ArduinoJson — Fast JSON Library for Embedded C++

Introduction

ArduinoJson is a C++ JSON library designed for embedded systems. It provides a simple API for parsing and generating JSON with predictable memory usage, making it the standard choice for IoT firmware that communicates via JSON APIs.

What ArduinoJson Does

• Parses JSON strings into a queryable document model
• Serializes C++ data structures to JSON or MessagePack
• Uses a fixed-size or dynamic memory pool to avoid heap fragmentation
• Supports JSON filtering to parse only the fields you need
• Handles nested objects, arrays, and streaming input

Architecture Overview

ArduinoJson uses a JsonDocument that owns a contiguous memory pool. Parsing writes all values into this pool, avoiding scattered heap allocations. The library is header-only with no external dependencies, compiling on any C++11 toolchain from AVR to 64-bit desktop.

Self-Hosting & Configuration

• Install via the Arduino Library Manager or PlatformIO
• Include the single header and create a JsonDocument with a size estimate
• Use the ArduinoJson Assistant at arduinojson.org to calculate memory requirements
• Configure stack vs heap allocation by choosing StaticJsonDocument or DynamicJsonDocument
• Works on Arduino, ESP8266, ESP32, STM32, Teensy, and standard Linux/macOS/Windows

Key Features

• Header-only with zero external dependencies
• Predictable memory usage with no fragmentation
• Supports both JSON and MessagePack binary format
• JSON filtering reduces memory use by skipping unneeded fields
• Extensive test suite with over 3,000 unit tests

Comparison with Similar Tools

• cJSON — plain C and smaller but lacks type safety and streaming; ArduinoJson is more ergonomic in C++
• RapidJSON — faster on desktop but uses more memory; ArduinoJson is optimized for MCUs
• nlohmann/json — beautiful API for desktop C++; too large for most embedded targets
• Jansson — C library with good API but no MessagePack support

FAQ

Q: How much RAM does ArduinoJson need? A: It depends on the document. A typical IoT payload with 5-10 fields uses 200-500 bytes. The online assistant calculates exact requirements.

Q: Does it work on 8-bit AVR boards? A: Yes. ArduinoJson supports AVR, ARM, RISC-V, and Xtensa architectures with as little as 2KB RAM.

Q: Can I parse streaming JSON? A: Yes. ArduinoJson can read from Stream objects (Serial, WiFiClient, File) without loading the entire string into memory.

Q: Is ArduinoJson free for commercial use? A: Yes. It is released under the MIT license.

Sources

• https://github.com/bblanchon/ArduinoJson
• https://arduinojson.org/