How To Append A JSON Object To A File? | ArduinoJson 6

Home » Add Element Json List » How To Append A JSON Object To A File? | ArduinoJson 6

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Scenario: you have a file (on SD, SPIFFS, LittleFS, or any other filesystem) that contains a list of JSON objects, and you want to add a new object to the list.

Question: What’s the best way to proceed? Should you load the complete file in memory, or is there a better solution?

There are two ways to append a JSON object to a file, depending on the format of the file.

Option 1: the file contains an array

Usually, we store a list of objects in a JSON array, like so:

[{"first_name":"Stan","last_name":"Marsh"},{"first_name":"Kyle","last_name":"Broflovski"},{"first_name":"Eric","last_name":"Cartman"}]

As you can see, appending an element to the array requires inserting the new object before the closing bracket (]).

This operation is not supported by the filesystem, so our only option is to load the file in memory and serialize it back, like so:

DynamicJsonDocument doc(8192); // Read the file File file = SPIFFS.open("/southpath.json", "r"); deserializeJson(doc, file); file.close(); // Append new element JsonObject obj = doc.createNestedObject(); obj["first_name"] = "Kenny"; obj["last_name"] = "McCormick"; // Write the file file = SPIFFS.open("/southpath.json", "w"); serializeJson(doc, file); file.close();

Option 2: the file contains line-separated objects

If you want to optimize the append-to-file scenario, you must change the format of the file. Instead of wrapping the list in a JSON array, you must write the object one after the other. Usually, we insert a line break between the objects, following the pseudo-standard JSONLines and ndjson.

Here is an example:

{"first_name":"Stan","last_name":"Marsh"}{"first_name":"Kyle","last_name":"Broflovski"}{"first_name":"Eric","last_name":"Cartman"}

As you can see, appending an element is just a matter of writing the new object at the end of the file; we don’t need to load the file in memory to do that.

// Create the new element StaticJsonDocument<256> doc; doc["first_name"] = "Kenny"; doc["last_name"] = "McCormick"; // Append to file File file = SPIFFS.open("/southpath.jsonl", "a"); serializeJson(doc, file); file.println(); file.close();

How to read a JSONL file?

If you want to read this file with ArduinoJson, you simply need to call deserializeJson() repeatedly:

File file = SPIFFS.open("/southpath.jsonl", "r"); while (true) { StaticJsonDocument<256> doc; DeserializationError err = deserializeJson(doc, file); if (err) break; Serial.print(doc["first_name"].as<const char*>); Serial.print(" "); Serial.println(doc["last_name"].as<const char*>); } file.close();

See also

  • JSONLines and ndjson
  • Line-delimited JSON on Wikipedia
  • “JSON streaming” in the “Advanced Techniques” chapter of Mastering ArduinoJson
  • serializeJson() and deserializeJson()
  • SPIFFS.open()
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