![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/Arduino-JSON-featured-image.png?resize=1146%2C588&quality=100&strip=all&ssl=1\")
<\/p>\nIn this blog post you’re going to learn how to decode (parse a JSON string) and encode (generate a JSON string) with the ArduinoJson library using the Arduino with the Ethernet shield. This guide also works with the ESP8266 and ESP32 Wi-Fi modules with small changes.<\/p>\n
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Important:<\/strong> this tutorial is only compatible with the ArduinoJSON library 5.13.5.<\/span><\/p>\n JSON stands for J<\/strong>avaS<\/strong>cript O<\/strong>bject N<\/strong>otation. JSON is a lightweight text-based open standard design for exchanging data.<\/p>\n JSON is primarily used for serializing and transmitting structured data over network connection – transmit data between a server and a client. It is often used in services like APIs (Application Programming Interfaces) and web services that provide public data.<\/p>\n In JSON, data is structured in a specific way. JSON uses symbols like { } , : ” ” [ ] <\/strong>and it has the following syntax:<\/p>\n For example, to represent data in JSON, the key\/value pairs come as follows:<\/p>\n In a real world example, you may want structure data about a user:<\/p>\n Or in a IoT project, you may want to structure data from your sensors:<\/p>\n In JSON, the values can be another JSON object (sports) or an array (pets) . For example:<\/p>\n Here we are structuring data about a user and we have several keys: “name”<\/strong>, “sports”<\/strong> and “pets”.<\/strong><\/p>\n The name has the value Rui assigned. Rui may practice different sports relating to where they are practiced. So, we create another JSON object to save Rui’s favorite sports. This JSON object is the value of the “sports<\/strong>” key.<\/p>\n The “pets”<\/strong> key has an array that contains Rui’s pets’ names and it has the values “Max<\/strong>” and “Dique<\/strong>” inside.<\/p>\n Most APIs return data in JSON and most values are JSON objects themselves. The following example shows the data provided by a weather API.<\/p>\n This API provides a lot of information. For example, the first lines store the coordinates with the longitude and latitude.<\/span><\/p>\n The examples in this post use an Arduino with an Ethernet shield. Just mount the shield onto your Arduino board and connect it to your network with an RJ45 cable to establish an Internet connection (as shown in the figure below).<\/p>\n Note:<\/strong> the examples provided in this tutorial also work with the ESP8266 and ESP32 with small changes.<\/p>\n The easiest way to decode and encode JSON strings with the Arduino IDE is using the ArduinoJson library 5.13.5 which was designed to be the most intuitive JSON library, with the smallest footprint and most efficiently memory management for Arduino.<\/p>\n It has been written with Arduino in mind, but it isn’t linked to Arduino libraries so you can use this library in any other C++ project. There’s also a documentation<\/a> website for the library with examples and with the API reference.<\/p>\n For this project you need to install the ArduinoJson library in your Arduino IDE:<\/p>\n Let’s start by decoding\/parsing the next JSON string:<\/p>\n Import the ArduinoJson library:<\/p>\n Arduino JSON uses a preallocated memory pool to store the JsonObject tree, this is done by the StaticJsonBuffer. You can use ArduinoJson Assistant<\/a> to compute the exact buffer size, but for this example 200 is enough.<\/p>\n Create a char array called json[]<\/strong> to store a sample JSON string:<\/p>\n Use the function parseObject()<\/strong> to decode\/parse the JSON string to a JsonObject called root<\/strong>.