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JSON parser/generator to/from Clojure data structures.

Key goals:

Releases and Dependency Information

This project follows the version scheme MAJOR.MINOR.PATCH where each component provides some relative indication of the size of the change, but does not follow semantic versioning. In general, all changes endeavor to be non-breaking (by moving to new names rather than by breaking existing names).

Latest stable release is 2.4.0

CLI/deps.edn dependency information:

org.clojure/data.json {:mvn/version "2.4.0"}

Leiningen dependency information:

[org.clojure/data.json "2.4.0"]

Maven dependency information:


Other versions:


API Documentation

Example usage:

(ns example
  (:require [ :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.

Converting Key/Value Types

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"}

Be aware that :value-fn only works on maps (JSON objects). If your root data structure is, for example, a vector of dates, you will need to pre- or post-process it outside of data.json. clojure.walk may be useful for this.

Order of key-fn / value-fn

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\"}"

Reading/Writing a Stream

You can also read JSON directly from a with json/read and write JSON directly to a with json/write.


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

Developer Information

Change Log

  • Release 2.4.0 on 2021-Jul-12
    • Fix DJSON-52: Remove Classloader workaround to support Clojure 1.2.x and below
    • Fix DJSON-53: Move deprecated API functions from compat ns into main ns
  • Release 2.3.1 on 2021-May-19
    • Fix DJSON-51: Fix possible read of x00's in quoted strings on partial stream read
  • Release 2.3.0 on 2021-May-14
    • Fix DJSON-48: Make array parsing match spec
    • Fix DJSON-18: Make pprint-json much faster
  • Release 2.2.3 on 2021-May-6
    • Fix DJSON-47: Make number parsing match spec (reject invalid numbers)
  • Release 2.2.2 on 2021-Apr-26
    • Perf DJSON-36: Reapplied updated refactored code in read-array and read-object
    • Add DJSON-45: Generative tests for read/write roundtrip
  • Release 2.2.1 on 2021-Apr-19
    • Revert DJSON-36: Problem with transient batching in that change
  • Release 2.2.0 on 2021-Apr-16
    • Add DJSON-41: New support for writing java.time.Instant and java.util.Date (+ subclasses)
      • New options to write functions:
        • :date-formatter - java.time.DateTimeFormatter to use (default DateTimeFormatter/ISO_INSTANT)
        • :sql-date-converter - fn to convert java.sql.Date to java.time.Instant (default provided)
    • Perf DJSON-36: Refactor code in read-array and read-object
  • Release 2.1.1 on 2021-Apr-15
    • Fix DJSON-43: Fix buffer overflow in pushbackreader
    • Update parent pom to latest (1.1.0)
  • Release 2.1.0 on 2021-Apr-6
    • Fix DJSON-39: Support writing UUIDs (as strings)
  • Release 2.0.2 on 2021-Mar-27
    • Fix DJSON-38: Type-hint return type of write-str
    • Fix DJSON-40: Make named argument passing compatible with new
  • Release 2.0.1 on 2021-Mar-19
    • Fix DJSON-37: Fix off-by-one error reading long strings, regression in 2.0.0
  • Release 2.0.0 on 2021-Mar-19
    • Perf DJSON-35: Replace PrintWriter with more generic Appendable, reduce wrapping
    • Perf DJSON-34: More efficient writing for common path
    • Perf DJSON-32: Use option map instead of dynamic variables (affects read+write)
    • Perf DJSON-33: Improve speed of reading JSON strings
    • Fix DJSON-30: Fix bad test
  • Release 1.1.0 on 2021-Mar-5
    • Fix DJSON-26: write-object should check seq on loop var, not param
    • Use latest parent pom (will bump default clojure dep to 1.8.0)
    • Use direct linking on "aot" classifier lib
  • Release 1.0.0 on 2020-Feb-18
  • Release 0.2.7 on 2019-Nov-18
    • Fix DJSON-29: throw exception on missing object entries (extra commas)
  • Release 0.2.6 on 2015-Mar-6
    • Modify build to produce an AOT package with classifier "aot"
  • Release 0.2.5 on 2014-Jun-13
    • Fix DJSON-17: throw exception on Infinite or NaN floating-point values. Old behavior could produce invalid JSON.
  • Release 0.2.4 on 2014-Jan-10
    • Small change in behavior: now adds a newline after its output just like clojure.core/pprint
    • Fix DJSON-13: flush output after pprint
    • Fix DJSON-14: handle EOF inside character escape
    • Fix DJSON-15: bad syntax in test
  • Release 0.2.3 on 2013-Aug-30
    • Enhancement DJSON-9: option to escape U+2028 and U+2029
    • Fix DJSON-11: printing unnecessary commas with value-fn
  • Release 0.2.2 on 2013-Apr-07
    • Fix DJSON-7: extra commas when removing key/value pairs)
    • Fix DJSON-8: wrong output stream in write-json
  • Release 0.2.1 on 2012-Oct-26
    • Restores backwards-compatibility with 0.1.x releases. The older 0.1.x APIs are marked as deprecated in their documentation. They will be removed in a future release.
  • Release 0.2.0 on 2012-Oct-12
    • Not recommended for use: this release introduced breaking API changes (renaming core functions) without any path for backwards-compatibility. Applications with transitive dependencies on both the 0.2.x and 0.1.x APIs cannot use this version.
    • 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 DJSON-1: reflection warnings
  • 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, Rich Hickey, and contriburos 2012-2020. All rights reserved. The use and distribution terms for this software are covered by the Eclipse Public License 1.0 ( 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.