ArduinoJson: Efficient JSON serialization for embedded C++

The best JSON library for

Quickstart

Serialization

JsonDocument doc;

doc["sensor"] = "gps";
doc["time"] = 1351824120;
doc["data"][0] = 48.756080;
doc["data"][1] = 2.302038;

serializeJson(doc, Serial);
// This prints:
// {"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}

See Serialization example and Serialization tutorial.

Deserialization

char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";

JsonDocument doc;
deserializeJson(doc, json);

const char* sensor = doc["sensor"];
long time          = doc["time"];
double latitude    = doc["data"][0];
double longitude   = doc["data"][1];

See Deserialization example and Deserialization tutorial.

Features

Intuitive Syntax

ArduinoJson has a simple and intuitive syntax to handle objects and arrays.

Fetch data:

const char* city = doc["city"];
double temp = doc["weather"]["temp"];

Set data:

doc["city"] = "Paris";
doc["weather"]["temp"] = 21.2;

Serialize and Deserialize

ArduinoJson supports both JSON serialization and deserialization.

Read JSON:

deserializeJson(doc, myInput);

…modify:

doc["last_modification"] = time();

…write back:

serializeJson(doc, myOutput);

Input filter

ArduinoJson can filter large inputs to keep only the fields that are relevant to your application, thereby saving a lot of memory.

JsonDocument filter;
filter["list"][0]["temperature"] = true;

deserializeJson(doc, input, DeserializationOption::Filter(filter));

Parse From Stream

ArduinoJson can parse directly from an input Stream or std::istream

Parse input from the serial port:

deserializeJson(doc, Serial);

…an Ethernet connection

deserializeJson(doc, ethernetClient);

…or a Wifi connection

deserializeJson(doc, wifiClient);

It also supports custom reader types.

Print to Stream

ArduinoJson is able to print directly to a Print or std::ostream

Send the JSON to the serial port:

serializeJson(doc, Serial);

…an Ethernet connection

serializeJson(doc, ethernetClient);

…or a Wifi connection

serializeJson(doc, wifiClient);

It also supports custom writer types.

Indented output

ArduinoJson can produce compact:

{"ports":[80,443]}

…or prettified documents:

{
  "ports": [
    80,
    443
  ]
}

Flash Strings

ArduinoJson works directly with strings stored in program memory (PROGMEM)

You can use a Flash String as JSON input

deserializeJson(doc, F("{\"city\":\"Paris\",\"temp\":18.5}"));

…as a value

doc["cond"] = F("Sunny");

…or as a key

float temp = doc[F("temp")];

String deduplication

ArduinoJson deduplicates strings in the JSON document.

When you have several identical keys or values, the JsonDocument only stores one of each.

In practice, this feature reduces memory consumption by 20-30%.

See this article for details.

Implicit or Explicit Casts

ArduinoJson supports two coding styles, with implicit or explicit casts

Implicit cast:

const char* city = doc["city"];
float temp = doc["temperature"];

Explicit cast:

auto city = doc["city"].as<const char*>();
auto temp = doc["temperature"].as<float>();

Unicode

ArduinoJson can decode UTF-16 escape sequences to UTF-8.

deserializeJson(doc, "{\"name\":\"Beno\u00EEt");
const char* name = doc["name"]; // <- "Benoît"

Comments

ArduinoJson supports comments in the input document.

{
  /* The WiFi configuration */
  "wifi": {
    "ssid": "The Bat Cave",
    "pass": "imbatman" // <- not secure!!
  }
}

Optional feature, see ARDUINOJSON_ENABLE_COMMENTS

NDJSON, JSON Lines

When parsing a JSON document from an input stream, ArduinoJson stops reading as soon as the document ends (e.g., at the closing brace).

This feature allows reading JSON documents one after the other; for example, you can use it to read line-delimited formats like NDJSON or JSON Lines.

{"event":"add_to_cart"}
{"event":"purchase"}

Efficient

Compared to the “official” Arduino_JSON library:

ArduinoJson is twice smaller,
ArduinoJson runs almost 10% faster,
ArduinoJson consumes roughly 10% less RAM.

See in-depth comparison

MessagePack

In addition to JSON, ArduinoJson supports MessagePack, an alternative serialization format.

MessagePack is like a binary version of JSON. You can switch from one format to the other with a single function call!

Reentrant

As long as you use a different JsonDocument in each thread, there is no risk of race condition.

Overflow Safe

ArduinoJson protects you against integer overflows. Before casting a value to an integer, it checks that the value is in range. If the value doesn’t fit, it returns 0 to prevent overflows.

doc["value"].as<int8_t>();   // 0
doc["value"].as<int16_t>();  // 20000

You can also safely test the type:

doc["value"].is<int8_t>();   // false
doc["value"].is<int16_t>();  // true

Custom converters

You can extend the types supported by ArduinoJson with custom converters.

