Clojure Q&A - Recent questions tagged json

An undocumented breaking change in data.json

Thu, 23 Oct 2025 17:18:23 +0000

between the versions 1.1.0 and 2.0.0, data.json introduced a breaking change: The JSONWriter#-write signature changed from [object out] to [object out options]
On this commit:
https://github.com/clojure/data.json/commit/8f070ba94dc7c469f303259ce5f5bfa0b53d50bd
In release notes Release 2.0.0 on 2021-Mar-19 , there is no mention to JSONWriter or -write protocol changes.

I know it's a very old problem, but I think this documentation could be present.

Separate text from JSON to process JSON

Fri, 18 Oct 2024 14:10:03 +0000

Hi,

I'm starting with Clojure, better yet, re-starting after trying it several years ago.
There are some new tools like Babashka that I'm trying to use for this.

The challenge is to do the following:

Start ansible virtual environment
Run an external process with 'ansible-inventory' against one host
at a time, or all hosts, let's see...
The interesting part of the output of 'ansible-inventory' goes to
stderr, so read it.
It consists of lines of text and then some JSON.
Parse the JSON contents.

It will be more elaborate than this, but it's enough for the case.

Let's forget about 1.

Instead of 2., just do a simple 'cat a.txt' file built with some text and some JSON like

line1
line 2
{
  'field': 'value'
}

What I've been able to build was this:

#!/usr/bin/env bb

(require '[babashka.process :refer [shell]])
(require '[cheshire.core :as json])
(require '[clojure.string :as str])

(-> (shell {:out :string} "cat" "a.txt")
    :out json/parse-string)

What I'do in an imperative language, that I use to use would be:

Read line by line. In Clojure (str/split-lines)
Discard any line until a line consisiting only of a '{' is found.
From there, accumulate all lines in a string until a '}' is found.
Parse the JSON from that string.

How can I approach this in Clojure?

Accidental breaking change in data.json 2.5.0: previously, read succeeded, now it isn't

Wed, 25 Sep 2024 15:46:20 +0000

Here is a snippet of code that fails on data.json 2.5.0:

clj -Sdeps '{:deps {org.clojure/data.json {:mvn/version "2.5.0"}}}' -M -e '((requiring-resolve (quote clojure.data.json/read)) (clojure.lang.LineNumberingPushbackReader. (java.io.StringReader. "[\n  {\n    \"name\": \"position\",\n    \"type\": \"float32\",\n    \"count\": 3,\n    \"data\": [0,0,0]\n  },\n  {\n    \"name\": \"normal\",\n    \"type\": \"float32\",\n    \"count\": 3,\n    \"data\": [0,0,0]\n  },\n  {\n    \"name\": \"texcoord0\",\n    \"type\": \"float32\",\n    \"count\": 2,\n    \"data\": [0,0]\n  }\n]")))'

Execution error (IOException) at java.io.PushbackReader/unread (PushbackReader.java:166).
Pushback buffer overflow

The same code succeeds on 2.4.0:

clj -Sdeps '{:deps {org.clojure/data.json {:mvn/version "2.4.0"}}}' -M -e '((requiring-resolve (quote clojure.data.json/read)) (clojure.lang.LineNumberingPushbackReader. (java.io.StringReader. "[\n  {\n    \"name\": \"position\",\n    \"type\": \"float32\",\n    \"count\": 3,\n    \"data\": [0,0,0]\n  },\n  {\n    \"name\": \"normal\",\n    \"type\": \"float32\",\n    \"count\": 3,\n    \"data\": [0,0,0]\n  },\n  {\n    \"name\": \"texcoord0\",\n    \"type\": \"float32\",\n    \"count\": 2,\n    \"data\": [0,0]\n  }\n]")))'
[{"name" "position", "type" "float32", "count" 3, "data" [0 0 0]} {"name" "normal", "type" "float32", "count" 3, "data" [0 0 0]} {"name" "texcoord0", "type" "float32", "count" 2, "data" [0 0]}]

The gist of the problem: previously, Clojure's line numbering pushback reader could be used as an input to read. Now it can't: it throws an exception.

For reference, this is a json repro (a valid json!):

[
  {
    "name": "position",
    "type": "float32",
    "count": 3,
    "data": [0,0,0]
  },
  {
    "name": "normal",
    "type": "float32",
    "count": 3,
    "data": [0,0,0]
  },
  {
    "name": "texcoord0",
    "type": "float32",
    "count": 2,
    "data": [0,0]
  }
]

And, the formatted code: that uses the json:

(clojure.data.json/read 
  (clojure.lang.LineNumberingPushbackReader.
    (java.io.StringReader. "...")))

It seems the problem is introduced in https://clojure.atlassian.net/browse/DJSON-50

Should clojure.data.json protect against duplicate keys when writing a combination of keyword and string keys?

Tue, 02 Jan 2024 22:16:45 +0000

When running (json/write-str {:a 1 "a" 2}) it outputs ;; => "{\"a\":1,\"a\":2}". Should there be a validation to assert no duplicate keys exist? Or is it on the user to: 1. Not to mix keyword keys and string keys, and 2. Validate if there are no duplicate keys?

REST API response body is not a string?

