I can’t run a compiled Kotlin class. When I try to run it like I would a normal Java class I get the following:

C:\Users\User\Desktop>java _DefaultPackage

Exception in thread "main" java.lang.NoClassDefFoundError: jet/runtime/Intrinsics
    at _DefaultPackage.main(Finder.kt)
Caused by: java.lang.ClassNotFoundException: jet.runtime.Intrinsics
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
    ... 1 more

This led me to believe that the Kotlin runtime was simply missing, given that output. So I tried the following:

C:\Users\User\Desktop>java -cp kotlin-runtime.jar _DefaultPackage

Error: Could not find or load main class _DefaultPackage

Which made me think that maybe I needed to add the class file to my declared classpath so:

C:\Users\User\Desktop>java -cp kotlin-runtime.jar';_DefaultPackage.class _DefaultPackage

Error: Could not find or load main class _DefaultPackage

What is missing?

Solution

To run a Kotlin class you are actually running a special class that is created at the file level that hold your main() and other functions that are top-level (outside of a class or interface). So if your code is:

// file App.kt
package com.my.stuff

public fun main(args: Array<String>) {
  ...
}

Then you can execute the program by running the com.my.stuff.AppKt class. This name is derived from your filename with Kt appended. You can change the name of this class within the file by adding this file-targeted annotation:

@file:JvmName("Foo")  

You can also put your main() into a class with a companion object and make it static using the JvmStatic annotation. Therefore your class name is the one you chose:

// file App.kt
package com.my.stuff

public class MyApp {
    companion object {
        @JvmStatic public fun main(args: Array<String>) {
          ...
        }
    }
}

Now you just run the class com.my.stuff.MyApp

What other JAR files do I need?

You need your application JAR and any dependencies. For Kotlin specific JARs when outside of Maven/Gradle you need a Kotlin distribution which contains:

  • kotlin-runtime.jar (combined runtime and stdlib)
  • kotlin-reflect.jar only if using Kotlin reflection
  • kotlin-test.jar for unit tests that use Kotlin assertion classes

Within Maven/Gradle currently there is also a separate kotlin-stdlib.jar

Running from Intellij

If in Intellij (if it is your IDE) you can right click on the main() function and select Run, it will create a runtime configuration for you and show the fully qualified class name that will be used. You can always use that if you are unsure of the name of the generated class.

Running from Gradle

You can also use the Gradle Application plugin to run a process from Gradle, or to create a runnable system that includes a zip/tgz of your JAR and all of its dependencies, and a startup script. Using the example class above, you would add this to your build.gradle:

apply plugin: 'application'

mainClassName = 'com.my.stuff.AppKt'

// optional:  add one string per argument you want as the default JVM args
applicationDefaultJvmArgs = ["-Xms512m", "-Xmx1g"] 

And then from the command-line use:

// run the program
gradle run

// debug the program
gradle run --debug-jvm

// create a distribution (distTar, distZip, installDist, ...)
gradle distTar

Running Directly from Java Command-line

If you have a runnable JAR, and assuming KOTLIN_LIB points to a directory where Kotlin runtime library files reside:

java -cp $KOTLIN_LIB/kotlin-runtime.jar:MyApp.jar com.my.stuff.AppKt

See the notes above about other JAR files you might need. A slight variation if you have a runnable JAR (with the manifest pointing at com.my.stuff.AppKt as the main class):

java -cp $KOTLIN_LIB/kotlin-runtime.jar -jar MyApp.jar

Running using the Kotlin command-line tool

If you install Kotlin tools via Homebrew or other package manager. (on Mac OS X brew update ; brew install kotlin) Then it is very simple to run:

kotlin -cp MyApp.jar com.my.stuff.AppKt

This command adds the runtime to the classpath provided, then runs the class. You may need to add additional Kotlin libraries as mentioned in the section above “Running from Java.”

Creating runnable JAR with the Kotlin compiler

This is not very common since most people use other build tools, but the Kotlin compiler can create a runnable Jar that solves this for you (see http://kotlinlang.org/docs/tutorials/command-line.html) when it bundles the runtime and your code together. Although this isn’t as common when using tools such as Maven and Gradle, or IDE builds. Then run using the normal Java:

java -jar MyApp.jar