Build a Native Image of Java and Kotlin Application

This example demonstrates how to compile a Java and Kotlin application ahead-of-time into a native executable, and what is the advantage.

Preparation #

This example requires Maven build tool.

1. Download or clone the repository and navigate into the java-kotlin-aot directory:

git clone https://github.com/graalvm/graalvm-demos
cd graalvm-demos/java-kotlin-aot

This is a simple Java and Kotlin application showing how easy it is to interoperate between JVM-based languages. A Java method accesses a String from Kotlin and calls a Kotlin function, which later accesses a String from a Java class. Before running this example, you need to build the application. Note that you can use any JDK for building the application, however javac from GraalVM in the build script is used to simplify the prerequisites and not to depend on another JDK installed.

2. Having installed GraalVM, export the home directory as the $GRAALVM_HOME and add $GRAALVM_HOME/bin to the path, using a command-line shell for Linux:

export GRAALVM_HOME=/home/${current_user}/path/to/graalvm

and for macOS:

export GRAALVM_HOME=/Users/${current_user}/path/to/graalvm/Contents/Home

Note that your paths are likely to be different depending on the download location.

3. Install Native Image to make use of the native-image utility.

4. Then execute:

./build.sh

Have a look at the build.sh script which creates a native image from a Java class. The native-image utility compiles the application ahead-of-time for faster startup and lower general overhead at runtime.

$GRAALVM_HOME/bin/native-image -cp ./target/mixed-code-hello-world-1.0-SNAPSHOT.jar -H:Name=helloworld -H:Class=hello.JavaHello -H:+ReportUnsupportedElementsAtRuntime --allow-incomplete-classpath

It takes a couple of parameters, the classpath, the main class of the application with -H:Class=... and the name of the resulting executable with -H:Name=....

After executing the native-image command, check the directory, it should have produced an executable file helloworld.

Running the Application #

To run the application, you need to execute the JAR file in the target dir. You can run it as a normal Java application using java. Or, since we have a native image prepared, you can run that directly. The run.sh file executes both, and times them with the time utility.

java -cp ./target/mixed-code-hello-world-1.0-SNAPSHOT-jar-with-dependencies.jar hello.JavaHello
./helloworld

The output close to the following should be produced:

→ ./run.sh
+ java -cp ./target/mixed-code-hello-world-1.0-SNAPSHOT-jar-with-dependencies.jar hello.JavaHello
Hello from Kotlin!
Hello from Java!

real	0m0.129s
user	0m0.094s
sys	0m0.034s
+ ./helloworld
Hello from Kotlin!
Hello from Java!

real	0m0.010s
user	0m0.003s
sys	0m0.004s

The performance gain of the native version is largely due to the faster startup.

License #

This sample application is taken from the JetBrains Kotlin-examples repository. It is distributed under the Apache License 2.0.