ArduinoJson

A malloc-free JSON parser for Arduino

The library is an thin C++ wrapper around the jsmn tokenizer: http://zserge.com/jsmn.html

It's design to be very lightweight, works without any allocation on the heap (no malloc) and supports nested objects.

It has been written with Arduino in mind, but it isn't linked to Arduino libraries so you can use this library on any other C++ project.

Example

char* json = "{\"Name\":\"Blanchon\",\"Skills\":[\"C\",\"C++\",\"C#\"],\"Age\":32,\"Online\":true}";

JsonParser<256> parser;

JsonHashTable hashTable = parser.parseHashTable(json);

if (!hashTable.success())
{
    return;
}

char* name = hashTable.getString("Name");

JsonArray skills = hashTable.getArray("Skills");

int age = hashTable.getLong("Age");

bool online = hashTable.getBool("Online");

How to use ?

1. Install the the library

Download the library and extract it to:

<your Arduino Sketch folder>/libraries/ArduinoJonsParser

2. Import in your sketch

Just add the following line on the to of your .ino file:

#include <JonsParser.h>

3. Create a parser

To extract data from the JSON string, you need to instanciate a JsonParser, and specify the number of byte your allocate for the parser itself:

JsonParser<128> parser;

How to choose the size ?

The more bytes you give to the parser, the more complex the JSON can be, so if you have free space on your stack you should increase the size of the parser. A size from 128 to 512 bytes is usually good.

Behind the scenes, all these bytes are used to store jsmn tokens. A token is 8 bytes long, so 128 to 512, allows to parse from 16 to 64 tokens. It may seem small but it's very descent for a processor with only 2KB of RAM, you won't get better results with other parsers.

As an example the char* json on the top of this page requires 12 tokens, so 96 bytes would be enough.

4. Extract data

To use this library, you need to know beforehand what is the type of data contained in the JSON string, which is extremely likely.

Hash table

Consider we have a char* json pointing to the following JSON string:

{
    "Name":"Blanchon",
    "Skills":[
        "C",
        "C++",
        "C#"],
    "Age":32,
    "Online":true
}

In this case the root object of the JSON string is a hash table, so you need to extract a JsonHashTable:

JsonHashTable root = parser.parseHashTable(json);

To check if the parsing was successful, you must check:

if (!root.success())
{
    // Parsing fail: could be an invalid JSON, or too many tokens
}

And then extract the member you need:

char* name = hashTable.getString("Name");

JsonArray skills = hashTable.getArray("Skills");

int age = hashTable.getLong("Age");

bool online = hashTable.getBool("Online");

Array

Consider we have a char* json pointing to the following JSON string:

[
    [ 1.2, 3.4 ],
    [ 5.6, 7.8 ]               
]

In this case the root object of the JSON string is an array, so you need to extract a JsonArray:

JsonArray root = parser.parseArray(json);

To check if the parsing was successful, you must check:

if (!root.success())
{
    // Parsing fail: could be an invalid JSON, or too many tokens
}

And then extract the content by its index in the array:

JsonArray row0 = root.getArray(0);
double a = row0.getDouble(0);

or simply:

double a = root.getArray(0).getDouble(0);

Code size

Theses tables has been created by analyzing the map file generated by AVR-GCC after adding -Wl,-Map,foo.map to the command line.

As you'll see the code size if between 1680 and 3528 bytes, depending on the features you use.

Minimum setup

Function	Size in bytes
strcmp(char*,char*)	18
jsmn_init(jsmn_parser*)	20
JsonParser::parse(char*)	106
JsonObjectBase::getNestedTokenCount(jsmntok_t*)	84
JsonObjectBase::getStringFromToken(jsmntok_t*)	68
JsonArray::JsonArray(char*, jsmntok_t*)	42
JsonArray::getToken(int)	112
JsonArray::getString(int)	18
JsonHashTable::JsonHashTable(char*, jsmntok_t*)	42
JsonHashTable::getToken(char*)	180
JsonHashTable::getString(char*)	18
TOTAL	1680

Additional space to parse nested objects

Function	Size in bytes
JsonArray::getArray(int)	42
JsonArray::getHashTable(int)	64
JsonHashTable::getArray(char*)	64
JsonHashTable::getHashTable(char*)	42
TOTAL	212

Additional space to parse bool values

Function	Size in bytes
JsonObjectBase::getBoolFromToken(jsmntok_t*)	82
JsonArray::getBool(int)	18
JsonHashTable::getBool(char*)	18
TOTAL	130

Additional space to parse double values

Function	Size in bytes
strtod(char*,int)	704
JsonObjectBase::getDoubleFromToken(jsmntok_t*)	44
JsonArray::getDouble(int)	18
JsonHashTable::getDouble(char*)	18
TOTAL	796

Additional space to parse long values

Function	Size in bytes
strtol(char*,char**,int)	606
JsonObjectBase::getLongFromToken(jsmntok_t*)	56
JsonArray::getLong(int)	18
JsonHashTable::getLong(char*)	18
TOTAL	710