Gradle jacoco coverage report with more than one submodule(s)?

Does anybody know how to configure a gradle file for java jacoco report that contain codecoverage of more than one gradle submodule?

my current approach only shows codecoverage of the current submodule but not codecoverage of a sibling-submodul.

I have this project structure

- build.gradle (1)
- corelib/
    - build.gradle (2)
    - src/main/java/package/Core.java
- extlib/
    - build.gradle (3)
    - src/main/java/package/Ext.java
    - src/test/java/package/Integrationtest.java

when i execute gradlew :extlib:check :extlib:jacocoTestReport the junit-test "Integrationtest.java" is executed and a codecoverage report is generated that does not contain codecoverage for corelib classes like Core.java

The result should include the codecoverage of Ext.java and Core.java

I already read

• https://docs.gradle.org/current/userguide/jacoco_plugin.html
• and Filter JaCoCo coverage reports with Gradle

but found no clues

here is content of the gradle files

// root build.gradle (1)
// Top-level build file where you can add configuration options 
// common to all sub-projects/modules.
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.0'
    }
}
allprojects {
    repositories {
        jcenter()
    }
}

// build.gradle (2) subproject build file for corelib.
apply plugin: 'java'
apply plugin: 'jacoco'

dependencies {
}

jacocoTestReport {
    reports {
        xml.enabled true
        html.enabled true
    }
}

// build.gradle (3) subproject build file for extlib.
apply plugin: 'java'
apply plugin: 'jacoco'

dependencies {
    compile project(':corelib')
    testCompile 'junit:junit:4.11'

    // this does not compile
    // jacocoTestReport project(':pixymeta-core-lib')
}

jacocoTestReport {
    reports {
        xml.enabled true
        html.enabled true
    }
}

// workaround because android-studio does not make the test resources available
// see https://code.google.com/p/android/issues/detail?id=64887#c13
task copyTestResources(type: Copy) {
    from sourceSets.test.resources
    into sourceSets.test.output.classesDir
}
processTestResources.dependsOn copyTestResources

[Update 2016-08-01]

thanks to @Benjamin Muschko i also tried in the root gradle file

// https://discuss.gradle.org/t/merge-jacoco-coverage-reports-for-multiproject-setups/12100/6
// https://docs.gradle.org/current/dsl/org.gradle.testing.jacoco.tasks.JacocoMerge.html
task jacocoMerge(type: JacocoMerge) {
   subprojects.each { subproject ->
      executionData subproject.tasks.withType(Test)
   } 

}

but got error message (with gradle-2.14)

* What went wrong:
Some problems were found with the configuration of task ':jacocoMerge'.
> No value has been specified for property 'jacocoClasspath'.
> No value has been specified for property 'executionData'.
> No value has been specified for property 'destinationFile'.

there is also the gradle plugin https://github.com/paveldudka/JacocoEverywhere where i have asked for mulit-submodule support https://github.com/paveldudka/JacocoEverywhere/issues/16

[update 2016-08-01] i found a working solution based on https://github.com/palantir/gradle-jacoco-coverage

See my own answer below