Assuming that the board is connected to the WiFi network, you can send an HTTP request like so:
HTTPClient http; // Send request http.begin("http://arduinojson.org/example.json"); http.GET(); // Print the response Serial.print(http.getString()); // Disconnect http.end();
While you can perform HTTP request without
HTTPClient (cf “See also” below), this class greatly simplifies your code.
How to parse a JSON document from an HTTP response?
deserializeJson(doc, http); // error: 'class HTTPClient' has no member named 'read'; did you mean 'end'?
We could pass the result of
http.getString() directly to
deserializeJson(), but it would be quite inefficient because it would copy the complete response in RAM before parsing.
We can do much better by letting ArduinoJson pull the bytes from the HTTP response.
To do that, we must get
Stream by calling
http.getStream() instead of
HTTPClient http; // Send request http.useHTTP10(true); http.begin("http://arduinojson.org/example.json"); http.GET(); // Parse response DynamicJsonDocument doc(2048); deserializeJson(doc, http.getStream()); // Read values Serial.println(doc["time"].as<long>()); // Disconnect http.end();
Notice that I used a
DynamicJsonDocument here because the document is quite large.
Feel free to use a
StaticJsonDocument if your document is smaller.
As always, use the ArduinoJson Assistant to compute the right capacity for your project.
How to send a JSON document in an HTTP request?
Unfortunately, there is no way to use the same trick as above, so we have to use a temporary buffer.
Here is how we can do so with a
// Prepare JSON document DynamicJsonDocument doc(2048); doc["hello"] = "world"; // Serialize JSON document String json; serializeJson(doc, json); HTTPClient http; // Send request http.begin("http://httpbin.org/post"); http.POST(json); // Read response Serial.print(http.getString()); // Disconnect http.end();
If you run this program, you’ll see that it prints information on the HTTP requests. That’s the response returned by httpbin.org; it’s very handy to debug your programs.
You can significantly improve this code’s performance by calling
Here are some other tips for using the
String class efficiently.
Why is the
JsonDocument empty? 🆕
This feature was added quite recently on ESP32, so make sure you are up-to-date.
BONUS: How to see the document when I use a
As we saw, it’s more efficient to pass the
deserializeJson() because it saves a large amount of memory.
However, we cannot directly print the content of the
Stream to see what was sent to ArduinoJson.
To see the content that is received from the HTTP response, we can use the
ReadLoggingStream from the StreamUtils library.
ReadLoggingStream loggingStream(http.getStream(), Serial); deserializeJson(doc, loggingStream);
ReadLoggingStream will forward everything to
deserializeJson() and will print the content to the serial port.