Build as a FAT JAR with Maven

About FAT JARs

With Pi4J V1 you can create a so-called FAT JAR, which packages all the dependencies into one jar-file. That way it is very easy to build your project on one computer and distribute your application as a single file to one or more clients.

Because of the modular approach and how Pi4J V.2 loads it dependencies at runtime, this approach can be achieved by using the maven-shade-plugin.

Example FAT JAR project

Check the example project (link on top of this page) for the full README, pom.xml and sources.

Maven plugins

Three plugins are used in pom.xml to create the FAT JAR:


        The Compiler Plugin is used to compile the sources of your project.

        This plugin provides the capability to build (executable) jars and is used here to set the mainClass
        which will start the application.

        This plugin provides the capability to package the artifact in an uber-jar, including its dependencies and
        to shade - i.e. rename - the packages of some of the dependencies. The transformer will combine the files
        in the directories of multiple Pi4J plugins with the same package name into one file.
                    <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>

By using maven-shade-plugin the correct files are generated.

As a jar-file is actually a zip-file, we can easily check the contents of the FAT JAR after it has been created with mvn package:

  • directories
    • com
    • lib
    • META-INF
    • org
  • files
    • LICENSE.txt
    • NOTICE.txt

Loading of the Pi4J modules

Pi4J V.2 uses ServiceLoader to detect which modules are available to communicate with the GPIOs. This allows to very dynamically extend the possibilities of the framework.

Code extract from pi4j-core/src/…/runtime/impl/

// detect available Pi4J Plugins by scanning the classpath looking for plugin instances
var plugins = ServiceLoader.load(Plugin.class);

Thanks to the maven-shade-plugin, each Pi4J plugin that is part of the project, is included in META-INF/services/com.pi4j.extension.Plugin:


When running this application we can indeed see the loaded platform and provider plugins in the logs:

[main] INFO com.pi4j.util.Console - 
[main] INFO com.pi4j.util.Console - ---------------------------
[main] INFO com.pi4j.util.Console - |  Pi4J DEFAULT PLATFORM  |
[main] INFO com.pi4j.util.Console - ---------------------------
[main] INFO com.pi4j.util.Console - 
PLATFORM: "RaspberryPi Platform" {raspberrypi} <com.pi4j.plugin.raspberrypi.platform.RaspberryPiPlatform> {Pi4J Platform for the RaspberryPi series of products.} 
[main] INFO com.pi4j.util.Console - 
[main] INFO com.pi4j.util.Console - --------------------
[main] INFO com.pi4j.util.Console - |  Pi4J PROVIDERS  |
[main] INFO com.pi4j.util.Console - --------------------
[main] INFO com.pi4j.util.Console - 
PROVIDERS: [12] "I/O Providers" <com.pi4j.provider.impl.DefaultProviders> 
├─SPI: [2] <> 
│ ├─PROVIDER: "PiGpio SPI Provider" {pigpio-spi} <com.pi4j.plugin.pigpio.provider.spi.PiGpioSpiProviderImpl> {com.pi4j.plugin.pigpio.provider.spi.PiGpioSpiProviderImpl} 
│ └─PROVIDER: "RaspberryPi SPI Provider" {raspberrypi-spi} <com.pi4j.plugin.raspberrypi.provider.spi.RpiSpiProviderImpl> {com.pi4j.plugin.raspberrypi.provider.spi.RpiSpiProviderImpl} 
├─ANALOG_INPUT: [0] <> 
├─SERIAL: [2] <> 
│ ├─PROVIDER: "PiGpio Serial Provider" {pigpio-serial} <com.pi4j.plugin.pigpio.provider.serial.PiGpioSerialProviderImpl> {com.pi4j.plugin.pigpio.provider.serial.PiGpioSerialProviderImpl} 
│ └─PROVIDER: "RaspberryPi Serial Provider" {raspberrypi-serial} <com.pi4j.plugin.raspberrypi.provider.serial.RpiSerialProviderImpl> {com.pi4j.plugin.raspberrypi.provider.serial.RpiSerialProviderImpl} 
├─DIGITAL_INPUT: [2] <> 
│ ├─PROVIDER: "RaspberryPi Digital Input (GPIO) Provider" {raspberrypi-digital-input} <> {} 
│ └─PROVIDER: "PiGpio Digital Input (GPIO) Provider" {pigpio-digital-input} <> {} 
├─I2C: [2] <> 
│ ├─PROVIDER: "RaspberryPi I2C Provider" {raspberrypi-i2c} <com.pi4j.plugin.raspberrypi.provider.i2c.RpiI2CProviderImpl> {com.pi4j.plugin.raspberrypi.provider.i2c.RpiI2CProviderImpl} 
│ └─PROVIDER: "PiGpio I2C Provider" {pigpio-i2c} <com.pi4j.plugin.pigpio.provider.i2c.PiGpioI2CProviderImpl> {com.pi4j.plugin.pigpio.provider.i2c.PiGpioI2CProviderImpl} 
├─ANALOG_OUTPUT: [0] <> 
│ ├─PROVIDER: "RaspberryPi Digital Output (GPIO) Provider" {raspberrypi-digital-output} <> {} 
│ └─PROVIDER: "PiGpio Digital Output (GPIO) Provider" {pigpio-digital-output} <> {} 
└─PWM: [2] <> 
  ├─PROVIDER: "RaspberryPi PWM Provider" {raspberrypi-pwm} <com.pi4j.plugin.raspberrypi.provider.pwm.RpiPwmProviderImpl> {com.pi4j.plugin.raspberrypi.provider.pwm.RpiPwmProviderImpl} 
  └─PROVIDER: "PiGpio PWM Provider" {pigpio-pwm} <com.pi4j.plugin.pigpio.provider.pwm.PiGpioPwmProviderImpl> {com.pi4j.plugin.pigpio.provider.pwm.PiGpioPwmProviderImpl} 
[main] INFO com.pi4j.util.Console -