Apply

MyModulePlugin_Java8.kt

import org.gradle.api.JavaVersion
import org.gradle.api.Plugin
import org.gradle.api.Project
import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

class MyModulePlugin : Plugin<Project> {
  override fun apply(project: Project) {
    ...
    // Configure common android build parameters.
    val androidExtension = project.extensions.getByName("android")
    if (androidExtension is BaseExtension) {
      androidExtension.apply {
        ...
        // Java 8
        compileOptions {
          sourceCompatibility = JavaVersion.VERSION_1_8
          targetCompatibility = JavaVersion.VERSION_1_8
        }
        project.tasks.withType(KotlinCompile::class.java).configureEach {
          kotlinOptions {
            jvmTarget = "1.8"
          }
        }
      }
    }
  }
}