JSON in Clojure
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JSON parser/generator to/from Clojure data structures.

Follows the specification on http://json.org/

Releases and Dependency Information

Latest stable release is 0.2.0

Leiningen dependency information:

[org.clojure/data.json "0.2.0"]

Maven dependency information:


Other versions:


API Documentation


(require '[clojure.data.json :as json])

To convert to/from JSON strings, use json/write-str and json/read-str:

(json/write-str {:a 1 :b 2})
;;=> "{\"a\":1,\"b\":2}"

(json/read-str "{\"a\":1,\"b\":2}")
;;=> {"a" 1, "b" 2}

Note that these operations are not symmetric: converting Clojure data into JSON is lossy.

You can specify a :key-fn to convert map keys on the way in or out:

(json/read-str "{\"a\":1,\"b\":2}"
               :key-fn keyword)
;;=> {:a 1, :b 2}

(json/write-str {:a 1 :b 2}
                :key-fn #(.toUpperCase %))
;;=> "{\"A\":1,\"B\":2}"

(json/read-str "{\"a\":1,\"b\":2}"
               :key-fn #(keyword "com.example" %))
;;=> {:com.example/a 1, :com.example/b 2}

You can specify a :value-fn to convert map values on the way in or out. The value-fn will be called with two arguments, the key and the value, and it returns the updated value.

(defn my-value-reader [key value]
  (if (= key :date)
    (java.sql.Date/valueOf value)

(json/read-str "{\"number\":42,\"date\":\"2012-06-02\"}"
               :value-fn my-value-reader
               :key-fn keyword) 
;;=> {:number 42, :date #inst "2012-06-02T04:00:00.000-00:00"}

Note that if you specify both a :key-fn and a :value-fn when reading, the value-fn is called after the key has been processed by the key-fn. The reverse is true when writing:

(defn my-value-writer [key value]
  (if (= key :date)
    (str (java.sql.Date. (.getTime value)))

(json/write-str {:number 42, :date (java.util.Date. 112 5 2)}
                :value-fn my-value-writer
                :key-fn name) 
;;=> "{\"number\":42,\"date\":\"2012-06-02\"}"

You can also read JSON directly from a java.io.Reader with json/read and write JSON directly to a java.io.Writer with json/write.

Other options are available. Refer to the API Documentation for details.

Developer Information

Change Log

  • Release 0.2.0 on 2012-Oct-12
    • Breaking API changes: renamed core functions
    • New :key-fn and :value-fn permit flexible transformation of values when reading & writing JSON
    • Support for reading large integers as BigInt
    • Optional support for reading decimals as BigDecimal
    • Performance improvements
  • Release 0.1.3 on 2012-Mar-09
    • Fix writing strings containing characters outside the BMP
  • Release 0.1.2 on 2011-Oct-14
    • Better parsing of hexadecimal character escapes
    • Fix EOF-handling bug
    • Fix reflection warnings DJSON-1
  • Release 0.1.1 on 2011-Jul-01
    • Ensure that printing to *out* always uses a PrintWriter.
  • Release 0.1.0 on 2011-Mar-18
    • Initial release.
    • Source-compatible with clojure.contrib.json, except for the name change.

Copyright and License

Copyright (c) Stuart Sierra, 2012. All rights reserved. The use and distribution terms for this software are covered by the Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php) which can be found in the file epl-v10.html at the root of this distribution. By using this software in any fashion, you are agreeing to be bound by the terms of this license. You must not remove this notice, or any other, from this software.