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Clojure library for fast http format negotiation, encoding and decoding. Standalone library, but ships with adapters for ring (async) middleware & Pedestal-style interceptors. Explicit & extendable, supporting out-of-the-box JSON, EDN and Transit (both JSON & Msgpack). Ships with optional adapters for MessagePack and YAML.

Based on ring-middleware-format, but a complete rewrite (and up to 30x faster).




Check the docs on for detailed API documentation as well as more guides on how to use Muuntaja.

Latest version

[metosin/muuntaja "0.6.4"]

Optionally, the parts can be required separately:

[metosin/muuntaja-cheshire "0.6.4"]
[metosin/muuntaja-msgpack "0.6.4"]
[metosin/muuntaja-yaml "0.6.4"]

Muuntaja requires Java 1.8+



Use default Muuntaja instance to encode & decode JSON:

(require '[muuntaja.core :as m])

(->> {:kikka 42}
     (m/encode "application/json"))
; => #object[]

(->> {:kikka 42}
     (m/encode "application/json")
; => "{\"kikka\":42}"

(->> {:kikka 42}
     (m/encode "application/json")
     (m/decode "application/json"))
; => {:kikka 42}


Automatic decoding of request body and response body encoding based on Content-Type, Accept and Accept-Charset headers:

(require '[muuntaja.middleware :as middleware])

(defn echo [request]
  {:status 200
   :body (:body-params request)})

; with defaults
(def app (middleware/wrap-format echo))

(def request
   {"content-type" "application/edn"
    "accept" "application/transit+json"}
   :body "{:kikka 42}"})
(app request)
; {:status 200,
;  :body #object[]
;  :headers {"Content-Type" "application/transit+json; charset=utf-8"}}

Automatic decoding of response body based on Content-Type header:

(-> request app m/decode-response-body)
; {:kikka 42}    

There is a more detailed Ring guide too. See also differences to ring-middleware-format & ring-json.


Muuntaja support Sieppari -style interceptors too. See muuntaja.interceptor for details.

Interceptors can be used with Pedestal too, all but the exception-interceptor which conforms to the simplified exception handling model of Sieppari.


Explicit Muuntaja instance with custom EDN decoder options:

(def m
      [:formats "application/edn" :decoder-opts]
      {:readers {'INC inc}})))

(->> "{:value #INC 41}"
     (m/decode m "application/edn"))
; => {:value 42}

Returning a function to encode transit-json:

(def encode-transit-json
  (m/encoder m "application/transit+json"))

(slurp (encode-transit-json {:kikka 42}))
; => "[\"^ \",\"~:kikka\",42]"

Encoding format

By default, encode writes value into a This can be changed with a :return option, accepting the following values:

| value | description |——————|———————————————————————————- | :input-stream | encodes into (default) | :bytes | encodes into byte[]. Faster than Stream, enables NIO for servers supporting it | :output-stream | encodes lazily into via a callback function

All return types satisfy the following Protocols & Interfaces:


(def m (m/create (assoc m/default-options :return :input-stream)))

(->> {:kikka 42}
     (m/encode m "application/json"))
; #object[]


(def m (m/create (assoc m/default-options :return :bytes)))

(->> {:kikka 42}
     (m/encode m "application/json"))
; #object["[B" 0x31f5d734 "[B@31f5d734"]


(def m (m/create (assoc m/default-options :return :output-stream)))

(->> {:kikka 42}
     (m/encode m "application/json"))
; <<StreamableResponse>>

Format-based return

(def m (m/create (assoc-in m/default-options [:formats "application/edn" :return] :output-stream)))

(->> {:kikka 42}
     (m/encode m "application/json"))
; #object[]

(->> {:kikka 42}
     (m/encode m "application/edn"))
; <<StreamableResponse>>

HTTP format negotiation

HTTP format negotiation is done using request headers for both request (content-type, including the charset) and response (accept and accept-charset). With the default options, a full match on the content-type is required, e.g. application/json. Adding a :matches regexp for formats enables more loose matching. See Configuration docs for more info.

Results of the negotiation are published into request & response under namespaced keys for introspection. These keys can also be set manually, overriding the content negotiation process.


When something bad happens, an typed exception is thrown. You should handle it elsewhere. Thrown exceptions have an ex-data with the following :type value (plus extra info to enable generating descriptive erros to clients):

Server Spec




Default options

{:http {:extract-content-type extract-content-type-ring
        :extract-accept-charset extract-accept-charset-ring
        :extract-accept extract-accept-ring
        :decode-request-body? (constantly true)
        :encode-response-body? encode-collections}

 :allow-empty-input? true
 :return :input-stream

 :default-charset "utf-8"
 :charsets available-charsets

 :default-format "application/json"
 :formats {"application/json" json-format/json-format
           "application/edn" edn-format/edn-format
           "application/transit+json" transit-format/transit-json-format
           "application/transit+msgpack" transit-format/transit-msgpack-format}}


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By Unknown. The drawing is signed “E. Ducretet”, indicating that the apparatus was made by Eugene Ducretet, a prominent Paris scientific instrument manufacturer and radio researcher. The drawing was undoubtedly originally from the Ducretet instrument catalog. [Public domain], via Wikimedia Commons.

Original Code (ring-middleware-format)

Copyright © 2011, 2012, 2013, 2014 Nils Grunwald
Copyright © 2015, 2016 Juho Teperi

This library

Copyright © 2016-2019 Metosin Oy

Distributed under the Eclipse Public License 2.0.