<\/p>\n To check if the decoding\/parsing was successful, you can call root.success()<\/strong>:<\/p>\n The result can be false for three reasons:<\/p>\n Now that the object or array is in memory, you can extract the data easily. The simplest way is to use the JsonObject root<\/strong>:<\/p>\n You can use the decoded variables sensor, time, latitude or longitude in your code logic.<\/p>\n For a real example using an Arduino with an Ethernet shield, we’re going to use a free API from OpenWeatherMap<\/a> to request the day\u2019s weather forecast for your chosen location.<\/p>\n Learning to use APIs is a great skill because it allows you access to a wide variety of constantly-changing information, such as the current stock price, the currency exchange rate, the latest news, traffic updates, and much more.<\/p>\n Using the API<\/strong><\/p>\n OpenWeatherMap\u2019s free plan provides everything you need for thins example. To use the API you need an API key, known as the APIID. To get an APIID:<\/p>\n This is a unique key you need to pull information from the site. Copy and paste this key somewhere, you\u2019ll need it in a moment.<\/p>\n To pull information on weather in your chosen location, enter the following URL with the sections in curly brackets replaced with your chosen location information and your unique API key:<\/p>\n Replace {your city}<\/strong><\/span> with the city you want data for, {your country code}<\/strong><\/span> with the country code for that city, and {your API key}<\/strong><\/span> with your unique API key we found previously. For example, our API URL for the town of Porto in Portugal, after replacing with the details, would be:<\/p>\n Note<\/strong>: more information on using the API to get weather information is available here<\/a>.<\/p>\n Copy your URL into your browser and it should give you a bunch of information that corresponds to your local weather information.<\/p>\n In our case, it returns the weather in Porto, Portugal on the day of writing:<\/p>\n Now that you have a URL that returns your local weather data. You can automate this task and access that data in your Arduino or ESP8266 projects. Here’s the full script that you need to upload to your Arduino with Ethernet shield to return the temperature in Kelvin and humidity:<\/p>\n View raw code<\/a><\/p>\n Note:<\/strong> make sure your replace the resource variable with your unique OpenWeatherMap URL resource:<\/p>\n In this example, the Arduino performs an HTTP GET request to a desired service (in this case the OpenWeatherMap API), but you could change it to request any other web service. We won’t explain the Arduino code line by line.<\/p>\n For this project it’s important that you understand what you need to change in the Arduino code to decode\/parse any JSON response. Follow these next three steps.<\/p>\n Create a data structure that can store the information that you’ll want to extract from the API. In this case, we want to store the temperature and humidity in char arrays:<\/p>\n Go to the ArduinoJson Assistant<\/a> and copy the full OpenWeatherMap API response to the Input field.<\/p>\n Copy the Expression<\/strong> generated (see the preceding figure), in my case:<\/p>\n You’ll need to edit the readReponseContent()<\/strong> function with your the generated JsonBuffer<\/strong> size from ArduinoJson Assistant to allocate the appropriate memory for decoding the JSON response from an API:<\/p>\n <\/p>\n Still inside the readReponseContent()<\/strong> function you need to copy the variables that you need for your project to your struct data:<\/p>\n Then, you can easily access the decoded JSON data in your Arduino code and do something with it. In this example we’re simply printing the temperature in Kelvin and humidity in the Arduino IDE serial monitor:<\/p>\n Open the Arduino IDE serial monitor at a baud rate of 9600 and you’ll see the temperature in Kelvin and the humidity in percentage being printed in the Serial monitor every 60 seconds.