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= GraalVM Native Images
:description: Helidon SE Native Images
:keywords: helidon, guide, graalvm
:helidon-tag: https://github.com/oracle/helidon/tree/{helidon-version}
:quickstart-example: {helidon-tag}/examples/quickstarts/helidon-quickstart-se
:docker-multi-stage-url: https://docs.docker.com/develop/develop-images/multistage-build/
:openjdk-docker-217: https://github.com/docker-library/openjdk/issues/217

This guide describes how to build a GraalVM native image for a Helidon SE application.

== Introduction

https://www.graalvm.org/docs/reference-manual/aot-compilation/[Native images] are ahead-of-time compiled Java code that result in a self
contained native executable. When used appropriately native images have dramatically faster
startup and lower runtime memory overhead compared to a Java VM.

In this guide you will learn how to build a native image locally on your machine, as well as using Docker.

== What You Need

|===
|About 10 minutes
| <<about/03_prerequisites.adoc,Helidon Prerequisites>>
| GraalVM CE https://www.graalvm.org/downloads[19.2+ or 19.3+]
|===

== Install GraalVM and the Native Image Command

After https://github.com/oracle/graal/releases[downloading and installing] GraalVM,
set the `GRAALVM_HOME` environment variable to point at your GraalVM installation.

[source,bash]
----
# Your path might be different
export GRAALVM_HOME=/usr/local/graalvm-ce-19.2.0/Contents/Home/
----

Then install the optional `native-image` command:

[source,bash]
----
$GRAALVM_HOME/bin/gu install native-image
----

And verify:

[source,bash]
----
$GRAALVM_HOME/bin/java -version
$GRAALVM_HOME/bin/native-image --version
----

== Generate The Project

Generate the project using the Helidon SE Quickstart Maven archetype.

[source,bash,subs="attributes+"]
----
mvn archetype:generate -DinteractiveMode=false \
    -DarchetypeGroupId=io.helidon.archetypes \
    -DarchetypeArtifactId=helidon-quickstart-se \
    -DarchetypeVersion={helidon-version} \
    -DgroupId=io.helidon.examples \
    -DartifactId=helidon-quickstart-se \
    -Dpackage=io.helidon.examples.quickstart.se
----

The archetype generates a Maven project in your current directory
(for example, `helidon-quickstart-se`). Change into this directory and build.

[source,bash]
----
cd helidon-quickstart-se
mvn package
----

At this point you can run the application using the JVM:

[source,bash]
----
java -jar target/helidon-quickstart-se.jar
----

In another shell test an endpoint:

[source,bash]
----
curl -X GET http://localhost:8080/greet
----

The application should respond with `{"message":"Hello World!"}`

Now stop the running application (by pressing Ctrl+C).

For more information about the Quickstart application and other enpoints it supports see the
<<guides/02_quickstart-se.adoc,Helidon SE Quickstart Guide>>.

== Building a Native Image

You can build a native executable in 2 different ways:

* With a local installation of GraalVM
* Using Docker

=== Local build

Make sure you have GraalVM locally installed:

[source,bash]
----
$GRAALVM_HOME/bin/native-image --version
----

Build the native image using the native image profile:

[source,bash]
----
mvn package -Pnative-image
----

[TIP]
This uses the `helidon-maven-plugin` to perform the native compilation using your installed
copy of GraalVM. It might take a little while to complete.

Once it completes start the application using the native executable (no JVM!):

[source,bash]
----
./target/helidon-quickstart-se
----

Yep, it starts fast. You can exercise the application's endpoints as before.

=== Multi-stage Docker build

Build the "native" Docker Image

[source,bash]
----
docker build -t helidon-quickstart-se-native -f Dockerfile.native .
----

[TIP]
This does a full build inside the Docker container. The first
time you run it, it will take a while because it is downloading all
of the Maven dependencies and caching them in a Docker layer.
Subsequent builds will be much faster as long as you don't change
the `pom.xml` file. If the pom is modified then the dependencies
will be re-downloaded.

Start the application:

[source,bash]
----
docker run --rm -p 8080:8080 helidon-quickstart-se-native:latest
----

Again, it starts fast. You can exercise the application's endpoints as before.

== How small and fast is this?

First let's take a look at the Docker image size:

[source,bash]
----
docker images helidon-quickstart-se-native:latest
----

----
REPOSITORY             TAG       IMAGE ID       CREATED        SIZE
quickstart-se-native   latest    1227ac82d199   5 days ago     21.4MB
----

That's much smaller than a Docker image with the application code plus
the JRE and operating system files needed to run it.

Startup times are quite impressive. These are just some informal numbers
to show the magnitude of speed-up. Your mileage may vary:

|===
|Helidon SE Quickstart Java VM|0.921 seconds
|Helidon SE Quickstart Native|0.026 seconds
|===

The startup time for Helidon SE with the normal Java VM is quite good
at under a second. But the native executable is an order of magnitude
faster. Memory footprint is similarly improved.

== When should I use Native Images?

Native images are ideal for applications with high horizontal scalability requirements where
the ability to rapidly scale out to numerous instances is important.

That said, native images do have some https://github.com/oracle/graal/blob/master/substratevm/LIMITATIONS.md[limitations],
and for long running applications where startup and footprint are less of a priority, the Java SE
HotSpot VM might be more appropriate.

== What about Helidon MP?

Currently GraalVM native image support is only available for Helidon SE.