For example, it allows you to add support for the (almost-)standard tm structure:

struct tm timeinfo = *gmtime(&time);

// set {"time":"2021-05-04T13:13:04Z"}
doc["time"] = timeinfo;

// parse {"time":"2021-05-04T13:13:04Z"}
timeinfo = doc["time"];

Custom allocator

ArduinoJson supports custom allocators, in case you need something special, like using the external PSRAM on your ESP32:

struct SpiRamAllocator : ArduinoJson::Allocator {
  void* allocate(size_t size) override {
    return heap_caps_malloc(size, MALLOC_CAP_SPIRAM);
  }

  void deallocate(void* pointer) override {
    heap_caps_free(pointer);
  }

  void* reallocate(void* ptr, size_t new_size) override {
    return heap_caps_realloc(ptr, new_size, MALLOC_CAP_SPIRAM);
  }
};

Portable

Despite its name, ArduinoJson is not limited to Arduino and can be used on any C++ project. All it requires a C++11-capable compiler, a requirement met by the major compilers since 2014.

ArduinoJson is continuously tested on:

GCC 5, 6, 8, 9, 10, 11, and 12
Clang 3.9, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15
Visual Studio 2017, 2019, and 2022

Support for older compilers is available up to ArduinoJson 6.20.

Self Contained

ArduinoJson doesn’t depend on any library; in fact, it doesn’t even depend on Arduino!

It means you can use ArduinoJson anywhere you can compile C++ code.

The best example is your test suite: you can comfortably test your program on your computer without touching a microcontroller.

Zero warning

ArduinoJson compiles without any warning, even with the highest compiler settings:

-Wall -Wextra -pedantic on GCC/Clang
/W4 on MSVC

It also has zero warning on Clang-Tidy.

Well tested

Because ArduinoJson is developed with TDD, it is thoroughly tested.

With more than 3600 assertions, ArduinoJson reaches a test coverage above 99%, making it the most tested Arduino library ever!

Header-only

ArduinoJson is a header-only library, meaning that all the code is in the headers.

This dramatically simplifies the compilation as you don’t have to worry about compiling and linking the library.

A single file distribution is also available; that way, you just need to download one header.

Try online

You can try ArduinoJson online on wandbox.org:

Documentation

Of all Arduino libraries, ArduinoJson is by far the one with the best documentation. On this website, you’ll find 728 pages of documentation and 45 news articles.

The site also hosts the ArduinoJson Assistant, an online tool that helps you to write the code for your project.

There is also the ArduinoJson Troubleshooter, another online tool that helps you solve common issues by asking a series of questions (there are about 300 pages in this tool).

Continuous fuzzing

Security is considered very seriously.

That is why ArduinoJson is integrated in Google’s OSS-Fuzz program.

As such, it is continuously fuzzed in search for security vulnerabilities.

MIT License

ArduinoJson is open-source and uses one of the most permissive licenses so you can use it on any project.

✔️️ Commercial use

✔️️ Modification

✔️️ Distribution

✔️️ Private use

They use ArduinoJson

Testimonials

Your library is a life-saver. Thank you for this excellent tool!

Nick Koumaris on Instructables

Arduino makers can use ArduinoJson in their projects to connect multiple Arduino gadgets, or Arduinos to their web services. The library includes a powerful parser which can deal with nested objects (great for customising the messaging format to fit with your objectives), it is light on memory (both RAM and flash), and it has a really nice API so it’s easy to use.

Peter Dalmaris on techexplorations.com

Thanks for your great work on this awesome library, I will definitely keep using it in my projects and recommend to others :)

Nuno Santos on GitHub

I love ArduinoJson! Keep up the great work!

AJ Keller on GitHub

ArduinoJson library should be used as a benchmark/reference for making libraries. Truly elegant.

Hemal Chevli on Twitter

You’ve clearly done a tremendous amount of very good work. In fact, the existence of ArduinoJson was a key input into the decision and design process for our system’s architecture a couple years ago.

Mike Karlesky on GitHub

It has a really elegant, simple API and it works like a charm on embedded and Windows/Linux platforms. We recently started using this on an embedded project and I can vouch for its quality.

thegreendroid on StackOverflow

I’ve been watching you consistently develop this library over the past six months, and I used it today for a publish and subscribe architecture designed to help hobbyists move into more advanced robotics. Your library allowed me to implement remote subscription in order to facilitate multi-processor robots.

zacsketches on GitHub

its a great lib:) and i use it in everything!

sticilface on GitHub

This is a great library and I wouldn’t be able to do the project I’m doing without it. I completely recommend it.

erm_what_ on Reddit

I am just starting an ESP8266 clock project and now I can output JSON from my server script and interpret it painlessly.

makerhacks on Reddit

I tried aJson and json-arduino before trying your library. I always ran into memory problem after a while. I have no such problem so far with your library. It is working perfectly with my web services. Thanks Benoit for a very well polished product!

jflaplante on Arduino Forum

That I can work through use of ArduinoJson without ever touching a MCU is a huge plus! Kudos on what must have been a non-trivial task!