<\/p>\n You can access the rest of the information in the OpenWeatherMap API response, but for demonstration purposes we only decoded the temperature and humidity.<\/p>\n Let’s learn how to encode\/generate the next JSON string:<\/p>\n You can read the docs about enconding here<\/a>.<\/p>\n Import the ArduinoJson library:<\/p>\n Arduino JSON uses a preallocated memory pool to store the object tree, this is done by the StaticJsonBuffer. You can use ArduinoJson Assistant<\/a> to compute the exact buffer size, but for this example 200 is enough.<\/p>\n![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/arduino-json-library-installed.png?resize=786%2C179&quality=100&strip=all&ssl=1\")
<\/p>\nWhat is JSON?<\/h2>\n
JSON syntax basics<\/h2>\n
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{\"key1<\/strong>\":\"value1<\/strong>\", \"key2<\/strong>\":\"value2<\/strong>\", \"key3<\/strong>\":\"value3<\/strong>\"}<\/pre>\nJSON examples<\/h2>\n
{\"name<\/strong>\":\"Rui<\/strong>\", \"country<\/strong>\": \"Portugal<\/strong>\", \"age<\/strong>\":24<\/strong>}<\/pre>\n{\"temperature<\/strong>\":27.23<\/strong>, \"humidity<\/strong>\":62.05<\/strong>, \"pressure<\/strong>\":1013.25<\/strong>}<\/pre>\n{\n \"name<\/strong>\": \"Rui<\/strong>\",\n \"sports<\/strong>\": {\n \"outdoor<\/strong>\": \"hiking<\/strong>\",\n \"indoor<\/strong>\": \"swimming<\/strong>\"\n },\n \"pets<\/strong>\": [\n \"Max<\/strong>\",\n \"Dique<\/strong>\"\n ]\n}<\/pre>\n{ <\/i> <\/span>\n \"coord\"<\/span>:<\/span>{ <\/i> <\/span>\n \"lon\"<\/span>:<\/span>-8.61<\/span>,<\/span>\n \"lat\"<\/span>:<\/span>41.15<\/span>\n }<\/span>,<\/span>\n \"weather\"<\/span>:<\/span>[ <\/i> <\/span>\n { <\/i> <\/span>\n \"id\"<\/span>:<\/span>803<\/span>,<\/span>\n \"main\"<\/span>:<\/span>\"Clouds\"<\/span>,<\/span>\n \"description\"<\/span>:<\/span>\"broken clouds\"<\/span>,<\/span>\n \"icon\"<\/span>:<\/span>\"04d\"<\/span>\n }<\/span>\n ]<\/span>,\n<\/span> \"base\"<\/span>:<\/span>\"stations\"<\/span>,<\/span>\n \"main\"<\/span>:<\/span>{ <\/i> <\/span>\n \"temp\"<\/span>:<\/span>288.15<\/span>,<\/span>\n \"pressure\"<\/span>:<\/span>1020<\/span>,<\/span>\n \"humidity\"<\/span>:<\/span>93<\/span>,<\/span>\n \"temp_min\"<\/span>:<\/span>288.15<\/span>,<\/span>\n \"temp_max\"<\/span>:<\/span>288.15<\/span>\n }<\/span>,<\/span>\n (...)\n}<\/span><\/pre>\nArduino with Ethernet shield<\/h2>\n
![\"\"](\"https:\/\/i1.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/03\/Ethernet-Shield.jpg?resize=700%2C478&ssl=1\")
<\/p>\nPreparing the Arduino IDE<\/h2>\n
Features<\/h3>\n
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Compatible with<\/h3>\n
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Installing the ArduinoJson library<\/h3>\n
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ArduinoJson-master<\/del> to ArduinoJson<\/strong><\/li>\nDecoding JSON – Parse JSON string<\/h2>\n
{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}<\/pre>\n<\/a>#include <ArduinoJson.h><\/strong><\/pre>\n
StaticJsonBuffer<200> jsonBuffer;<\/strong><\/pre>\n
char json[] = \"{\\\"sensor\\\":\\\"gps\\\",\\\"time\\\":1351824120,\\\"data\\\":[48.756080,2.302038]}\"; <\/strong><\/pre>\nJsonObject& root = jsonBuffer.parseObject(json);<\/strong><\/pre>\n
if(!root.success())<\/strong> {\n Serial.println(\"parseObject() failed\");\n return false<\/strong>;\n}<\/pre>\n\n
const char* sensor<\/strong> = root[\"sensor\"];\nlong time<\/strong> = root[\"time\"];\ndouble latitude<\/strong> = root[\"data\"][0];\ndouble longitude<\/strong> = root[\"data\"][1];<\/pre>\n
OpenWeatherMap API<\/h3>\n
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![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/openWeatherMap-api-key.png?resize=998%2C474&quality=100&strip=all&ssl=1\")
