the following line in src/fupload/handler.clj under :require.

[clojure.java.io :as io]

Let’s check if it works. We could build a form with file field and proper, but instead, for simplicity, we will simulate a file upload using curl (a file can be referenced with @ sign). In the following example we will try to upload project.clj from the current directory:

curl -XPOST -F [email protected] localhost:3000/upload
Success

head -n 2 resources/public/project.clj
(defproject fupload "0.1.0-SNAPSHOT"
  :description "FIXME: write description"