What is HTTPClient?

HTTPClient is a class that performs HTTP requests on ESP8266 and ESP32 .

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.

On ESP8266 , this class is provided by the ESP8266HTTPClient library, which is bundled with the ESP8266 core for Arduino.

On ESP32 , it’s provided by the HTTPClient library, which is bundled with the Arduino core for the ESP32.

How to parse a JSON document from an HTTP response?

Unfortunately, HTTPClient doesn’t implement the Stream interface, so you cannot pass it directly to deserializeJson() like so:

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 HTTPClient’s underlying Stream by calling http.getStream() instead of http.getString().

Unfortunately, by using the underlying Stream, we bypass the code that handles chunked transfer encoding, so we must switch to HTTP version 1.0.

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();

Remember to call useHTTP10(true) when you use getStream().

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 String:

// 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 String::reserve() before serializeJson().
Here are some other tips for using the String class efficiently.

BONUS: How to see the document when I use a Stream?

As we saw, it’s more efficient to pass the Stream to 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.

Replace:

deserializeJson(doc, http.getStream());

with:

ReadLoggingStream loggingStream(http.getStream(), Serial);
deserializeJson(doc, loggingStream);

ReadLoggingStream will forward everything to deserializeJson() and will print the content to the serial port.

StreamUtils is a powerful library that deserves more attention. Please give it a star to spread the word.

See also

  • The GitHub example in chapter 3 of Mastering ArduinoJson uses HTTPClient to interact with GitHub’s API.
  • The Reddit case study in chapter 8 of Mastering ArduinoJson shows how to perform HTTP requests without HTTPClient.