<\/p>\nhttp:\/\/api.openweathermap.org\/data\/2.5\/weather?q={your city}<\/strong><\/span>,{your country code}<\/strong><\/span>&APPID={your API Key}<\/span><\/strong><\/pre>\nhttp:\/\/api.openweathermap.org\/data\/2.5\/weather?q=Porto,PT&APPID=801d2603e9f2e1c70e042e4------<\/pre>\n
![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/open-weather-map-test-api.png?resize=841%2C319&quality=100&strip=all&ssl=1\")
<\/p>\n{\n \"coord\": {\n \"lon\": -8.61,\n \"lat\": 41.15\n },\n \"weather\": [\n {\n \"id\": 701,\n \"main\": \"Mist\",\n \"description\": \"mist\",\n \"icon\": \"50d\"\n }\n ],\n \"base\": \"stations\",\n \"main\": {\n \"temp\": 290.86,\n \"pressure\": 1014,\n \"humidity\": 88,\n \"temp_min\": 290.15,\n \"temp_max\": 292.15\n },\n (...)\n}<\/pre>\nMaking an API Request with Arduino<\/h3>\n
\/*\n * Rui Santos \n * Complete Project Details http:\/\/randomnerdtutorials.com\n * Based on the Arduino Ethernet Web Client Example\n * and on the sketch "Sample Arduino Json Web Client" of the Arduino JSON library by Benoit Blanchon (bblanchon.github.io\/ArduinoJson)\n *\/\n\n#include <ArduinoJson.h>\n#include <Ethernet.h>\n#include <SPI.h>\n\nEthernetClient client;\n\n\/\/ Name address for Open Weather Map API\nconst char* server = "api.openweathermap.org";\n\n\/\/ Replace with your unique URL resource\nconst char* resource = "REPLACE_WITH_YOUR_URL_RESOURCE";\n\n\/\/ How your resource variable should look like, but with your own COUNTRY CODE, CITY and API KEY (that API KEY below is just an example):\n\/\/const char* resource = "\/data\/2.5\/weather?q=Porto,pt&appid=bd939aa3d23ff33d3c8f5dd1";\n\nconst unsigned long HTTP_TIMEOUT = 10000; \/\/ max respone time from server\nconst size_t MAX_CONTENT_SIZE = 512; \/\/ max size of the HTTP response\n\nbyte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};\n\n\/\/ The type of data that we want to extract from the page\nstruct clientData {\n char temp[8];\n char humidity[8];\n};\n\n\/\/ ARDUINO entry point #1: runs once when you press reset or power the board\nvoid setup() {\n Serial.begin(9600);\n while (!Serial) {\n ; \/\/ wait for serial port to initialize\n }\n Serial.println("Serial ready");\n if(!Ethernet.begin(mac)) {\n Serial.println("Failed to configure Ethernet");\n return;\n }\n Serial.println("Ethernet ready");\n delay(1000);\n}\n\n\/\/ ARDUINO entry point #2: runs over and over again forever\nvoid loop() {\n if(connect(server)) {\n if(sendRequest(server, resource) && skipResponseHeaders()) {\n clientData clientData;\n if(readReponseContent(&clientData)) {\n printclientData(&clientData);\n }\n }\n }\n disconnect();\n wait();\n}\n\n\/\/ Open connection to the HTTP server\nbool connect(const char* hostName) {\n Serial.print("Connect to ");\n Serial.println(hostName);\n\n bool ok = client.connect(hostName, 80);\n\n Serial.println(ok ? "Connected" : "Connection Failed!");\n return ok;\n}\n\n\/\/ Send the HTTP GET request to the server\nbool sendRequest(const char* host, const char* resource) {\n Serial.print("GET ");\n Serial.println(resource);\n\n client.print("GET ");\n client.print(resource);\n client.println(" HTTP\/1.1");\n client.print("Host: ");\n client.println(host);\n client.println("Connection: close");\n client.println();\n\n return true;\n}\n\n\/\/ Skip HTTP headers so that we are at the beginning of the response's body\nbool skipResponseHeaders() {\n \/\/ HTTP headers end with an empty line\n char endOfHeaders[] = "\\r\\n\\r\\n";\n\n client.setTimeout(HTTP_TIMEOUT);\n bool ok = client.find(endOfHeaders);\n\n if (!ok) {\n Serial.println("No response or invalid response!");\n }\n return ok;\n}\n\n\/\/ Parse the JSON from the input string and extract the interesting values\n\/\/ Here is the JSON we need to parse\n\/*{\n "coord": {\n "lon": -8.61,\n "lat": 41.15\n },\n "weather": [\n {\n "id": 800,\n "main": "Clear",\n "description": "clear sky",\n "icon": "01d"\n }\n ],\n "base": "stations",\n "main": {\n "temp": 296.15,\n "pressure": 1020,\n "humidity": 69,\n "temp_min": 296.15,\n "temp_max": 296.15\n },\n "visibility": 10000,\n "wind": {\n "speed": 4.6,\n "deg": 320\n },\n "clouds": {\n "all": 0\n },\n "dt": 1499869800,\n "sys": {\n "type": 1,\n "id": 5959,\n "message": 0.0022,\n "country": "PT",\n "sunrise": 1499836380,\n "sunset": 1499890019\n },\n "id": 2735943,\n "name": "Porto",\n "cod": 200\n}*\/\n\nbool readReponseContent(struct clientData* clientData) {\n \/\/ Compute optimal size of the JSON buffer according to what we need to parse.\n \/\/ See https:\/\/bblanchon.github.io\/ArduinoJson\/assistant\/\n const size_t bufferSize = JSON_ARRAY_SIZE(1) + JSON_OBJECT_SIZE(1) + \n 2*JSON_OBJECT_SIZE(2) + JSON_OBJECT_SIZE(4) + JSON_OBJECT_SIZE(5) + \n JSON_OBJECT_SIZE(6) + JSON_OBJECT_SIZE(12) + 390;\n DynamicJsonBuffer jsonBuffer(bufferSize);\n\n JsonObject& root = jsonBuffer.parseObject(client);\n\n if (!root.success()) {\n Serial.println("JSON parsing failed!");\n return false;\n }\n\n \/\/ Here were copy the strings we're interested in using to your struct data\n strcpy(clientData->temp, root["main"]["temp"]);\n strcpy(clientData->humidity, root["main"]["humidity"]);\n \/\/ It's not mandatory to make a copy, you could just use the pointers\n \/\/ Since, they are pointing inside the "content" buffer, so you need to make\n \/\/ sure it's still in memory when you read the string\n\n return true;\n}\n\n\/\/ Print the data extracted from the JSON\nvoid printclientData(const struct clientData* clientData) {\n Serial.print("Temp = ");\n Serial.println(clientData->temp);\n Serial.print("Humidity = ");\n Serial.println(clientData->humidity);\n}\n\n\/\/ Close the connection with the HTTP server\nvoid disconnect() {\n Serial.println("Disconnect");\n client.stop();\n}\n\n\/\/ Pause for a 1 minute\nvoid wait() {\n Serial.println("Wait 60 seconds");\n delay(60000);\n}\n<\/code><\/pre>\n\tconst char* resource = \"REPLACE_WITH_YOUR_URL_RESOURCE<\/span><\/strong>\";<\/pre>\nModifying the code for your project<\/h4>\n
STEP #1 – struct<\/h4>\n
struct clientData {<\/strong>\n char temp[8];<\/strong>\n char humidity[8];<\/strong>\n};<\/strong><\/pre>\nSTEP #2 – JsonBuffer size<\/h4>\n
![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/ArduinoJson-Assistant-example.png?resize=740%2C379&quality=100&strip=all&ssl=1\")
<\/p>\nJSON_ARRAY_SIZE(1) + JSON_OBJECT_SIZE(1) + 2*JSON_OBJECT_SIZE(2) + JSON_OBJECT_SIZE(4) + JSON_OBJECT_SIZE(5) + JSON_OBJECT_SIZE(6) + JSON_OBJECT_SIZE(12)<\/strong><\/pre>\n
bool readReponseContent<\/strong>(struct clientData* clientData) {\n const size_t bufferSize = JSON_ARRAY_SIZE(1) + JSON_OBJECT_SIZE(1) + <\/strong>\n 2*JSON_OBJECT_SIZE(2) + JSON_OBJECT_SIZE(4) + JSON_OBJECT_SIZE(5) + <\/strong>\n JSON_OBJECT_SIZE(6) + JSON_OBJECT_SIZE(12) + 390; \n\n<\/strong><\/span> DynamicJsonBuffer jsonBuffer(bufferSize);\n JsonObject& root = jsonBuffer.parseObject(client);<\/pre>\nstrcpy(clientData->temp, root[\"main\"][\"temp\"]<\/strong>);\nstrcpy(clientData->humidity, root[\"main\"][\"humidity\"]<\/strong>);<\/pre>\n
STEP #3 – accessing the decoded data<\/h4>\n
void printclientData<\/strong>(const struct clientData* clientData) {\n Serial.print(\"Temp = \");\n Serial.println(clientData->temp<\/strong>);\n Serial.print(\"Humidity = \");\n Serial.println(clientData->humidity<\/strong>);\n}<\/pre>\nDemonstration<\/h3>\n
![\"\"](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/08\/arduino-ide-serial-monitor-open-weather-map.png?resize=696%2C438&quality=100&strip=all&ssl=1\")
<\/p>\nEncoding JSON – Generate JSON string<\/h2>\n
{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}<\/pre>\n<\/a>#include <ArduinoJson.h><\/strong><\/pre>\n
StaticJsonBuffer<200> jsonBuffer;<\/strong><\/pre>\n