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plugin

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plugin

Write and apply a custom Gradle plugin. This project also showcases Gradle's buildSrc feature.

Overview

Specifically this subproject defines a custom Gradle plugin that defines a task to list the full path to each runtime dependency of the project.

The task prints the content to a file and separates each path by the colon character (':') so that the file's contents can be used conveniently as the classpath argument to the java command. The availability of this classpath file is a kind of "escape hatch" from Gradle so that we can use handwritten java commands to execute our program. Effectively, we are relying on Gradle as a build tool but not as a run tool (so to speak). Why would we ever want that? Here are some examples where we want to execute a program's source code but not in a traditional ./gradlew run-way:

Instructions

Follow these instructions to build the plugin and use it in the Gradle project.

  1. Pre-requisite: Java 21
  2. Compile the program's source code: 
    • ./gradlew compileJava
    • Notice the class files in build/classes/java/main/
  3. Build the classpath reference file: 
    • ./gradlew listDependencies
    • Notice the file build/runtime-dependencies.txt. Thanks Gradle!
  4. Run the program: 
    • ./run.sh
    • You should see something like this: 
      [main] INFO dgroomes.Main - Hello world from the 'main' method!

Wish List

General clean-ups, TODOs and things I wish to implement for this project:

  • DONE Make the Gradle buildSrc content more idiomatic. It should probably be a plugin.
    • DONE make it a plugin
    • SKIPPED (it's better to show the distinct task executions for illustrative purposes I think) Make the task depend on build for convenience?
    • DONE Use the "plugins" block to apply the plugin
    • SKIPPED (there's no config necessary for this plugin so it doesn't make sense) Use the proper file APIs. See Working with files in custom tasks and plugins
    • DONE Identify tasks inputs and outputs whihc enables Gradle's Incremental Build capability.
  • DONE Delete the JShell component of the example because it was migrated to my jshell-playground repository.

Reference