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:

WiFiClient client;  // or WiFiClientSecure for HTTPS
HTTPClient http;

// Send request
http.begin(client, "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.

For HTTPS, you need to use WiFiClientSecure instead of WiFiClient.
You must also choose the certificate validation strategy by calling WiFiClientSecure::setInsecure(), for example.

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.

WiFiClient client;  // or WiFiClientSecure for HTTPS
HTTPClient http;

// Send request
http.useHTTP10(true);
http.begin(client, "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(), or deserializeJson() will return InvalidInput.

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, HTTPClient doesn’t let us 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);

WiFiClient client;  // or WiFiClientSecure for HTTPS
HTTPClient http;

// Send request
http.begin(client, "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.

If you want to send the payload as a stream, you need to use WiFiClient directly, as shown in the Serialization Tutorial (around page 127).

Why is the JsonDocument empty?

The most likely explanation is that the server returned a redirection code, like 301 Moved Permanently or 302 Found.

http.setFollowRedirects(HTTPC_FORCE_FOLLOW_REDIRECTS);

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 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