Sat, 20 Feb 2021 08:42:46 +0000

It's been a while since I've written Clojure, so I am rusty.

Basically, I'm trying to write code which does an HTTP GET on a REST API and converts that JSON array of objects into clojure data (list of Maps?). The JSON data looks like this:

[
    {
        "id": 1,
        "category": "technical",
        "asin": "0131103628",
        "title": "C Programming Language, 2nd Edition",
        "publication_date": "1988-04-01T00:00:00Z",
        "review": "The C book.",
        "created_at": "2020-03-06T20:48:00Z"
    },
    {
        "id": 2,
        "category": "technical",
        "asin": "0387966080",
        "title": "The Science of Fractal Images",
        "publication_date": "1988-07-19T00:00:00Z",
        "review": "The book on fractal algorithms.  Sadly, no longer in print.  I had to search high and low for a used copied.",
        "created_at": "2020-03-06T20:48:00Z"
    },
    // etc..
]

I'm using http-kit (I've also tried clj-http) to call the REST API and return the body. Later, I try converting the body to Clojure data. I've tried using both org.clojure/data.json and cheshire. However, I don't belive the data is being converted correctly.

(defn get-books-body
  []
  (:body @(client/get books-url api-options)))

(defn get-books
  []
  (parse-string get-books-body true))

When I try to display the type of the body, it shows up as an address instead of a string:

clojure-json-test.core=> (type get-books-body)
clojure_json_test.core$get_books_body

And I cannot look at the first record, after converting from JSON:

clojure-json-test.core=> (first get-books)
Execution error (IllegalArgumentException) at clojure-json-test.core/eval3169 (form-      init15378865171954877190.clj:1).
Don't know how to create ISeq from: clojure_json_test.core$get_books

Any idea what I am doing wrong?

json writer and namespaces

Thu, 05 Nov 2020 11:50:53 +0000

Hello

I'm new to Clojure and didn't find answer so please sorry if this is not issue at all.

lets say we have data:

{:space1/value 123 :space2/value 456}

and use clojure.data.json as serialiser:

(clojure.data.json/write-str {:space1/value 123 :space2/value 456})
=> "{\"value\":123,\"value\":456}"

As you see result is not what one can expect.
Obvious solution would be to provide own key-fn and forget about it. But I spend a lot of time catching bug caused by default implementation and like to save time of other

Reading a JSON stream over HTTP, indefinitely?

Sat, 29 Feb 2020 07:59:50 +0000

I am trying to read a json HTTP stream and use the json elements, indefinitely.

I came up with this code below, but it does close the stream and only returns 30 results - how do I indefinitely use the HTTP stream?

Thanks for your help!

(ns core
  (:require [clj-http.client :as http]
            [cheshire.core :as json]
            [clojure.java.io :as io]
            [clojure.core.async :as async]))

(def gh-url "https://api.github.com/events")
(def chan (async/chan 100))

(async/go
  (loop [r (async/<! chan)]
    (when (not-empty r) (println (:type r)))
    (recur (async/<! chan))))

(defn read-gh-stream [url]
  (with-open [stream (-> url (http/get {:as :stream}) :body)]
    (let [lines (-> stream io/reader (json/parse-stream true))]
      (doseq [l lines]
        (async/go
          (async/>! chan l))))))

Any good tutorials on clojure-walk ?

Sun, 23 Feb 2020 21:06:03 +0000

I want to develop a tool that ingests json into native clojure data-structure and then provides a jq /json-pointer like select/map/filter api or capabilities. I believe clojure-walk might solve this.

Best way to write a unix(y) tool with Clojure ?

Mon, 10 Feb 2020 00:08:35 +0000

I was looking for a way to use Clojure data/json to convert back and forth from json but I cannot find any easy way to take unescaped json from stdin and output lisp like expression tree to stdout or vice-versa.

P.S: 'Hello World' is the 1st program people teach, I think 'Echo back' should be the second one.
Or something like
> This is Houston. Say again, please.
> Houston, we've had a problem.

Is the standard JSON parser broken ?

Mon, 18 Nov 2019 16:41:29 +0000

It seems that the "standard" json parser is happily parsing invalid json as shown below
user=> (require '[clojure.data.json :as j]) nil user=> (j/read-str "{\"a\": 123,,}") {"a" 123} user=> (j/read-str "{\"a\": 123,\"b\",}") {"a" 123}
I know other libraries exist (eg jsonista, cheshire) but it doesn't look good that what seems to be the "standard" json parser (as per the link on http://json.org/ and the fact that it lives inside the main clojure github org) is broken.

(Logged as https://clojure.atlassian.net/browse/DJSON-29)

Why does Transit not allow serialization of clojure(script) functions ? What is the inherent difficulty there ?

Mon, 29 Jul 2019 23:10:55 +0000

I am trying to serialize my datascript database to JSON for storage on a filesystem.
Some of the data inside my database includes functions. For reasons that are not interesting, I need to store them in the database for now with other session info for the application. But I get an exception that transit "cannot write Function"

I looked at the code and it simply tries to handle data structures only - maps, vectors, etc. But there is no handler for functions. So I KNOW WHY I get the error, but the gist of my question is why it was not implemented to be able to serialize functions as well ?

Is it just too novel an thought ?

thanks