BasicJsonDocument<T>::shrinkToFit() reduces the capacity of the memory pool to match the current usage. Use this function to release some memory.

There is no reverse operation to resize the memory pool; you can only shrink it.

Unlike JsonDocument::garbageCollect(), this function doesn’t reclaim leaked memory blocks.

This function was added in ArduinoJson 6.14.0


Here is how you can deserialize using as a lot of memory, and then release what’s unused.

DynamicJsonDocument doc(8192);
deserializeJson(doc, json);

On ESP8266, you can call ESP.getMaxFreeBlockSize() to know how much memory you can use:

DynamicJsonDocument doc(ESP.getMaxFreeBlockSize() - 512);
deserializeJson(doc, json);

Similarly, on ESP32, you can call ESP.getMaxAllocHeap():

DynamicJsonDocument doc(ESP.getMaxAllocHeap() - 1024);
deserializeJson(doc, json);

These two functions return the size of the largest block of free memory. The result may be significantly lower than the total available memory (ESP.getFreeHeap()) if the heap is fragmented.

See also