A modern MAVLink library for the JVM written in Kotlin

Why another MAVLink implementation?

MAVLink is a lightweight, efficient and reliable communication protocol for drones. But over the years it has expanded its horizons to other non-aerial vehicles like rovers, submarines and other robotic platforms.

Since 2020, the drone industry has seen a sudden boom. To keep up with the demand for such robots, the developer community needs to match the requirements with control software that is performant, easy to develop and maintain. Keeping this in mind, at UrbanMatrix Technologies, we chose the Android platform to build our ground control station software. We believe that Android is the most readily available platform for users and hence the easiest to build for the masses, which is where the drone industry is heading. Drones are becoming a commodity and we need to be ready with easy-to-adopt user-facing tools.

Overview

mavlink-kotlin was created out of the need for a type-safe, high-performance and flexible MAVLink library for the JVM. For an end-to-end drone tech company like UrbanMatrix Technologies, MAVLink forms the heart of communication between the drone, the GCS and the companion computer. Apart from our proprietary Android-based GCS, UMT LaunchPad, we have developed several JVM applications that run on Matrix-OS, our proprietary Linux-based OS for our Companion Computer and our Management Server.

We had realized several problems with existing JVM MAVLink implementations. These include:

• Performance problems at very high message rates
• Hassle to generate and maintain MAVLink message objects from custom XML definitions
• Difficulties in understanding and fixing reflection-based code
• Difficulties in extending existing code-base due to lack of modularity

Through mavlink-kotlin we want to address the problems that we had faced while adopting and understanding MAVLink libraries not only for the JVM but also C/C++, Golang, Rust and Python. This library takes inspiration from several great features of these libraries. We want to make the adoption of this library as easy as possible by being flexible in terms of creating a modular and easy-to-extend API that allows the users to use the same base MAVLink and connection classes to create adapters of their own to fit whatever pub-sub library they want to use. Apart from this, the MAVLink generator plugin is designed to fit well with stand-alone Android and JVM projects while keeping the setup quick and hassle-free.

Key Design Considerations

Code generation instead of reflection

Reflection-based code is prone to errors, difficult to understand and difficult for compilers to optimize.

On the other hand, generated code is much easier for the users to go through and understand. And apart from the obvious possibilities of compile-time and run-time optimizations, it is free from the fragility that reflection brings in. mavlink-kotlin heavily uses code generation to avoid reflection and provide compile-time safety.

In fact, we’ve even tested it against other available MAVLink JVM implementations, and this library is 20,000x to 45,000x faster in terms of serialization and deserialization speeds and provides a much better memory footprint. Don’t believe us? try out the comparison tests in the mavlink-kotlin module.

Interface-based API and modular design

Don’t think the in-built implementation is good enough? No issues. Write your own code generators for the messages, enums, dialects etc. and the rest of the modules will work without requiring a reimplementation.

Message generator Gradle plugin

You can provide your own dialect files and the Gradle plugin will take care of generating the messages. Use it to generate and maintain your own private MAVLink message implementations for your drone systems by simply providing the MAVLink XMLs.

Support for Kotlin Multiplatform (future)

Target Android, Native, iOS and Desktop applications using the same codebase.

Project Structure

api

Interfaces, annotations and wrapper/helper classes for the MAVLink protocol. These are used by the generator module for creating the enums, messages and dialects. MAVLink classes generated by any other generator would still work with the other modules given that api is implemented correctly.

serialization

Tools for serializing and deserializing MAVLink message/frame Kotlin objects to and from a byte stream. More specifically, it contains extension functions on ByteArray and ByteBuffer to encode and decode different sizes of int, uint, float, strings/characters, arrays, etc.

generator

MAVLink generator Gradle Plugin that uses the api and the serialization modules to generate the MAVLink classes. This plugin is available at Gradle Plugin Portal with ID xyz.urbanmatrix.mavlink.generator. It can be used by anyone to generate MAVLink classes from their own XML files. No need to fork this project.

The instructions on how to use the plugin are available in the generator module.

definitions

Implementation of the standard MAVLink messages from the official MAVLink repository generated using the Gradle plugin.

mavlink-kotlin

Implementation of the api module for the handling of data streams and connections for different protocols. TCP is ready for use, while UDP and serial are in development. It contains the interface MavConnection which acts as the base for various connection types. It is later used by the different adapters to provide a simple end-user API.

adapters

The mavlink-kotlin module itself only handles the connections and provides reader and writer methods to and from byte streams. The adapters module wraps the mavlink-kotlin connection classes to provide an abstraction layer that makes the handling of the connections easy.

Users can use the MavConnection interface to create an adapter of their own. Some adapters already available are:

• RxJava2
• RxJava3
• Kotlin Coroutines

Detailed instructions on how to use these are available in the respective adapter directories.