Jeff Kletsky on GitHub

Your library has greatly simplified my parsing and will make maintaining my code vastly easier in the future. I’ll certainly be buying your book to shorten my learning curve and begin to take full advantage of your knowledge and library.

Bon Shaw on GitHub

It’s a great library and I have found it to be very useful in my Arduino projects. Wanted to test it on Raspberry Pi as well. Included the Headers in my project and it works flawlessly. Keep up the good work!!

umar14 on GitHub

Thanks again for making such a great library!

Jason Meredith on GitHub

ArduinoJson is for me the best json library for microcontrollers! I’m a huge fan of it!

vaz83 on GitHub

No other implementation was/is so memory efficient!

Ewald Comhaire on GitHub

Very impressive library, very useful indeed. Thank you!

mikee47 on GitHub

Thanks for creating this library and supporting material - it’s incredibly well written and useful, and I appreciate it.

Richard Lyon

My son and I are working on computer game using the Unreal engine, and while the engine has JSON support, their implementation is clunky and slow. I have decided I am going to use ArduinoJSON as my JSON engine in the C++ code for the game. I am also developing a computer system for my 1966 Mustang using an ESP32 board, and I’m going to use AJSON as both the data structure to hold the car’s current status info and to send REST API calls to the Adafruit.IO IOT service. Mostly what I like best is the focus on careful memory usage. Your article about heap fragmentation is very well done, and that is what sold me on using your library.

Morris Lewis

Thanks for your great work on this library. I experimented with the hornbill library as well, it also works but I prefer yours because you’re using more of the native esp32-arduino libraries, and there’s activity on this library.

TLS1000 on GitHub

I still like ArdunioJson over the alternatives, with the fact that it will never fragment malloc memory being a clear winning advantage. Careful consideration of ArduinoJson’s limitations should allow me to use it for what I want without disadvantage.

nsbawden on GitHub

I recently stumbled onto your library after dealing with JSON on my Particle projects. Fantastic job!

Georges Auberger

Thanks very much for the lib: I have been trying to squeeze an ethernet data collector onto a nano, and I was really struggling with memory. Your implementation of json seems magic in its memory useage! What makes it stand out is the simplicity of use, and the excellent documentation.

Dr Hugh Potts - Glasgow University

Keep up the good work with ArduinoJson! It is an awesome library and a kind of ”beacon” for me, really. The documentation, the website in general and your book is exemplary, I love it! I also provide and maintain several open source software projects (mostly Python packages or SailfishOS apps) and try to document them as good as possible, I know how time-consuming and hard it can be.

Yann Büchau on GitHub

Arduino would not be that good without your lib.

Davide Perini on GitHub

Fantastic library. I use it not only for embedded software but also for server stuff.

Carlos Cardeñoso

ArduinoJSON does bring the best features of many such json libraries into one umbrella. Thank you for an awesome library!! :-)

Sri Krishna Chaitanya Narumanchi on GitHub

Thank you again for your marvellous library which due to your book is probably one of the best documented libraries that exist. Please continue this excellent service that you provide to so many by your strive to improve this library still further and by keeping it up to date.

Dieter Grientschnig

Just wanted to say thank you for writing these docs so well. These are probably one of the easiest to understand well written docs I’ve ever browsed :)

Brad Matias

Thank you for such a great library! My students and I use it every year.

Daniel Law on GitHub

This is a fantastic library and the eBook has been a huge game changer for me with respect to optimizing the use of this library in my projects, so thanks 👍.

Aaron Neal on GitHub

Just to send my appreciation for the good work and congratulate Benoit et al for the quality of the book “Mastering ArduinoJson”, a very good value.

Jean-Marie Locci

With regards to the documentation I don’t think I’ve seen any non-commercial library that compares in terms of documentation so I think you’re doing well! Compared to your documentation stuff like the Linus kernel is incomprehensible! Thanks again for a great library!

OOHehir on GitHub

Just want to say that this is an impressive piece of coding. I noticed code I have never seen before, pushing the platform to it’s limits. Other languages could learn from this, complex on the inside, but how simple it is to use. Very nice :-)

Andreotti100 on GitHub

Thanks again for your availability an patience on this marvelous project 👍

tobozo on GitHub

ArduinoJson is a good example for great engineering. If you rely heavily on JSON, then ArduinoJson will not only give you the tools to handle data processing efficiently, but it will also be a framework to build better products upon.

Matthias Akstaller on GitHub

Thank you for an AMAZING library. ArduinoJson is a masterpiece.

Scott Baker on GitHub

I am very impressed with you work in doing the ArduinoJSON library, really, I think this is the most organized and documented library I have seen doing my project.

Juan Carlos Gomez Casal on GitHub