How to Deserialize JSON Objects from MQTT Payloads Using ArduinoJson Library

Learn how to effectively `deserialize JSON` objects received via MQTT on your Arduino using the ArduinoJson library. This post explores common issues and solutions to ensure you retrieve values accurately.
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How to Deserialize JSON Objects from MQTT Payloads Using ArduinoJson Library

In the world of Internet of Things (IoT), exchanging data efficiently is crucial. One common format for transmitting data is JSON (JavaScript Object Notation), which is lightweight and easy to read. When integrating MQTT (Message Queuing Telemetry Transport) and the ArduinoJson library, you might encounter issues while trying to deserialize JSON objects. In this guide, we'll explore a problem involving JSON deserialization and how to resolve it effectively.

The Problem

A user is attempting to deserialize a JSON object transmitted through MQTT, but is encountering difficulties. The payload in their example contains this JSON object:

[[See Video to Reveal this Text or Code Snippet]]

When using the ArduinoJson library, the user's code only returns zeros when trying to extract the "amount" value. Here’s the relevant code:

[[See Video to Reveal this Text or Code Snippet]]

Surprisingly, a similar piece of code that directly uses a string representation of the JSON works well:

[[See Video to Reveal this Text or Code Snippet]]

This inconsistency raises a significant question: What is preventing the retrieval of the value directly from the payload?

The Solution

Upon further investigation, the issue was traced back to how the data was sent from the Lambda function, which was not properly formatted. The user had initially overlooked that the payload was being sent as a string instead of a properly parsed JSON object. This plays a major role in successful deserialization.

Here’s how the issue was resolved:

Identifying the Wrong Data Format:

The original payload being sent was a string representation, which could lead to problems during deserialization.

Using JSON.stringify:

The solution was to use JSON.stringify in the Lambda function to serialize the response properly before sending it.

Here’s the critical part of the updated Lambda function code:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By ensuring that your data is properly formatted as a JSON object instead of a string, you can effectively use the ArduinoJson library to deserialize data received via MQTT. Making these adjustments can save you time and effort while troubleshooting communication issues between your devices.

Now, with these insights, you should be ready to tackle JSON deserialization in your Arduino projects seamlessly. Happy coding!