plugins {
    id 'jacoco'
}

subprojects {
    apply plugin: 'java'
    apply plugin: 'jacoco'

    repositories {
        mavenCentral()
    }

    dependencies {
        testImplementation 'org.junit.jupiter:junit-jupiter-api:5.10.0'
        testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.10.0'
    }

    test {
        useJUnitPlatform()
        finalizedBy jacocoTestReport
    }

    jacoco {
        toolVersion = "0.8.11"
    }

    jacocoTestReport {
        dependsOn test
        reports {
            html.required = true
            xml.required = true
            csv.required = false
            html.outputLocation = file("${rootProject.buildDir}/reports/jacoco/${project.name}")
            xml.outputLocation = file("${rootProject.buildDir}/reports/jacoco/${project.name}/report.xml")
        }
    }

    jacocoTestCoverageVerification {
        violationRules {
            rule {
                limit {
                    minimum = 0.8
                }
            }
        }
    }

    check.dependsOn jacocoTestCoverageVerification
}

実行

./gradlew test jacocoTestReport

サブプロジェクトをまたいで共通部品が使用されている場合
今回の場合はhoge,fugaがpiyoを呼び出している

task jacocoMergeAll(type: JacocoMerge) {
    executionData fileTree(dir: '.', includes: [
        'hoge/build/jacoco/*.exec',
        'fuga/build/jacoco/*.exec'
    ])
    destinationFile = file("$buildDir/jacoco/merged.exec")
}

task jacocoPiyoAggregatedReport(type: JacocoReport, dependsOn: jacocoMergeAll) {
    executionData file("$buildDir/jacoco/merged.exec")

    classDirectories.from = fileTree(dir: "piyo/build/classes/java/main") {
        include '**/*.class'
    }

    sourceDirectories.from = files("piyo/src/main/java")

    reports {
        html.required = true
        xml.required = true
        html.outputLocation = file("$buildDir/reports/jacoco/piyo-aggregated")
        xml.outputLocation = file("$buildDir/reports/jacoco/piyo-aggregated/report.xml")
    }
}

実行

./gradlew :hoge:test :fuga:test
./gradlew jacocoPiyoAggregatedReport

※JacocoMergeをなぜか認識してくれなかったので...

task jacocoPiyoAggregatedReport(type: JacocoReport) {
    dependsOn ':hoge:test', ':fuga:test'

    // ✅ Hoge・Fugaの .exec ファイルを複数読み込む
    executionData.from = files(
        fileTree(dir: 'hoge/build/jacoco', includes: ['*.exec']),
        fileTree(dir: 'fuga/build/jacoco', includes: ['*.exec'])
    ).filter { it.exists() && it.length() > 0 }

    // ✅ Piyo のクラスファイル
    classDirectories.from = files("piyo/build/classes/java/main")
    .asFileTree
    .matching {
        include '**/*.class'
    }


    // ✅ Piyo のソースファイル
    sourceDirectories.from = files("piyo/src/main/java")

    // ✅ 出力先をルート build 配下に集約
    reports {
        html.required = true
        xml.required = true
        html.outputLocation = file("$buildDir/reports/jacoco/piyo-aggregated")
        xml.outputLocation = file("$buildDir/reports/jacoco/piyo-aggregated/report.xml")
    }
}