Java UIO offers robust Java interfaces optimized for Linux Userspace IO, emphasizing high performance. Engineered from scratch, it leverages contemporary kernel APIs, libraries, and cutting-edge code generation methodologies, embodying a state-of-the-art cross-platform solution. Rather than reinventing established paradigms, Java UIO harnesses existing standards, avoiding redundant development commonly observed in other IO libraries. Seamlessly supporting JDK 21 LTS, this framework integrates Project Lombok to streamline code, enhancing readability and conciseness.

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• Periphery API for GPIO, LED, PWM, SPI, I2C, MMIO and Serial peripheral I/O interface access. Based on c-periphery API which also covers C, C++, Python, Lua and Dart. Cross platform high speed MMIO based GPIO included.

• U8g2 API for monochrome displays. Based on U8g2: Library for monochrome displays, version 2. I added ability to use multiple displays in a thread safe way and dramatically improved software driven I2C and SPI performance at the C level. You can also use SDL 2 based display to develop on a desktop without physical display attached. You can choose display type and fonts at runtime.

• Tools provides tools for mapping MMIO GPIO register mapping, code generation, etc.

• Demo provides CLI based demos instead of using mocks or hard coded pins, busses, etc.

• An install script gives you a complete install of JDK, Maven, HawtJNI fork and any required projects, so there is no guessing. This can be run on the target platform such as Armbian using Ubuntu. All testing has been done on Armbian and Ubuntu targets while development is done on x86 and Ubuntu.

• HawtJNI generates JNI code to reduce errors and time hand coding. Updating code is also much easier.

  • Generates JNI source code.
  • Generates an autoconf and msbuild source project to build the native library. This gets attached to the Maven project as as the native source zip file.
  • Builds the native source tar for the current platform.
  • Built native library is stored in a platform specific jar. This gets attached to the Maven project as a platform specific jar file.

• Why Linux userspace? This is really the only way to get cross platform libraries to work since most SBCs have different chip sets. The trade off is performance compared to native C written to specific chip sets. However, since I'm wrapping C with JNI it guarantees the fastest userspace experience for Java.

• Why Armbian? Because Armbian supports many SBCs and the idea is to be truly SBC cross platform. See downloads.

• Why Java 21? Because Java 21 is the current LTS version of Java.I'm only moving forward with Java. You can always create a fork and make a version of Java UIO based on another version.

• Why OpenJDK? Because it's easy to download without all the crap Oracle puts you through. You can always use another JDK 21 vendor, but you will have to do that manually.

SBC configuration

• If you are using Armbian then use armbian-config or edit /boot/armbianEnv.txt to configure various devices. Userspace devices are exposed through /dev or /sys. Verify the device is showing up prior to trying demo apps.
  • sudo apt install armbian-config
• If you are not using Armbian then you will need to know how to configure devices to be exposed to userspace for your Linux distribution and SBC model. Check each log in scripts directory to be sure there were no errors after running install.sh.
• Since linux 4.8 the GPIO sysfs interface is deprecated. Userspace should use the character device instead.
• I have tested 32 bit and 64 bit boards using the latest Armbian release or in the case of the Raspberry Pi Ubuntu Server. The ability to switch seamlessly between boards gives you a wide range of SBC choices.

Non-root access

Non-root access is provided by a systemd service called uio-permissions that applies permissions for GPIO, I2C, SPI, serial and system LEDs. GPIO sysfs (deprecated) and PWM permissions are provided by udev rules. Tweak uio-permissions.sh 98-sysfs.rules and 99-pwm.rules as needed.

Download project

• sudo apt install git
• cd ~/
• git clone --depth 1 https://github.com/sgjava/javauio.git

Install script

The install script assumes a clean OS install. If you would like to install on a OS with your own version of Java 21, etc. then you can look at what install.sh does and do it manually. What does the script do?

• Install build dependencies
• Install UIO Permissions Service and udev rules
• Installs Open JDK 21 to /usr/lib/jvm
• Installs Maven to /opt
• Build HawtJNI (using my fork that works with JDK 21)
• Download and copy c-periphery source to Maven module
• Download and copy U8g2 source to Maven module
• Build Java UIO

Run script

• cd ~/javauio/scripts
• ./install.sh
• Check various log files if you have issues running the demo code. Something could have gone wrong during the build/bindings generation processes.
  • tail -n 1 install.log total build time
  • tail -n 12 javauio.log make sure all modules show SUCCESS
• Make sure you use latest gpio.h
• If you need to recompile use
  • cd ~/javauio
  • mvn clean install

How to work on Java UIO project

I'm using NetBeans to develop, but this is a standard Maven multi-module project. The easiest way to setup your environment is create a Ubuntu 22.04 desktop VM (VirtualBox, etc.) and follow install process since it installs JVM, Maven, all projects, etc. Then you just need to install NetBeans (or Eclipse, etc.). Since Java UIO relies on c-periphery and u8g2 those files are not included in the project. You can see them ignored in .gitignore.

• Copy the c-periphery files to javauio/periphery/src/main/native-package/src
• Copy u8g2 files to javauio/u8g2/src/main/native-package/src

Now you should be ready to compile the project with mvn clean install. To use NetBeans right-click Run on a program in Demo project modify project properties:

• Click actions
• Click Run file via main()
  • exec.classpath=/home/youruser/NetBeansProjects/javauio/u8g2/target/u8g2-1.0.0-SNAPSHOT-linux64.jar:/home/youruser/NetBeansProjects/javauio/periphery/target/periphery-1.0.0-SNAPSHOT-linux64.jar
  • exec.appArgs=--type=SDL
• Click OK

Now if you run SimpleText it will use SDL instead of a real display. You will need to tweak for Periphery demo programs as well. Basically look at the command line arguments in the examples provided.