Configure Jacoco with gradle and kotlin DSL

src/main/java/org/example/A.java:

package org.example;

class A {
}

src/main/java/org/example/B.java:

package org.example;

class B {
}

src/test/java/org/example/ExampleTest.java:

package org.example;

public class ExampleTest {
  @org.junit.Test
  public void test() {
    new A();
    new B();
  }
}

build.gradle.kts:

plugins {
  java
  jacoco
}

repositories {
  mavenCentral()
}

dependencies {
  testCompile("junit:junit:4.12")
}

using Gradle 5.4.1 execution of gradle test jacocoTestReport produces following report

after addition to build.gradle.kts

tasks.withType<JacocoReport> {
  classDirectories.setFrom(
    sourceSets.main.get().output.asFileTree.matching {
      exclude("org/example/B.class")
    }
  )
}

execution of the same command produces following report

Just to add to @Godin's awesome answer:

The way @Godin explained it, you would have to run gradle test jacocoTestReport which isn't bad but If you want jacoco to run when you run just with gradle test add this to your build.gralde.kts:

tasks.test {
    finalizedBy("jacocoTestReport")
    doLast {
        println("View code coverage at:")
        println("file://$buildDir/reports/jacoco/test/html/index.html")
    }
}