Java To Pi Cross Development With Gradle and Idea

IntelliJ IDEA cross development targeting RaspberryPi (or other Linux Systems) for Java and other JVM languages ( Groovy, Kotlin, ...) using Gradle. This works well for IntelliJ IDEA (community and ultimate), tested on Windows 10 and Linux.

Preparing the Environment

Prerequisites on Pi Site

Scripts on development computer need to run commands on target Pi system:

• Existing ssh login on Pi with private key authentication (see e.g. here)
  Login with ssh at least once from the command line to make sure that the target host key is accepted.

Pi programs often access GPIO hardware or expose privileged ports which needs root privileges:

• Pi user can run sudo commands without entering a password (see e.g. here)

Prerequisites on Windows development system

The provided build scripts use several standard unix tools that have to be made available on Windows. Scripts use ssh, scp, sed and unzip.

The easiest way found to make these available is installing Git tools and adding the Git usr\bin subdirectory to your path. In a typical installation this is C:\Program Files\Git\usr\bin. (See e.g. here)

Prerequisites on linux development system

Scripts should work out of the box

Installing the Gradle Tasks

We are using a quite basic way to integrate cross development tasks into the Gradle build script build.gradle:

• Tasks and helper methods are coded in crossdev.gradle
• Site specific parameters are set in gradle.properties

The easiest way to use the cross development tasks is to apply crossdev.gradle from this project repository into your project. This is done by including the following line into your build.gradle configuration file.

apply from: 'https://raw.githubusercontent.com/geobe/Java2PiWithIdea/master/crossdev.gradle'

Alternatively, you can also clone this repository, test, if the simple RaspiHello program is running on your target and substitute the example specific code with your development.

Make distZip gradle task available

Be sure that the gradle standard task distZip is available to your builds. It is available, if your plugins closure contains the application plugin, e.g.:

 plugins {
     id 'application'
     id 'java'
 }

It is missing if only the java plugin is configured like this, so change it:

plugins {
     id 'java'
}

The distZip task needs to know your main class to generate a shell script or .bat file to start your program on the target. So make sure your build.gradle defines a main class like in the example:
mainClassName = 'de.geobe.java2pi.hello.HelloPigpio'

Configuration and Use

Project Specific Values in gradle.properties

You will have to set at least remoteHome and keyfilepath to your local requirements

# user@ip or user@url of remote machine*
remoteHome = [email protected]
# path to ssh key file for target login 
# remember to use / as file separators
# on linux typically ~/.ssh/id_rsa
keyfilepath= d:/usr/geb/.ssh/id_rsa
# distribution path on target machine, will be created if not existing
targetpath = xdev/runjava
# output directory of build process
buildOutput = build/distributions
# set true for running with sudo on target machine, 
# any other value will run as normal user
runsudo = true

Cross Development Tasks in crossdev.gradle

The following tasks are provided:

1. testRemoteLogin
   Login to target machine and show some identifying output on standard output.
   Intended to check if target can be reached with given configuration. Remember to login using ssh at least once from the command line!
2. distAll
   Distribute built java program and all needed libraries to the target:
   • Create target path on target machine, if needed
   • Unzip the distribution zip file in the buildOutput directory
   • Remote copy all the unzipped files from buildOutput to targetpath on target machine
   • Edit the generated startup shell script to run java program as sudo on target
   • Make startup shell script executable
3. distAllAndRunRemote
   Like distAll, but also run program on target. The number shown on standard output is the process id (pid) of the remote program. You can use it to sudo kill -9 pid kill the program if it hangs. If developing a web application, it is very helpful to make it have a shutdown menu (at least during development) because remote runs
   (xxxRunRemote tasks) cannot be stopped from the development machine.
4. distBuild
   Distribute only the jar file of latest build to the target, assuming that everything else was already distributed before and has not changed (i.e. no changed resources or added library jars). This task will only work if distAll had already been used before.
5. distBuildAndRunRemote
   Distribute the latest build and run it, similar to distAllAndRunRemote.