Build and run Kotlin from command line using gradle

Install Kotlin if it’s not installed yet.

Install gradle if it’s not installed yet. This is for installing gradle on mac os.

brew install gradle

Create a root folder for this sample project, and create directories for Kotlin classes and Kotlin test classes.

mkdir gradle_example
cd gradle_example
mkdir -p src/test/kotlin/com/example
mkdir -p src/main/kotlin/com/example/math

Create build.gradle at the root folder gradle_example, this is the configuration file to tell gradle command line tool how to build and run things in this gradle_example folder.

buildscript {
    ext.kotlin_version = '1.2.60'
    repositories {
        mavenCentral()
        jcenter()
    }

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

apply plugin: 'kotlin'
apply plugin: 'application'

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    testImplementation("junit:junit:4.12") 
}

sourceSets {
    main.kotlin.srcDirs += 'src/main/kotlin'
    main.java.srcDirs += 'src/main/java'
}

// This tells gradle to execute src/test/kotlin/com/example/Hello.kt 
// when 'gradle run' is entered on the command line at the root folder
mainClassName = 'com.example.HelloKt'

// This tells gradle to execute src/test/kotlin/com/example/Double.kt
// when 'gradle getDouble' entered on the command line at the root folder 
// If there are more than one args needed, enter them like this: args "one", "two", "3"
// If the class name is Double.kt, it need to be specified as DoubleKt
task getDouble(type: JavaExec) {
  classpath sourceSets.main.runtimeClasspath
  main = "com.example.DoubleKt"
  args "5"
}

// Another task, just like the getDouble above
task getSquare(type: JavaExec) {
  classpath sourceSets.main.runtimeClasspath
  main = "com.example.SquareKt"
  args "5"
}

// Another task, just like the getDouble above
task getRandom(type: JavaExec) {
  classpath sourceSets.main.runtimeClasspath
  main = "com.example.GetRandomKt"
}

Try these gradle commands from the root folder gradle_example. We have defined the tasks but haven’t create any Kotlin code for these tasks. Hence most of them should fail until the following files are created.

gradle clean
gradle build
gradle tasks --all
gradle run
gradle test
gradle getDouble
gradle getSquare
gradle getRandom
gradle run getDouble getSquare getRandom

Create src/test/kotlin/com/example/Hello.kt

package com.example
fun main(args: Array) {
  println("Hello Gradle!")
  println("Build          : gradle build")
  println("Clean          : gradle clean")
  println("List tasks     : gradle tasks --all")
  println("Run main       : gradle run")
  println("Run test       : gradle test")
  println("Run getDouble  : gradle getDouble")
  println("Run getSquare  : gradle getSquare")
  println("Run getRandom  : gradle getRandom")
  println("Run mulitasks  : gradle run getDouble getSquare getRandom")
}

Create src/test/kotlin/com/example/math/MyMathUtil.kt, this Kotlin class will be imported into the other Kotlin classes.

package com.example.math

import java.util.Random

class MyMathUtil {
  
  fun square(n : Int) : Int {
    return n*n
  }

  fun double(n : Int) : Int {
    return n*2
  }

  fun randomInt() : Int {
    return Random().nextInt();
  }
}

Create src/test/kotlin/com/example/math/Double.kt

package com.example

import com.example.math.MyMathUtil

fun main(args : Array) {
  val num = if (args.isNotEmpty()) args[0].toIntOrNull() else null
  if (num == null) {
    println("Please provide a number!")
    return
  }

  val doubleOfNum = MyMathUtil().double(num)
  println("The double of $num is $doubleOfNum")
}

Create src/test/kotlin/com/example/math/Square.kt

package com.example

import com.example.math.MyMathUtil

fun main(args : Array) {
  val num = if (args.isNotEmpty()) args[0].toIntOrNull() else null
  if (num == null) {
    println("Please provide a number!")
    return
  }

  val squareOfNum = MyMathUtil().square(num)
  println("The square of $num is $squareOfNum")
}

Create src/test/kotlin/com/example/math/GetRandom.kt

package com.example

import com.example.math.MyMathUtil

fun main(args : Array) {
  println("Here is a random number: ${MyMathUtil().randomInt()}")
}

Create src/test/kotlin/com/example/MyMathUtilTest.kt

package com.example

import org.junit.Assert.assertEquals
import org.junit.Test
import com.example.math.MyMathUtil

class MyMathUtilTest {
    
    @Test 
    fun testSquare() {
        assertEquals(100, MyMathUtil().square(10))
    }

    @Test
    fun testDouble() {
      assertEquals(20, MyMathUtil().double(10))
    }

    @Test
    fun testRandom() {
      val randomInt1 = MyMathUtil().randomInt()
      val randomInt2 = MyMathUtil().randomInt()
      assertEquals(true, randomInt1 != randomInt2)
    }

    @Test
    fun failTest() {
        assertEquals(1000, MyMathUtil().square(10))
    }

}

Now, if you enter gradle run getDouble getSquare getRandom from the command line, it should print the following. If you just want to run getDoble, enter gradle getDouble. Any news changes in the corresponding Kotlin file should be reflected, no need to do gradle build every time the Kotlin code is updated.

> Task :run
Hello Gradle!
Build          : gradle build
Clean          : gradle clean
List tasks     : gradle tasks --all
Run main       : gradle run
Run test       : gradle test
Run getDouble  : gradle getDouble
Run getSquare  : gradle getSquare
Run getRandom  : gradle getRandom
Run mulitasks  : gradle run getDouble getSquare getRandom

> Task :getDouble
The double of 5 is 10

> Task :getSquare
The square of 5 is 25

> Task :getRandom
Here is a random number: -2032204043

BUILD SUCCESSFUL in 1s
5 actionable tasks: 4 executed, 1 up-to-date

Complete example in Github