path: root/compojure.html.markdown

---
category: tool
tool: compojure
contributors:
    - ["Adam Bard", "http://adambard.com/"]
filename: learncompojure.clj
---

## Getting Started with Compojure

Compojure is a DSL for *quickly* creating *performant* web applications
in Clojure with minimal effort:

```clojure
(ns myapp.core
  (:require [compojure.core :refer :all]
            [org.httpkit.server :refer [run-server]])) ; httpkit is a server

(defroutes myapp
  (GET "/" [] "Hello World"))

(defn -main []
  (run-server myapp {:port 5000}))
```

**Step 1:** Create a project with [Leiningen](http://leiningen.org/):

```
lein new myapp
```

**Step 2:** Put the above code in `src/myapp/core.clj`

**Step 3:** Add some dependencies to `project.clj`:

```
[compojure "1.1.8"]
[http-kit "2.1.16"]
```

**Step 4:** Run:

```
lein run -m myapp.core
```

View at: <http://localhost:5000/>

Compojure apps will run on any ring-compatible server, but we recommend
[http-kit](http://http-kit.org/) for its performance and
[massive concurrency](http://http-kit.org/600k-concurrent-connection-http-kit.html).

### Routes

In compojure, each route is an HTTP method paired with a URL-matching pattern,
an argument list, and a body.

```clojure
(defroutes myapp
  (GET "/" [] "Show something")
  (POST "/" [] "Create something")
  (PUT "/" [] "Replace something")
  (PATCH "/" [] "Modify Something")
  (DELETE "/" [] "Annihilate something")
  (OPTIONS "/" [] "Appease something")
  (HEAD "/" [] "Preview something"))
```

Compojure route definitions are just functions which
[accept request maps and return response maps](https://github.com/mmcgrana/ring/blob/master/SPEC):

```clojure
(myapp {:uri "/" :request-method :post})
; => {:status 200
;     :headers {"Content-Type" "text/html; charset=utf-8}
;     :body "Create Something"}
```

The body may be a function, which must accept the request as a parameter:

```clojure
(defroutes myapp
  (GET "/" [] (fn [req] "Do something with req")))
```

Or, you can just use the request directly:

```clojure
(defroutes myapp
  (GET "/" req "Do something with req"))
```

Route patterns may include named parameters:

```clojure
(defroutes myapp
  (GET "/hello/:name" [name] (str "Hello " name)))
```

You can adjust what each parameter matches by supplying a regex:

```clojure
(defroutes myapp
  (GET ["/file/:name.:ext" :name #".*", :ext #".*"] [name ext]
    (str "File: " name ext)))
```

### Middleware

Clojure uses [Ring](https://github.com/ring-clojure/ring) for routing.
Handlers are just functions that accept a request map and return a
response map (Compojure will turn strings into 200 responses for you).

You can easily write middleware that wraps all or part of your
application to modify requests or responses:

```clojure
(defroutes myapp
  (GET "/" req (str "Hello World v" (:app-version req))))

(defn wrap-version [handler]
  (fn [request]
    (handler (assoc request :app-version "1.0.1"))))

(defn -main []
  (run-server (wrap-version myapp) {:port 5000}))
```

[Ring-Defaults](https://github.com/ring-clojure/ring-defaults) provides some handy
middlewares for sites and apis, so add it to your dependencies:

```
[ring/ring-defaults "0.1.1"]
```

Then, you can import it in your ns:

```
(ns myapp.core
  (:require [compojure.core :refer :all]
            [ring.middleware.defaults :refer :all]
            [org.httpkit.server :refer [run-server]]))
```

And use `wrap-defaults` to add the `site-defaults` middleware to your
app:

```
(defn -main []
  (run-server (wrap-defaults myapp site-defaults) {:port 5000}))
```

Now, your handlers may utilize query parameters:

```clojure
(defroutes myapp
  (GET "/posts" req
    (let [title (get (:params req) "title")
          author (get (:params req) "author")]
      (str "Title: " title ", Author: " author))))
```

Or, for POST and PUT requests, form parameters as well

```clojure
(defroutes myapp
  (POST "/posts" req
    (let [title (get (:params req) "title")
          author (get (:params req) "author")]
      (str "Title: " title ", Author: " author))))
```


### Return values

The return value of a route block determines at least the response body
passed on to the HTTP client, or at least the next middleware in the
ring stack. Most commonly, this is a string, as in the above examples.
But, you may also return a [response map](https://github.com/mmcgrana/ring/blob/master/SPEC):

```clojure
(defroutes myapp
  (GET "/" []
    {:status 200 :body "Hello World"})
  (GET "/is-403" []
    {:status 403 :body ""})
  (GET "/is-json" []
    {:status 200 :headers {"Content-Type" "application/json"} :body "{}"}))
```

### Static Files

To serve up static files, use `compojure.route.resources`.
Resources will be served from your project's `resources/` folder.

```clojure
(require '[compojure.route :as route])

(defroutes myapp
  (GET "/")
  (route/resources "/")) ; Serve static resources at the root path

(myapp {:uri "/js/script.js" :request-method :get})
; => Contents of resources/public/js/script.js
```

### Views / Templates

To use templating with Compojure, you'll need a template library. Here are a few:

#### [Stencil](https://github.com/davidsantiago/stencil)

[Stencil](https://github.com/davidsantiago/stencil) is a [Mustache](http://mustache.github.com/) template library:

```clojure
(require '[stencil.core :refer [render-string]])

(defroutes myapp
  (GET "/hello/:name" [name]
    (render-string "Hello {{name}}" {:name name})))
```

You can easily read in templates from your resources directory. Here's a helper function

```clojure
(require 'clojure.java.io)

(defn read-template [filename]
  (slurp (clojure.java.io/resource filename)))

(defroutes myapp
  (GET "/hello/:name" [name]
    (render-string (read-template "templates/hello.html") {:name name})))
```

#### [Selmer](https://github.com/yogthos/Selmer)

[Selmer](https://github.com/yogthos/Selmer) is a Django and Jinja2-inspired templating language:

```clojure
(require '[selmer.parser :refer [render-file]])

(defroutes myapp
  (GET "/hello/:name" [name]
    (render-file "templates/hello.html" {:name name})))
```

#### [Hiccup](https://github.com/weavejester/hiccup)

[Hiccup](https://github.com/weavejester/hiccup) is a library for representing HTML as Clojure code

```clojure
(require '[hiccup.core :as hiccup])

(defroutes myapp
  (GET "/hello/:name" [name]
    (hiccup/html
      [:html
        [:body
          [:h1 {:class "title"}
            (str "Hello " name)]]])))
```

#### [Markdown](https://github.com/yogthos/markdown-clj)

[Markdown-clj](https://github.com/yogthos/markdown-clj) is a Markdown implementation.

```clojure
(require '[markdown.core :refer [md-to-html-string]])

(defroutes myapp
  (GET "/hello/:name" [name]
    (md-to-html-string "## Hello, world")))
```

Further reading:

* [Official Compojure Documentation](https://github.com/weavejester/compojure/wiki)

* [Clojure for the Brave and True](http://www.braveclojure.com/)