GitRevisionPlugin.gradle

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath("org.eclipse.jgit:org.eclipse.jgit:4.0.0.201505050340-m2")
    }
}

apply plugin: GitRevisionPlugin

import org.eclipse.jgit.storage.file.*
import org.eclipse.jgit.api.*
import org.eclipse.jgit.lib.*
import org.eclipse.jgit.revwalk.*

class GitRevisionTask extends DefaultTask {

    def revisionFile = "revision.properties"

    def getOutputFile() {
        def dir = project.convention.plugins['java'].sourceSets.main.output.resourcesDir
        return new File(dir, revisionFile)
    }

    def getGitRepo() {
        try {
            return new FileRepositoryBuilder()
                .findGitDir()
                .build()
        } catch(IllegalArgumentException e) {
            logger.warn '=' * 64
            logger.warn '[warn] git-repo not found. skip to generate the revision file'
            logger.warn '=' * 64
        }
        return null
    }

    @TaskAction
    def action() {
        def repo = getGitRepo()
        if (repo == null) { return }

        def head = repo.resolve(Constants.HEAD);
        def git = new Git(repo)
        Iterable<RevCommit> commits = git.log().add(head).setMaxCount(1).call();
        def revision = commits.iterator().next().id.toString().split("[\t ]+")[1]
        getOutputFile().withWriter { it << "revision=${revision}\n" }
    }
}


class GitRevisionPlugin implements Plugin<Project> {
    void apply(Project project) {
        GitRevisionTask task = project.getTasks()
            .create("generateGitRevisionProperties", GitRevisionTask.class);
        task.setDescription("Generate the git revision properties file into output resource directory")
        task.setGroup(BasePlugin.BUILD_GROUP);
        project.getTasks().getByName(JavaPlugin.JAR_TASK_NAME).dependsOn(task);
    }
}