I’ve been working with GraalVM for a while now mostly on the Polyglot aspect, but some work has been done in the native image support. Today I’d like to share what it is possible right now, spoiler: run a Docker image with size of 38MB and 10MB of RAM!

Russian translation of this article is available here.

A bit of history

With the 3.x series of Eclipse Vert.x lots of work was done to reduce the ammount of Classloaders, Reflection magic (this is why there are no annotations, for example). This usually is seen as a contra sense in the java world, but it paid off, as we can already do some amazing stuff without having to thinkering much on hacks or workarounds to circunvent the substrate limitations.

Proof of Concept

As a proof of concept, I’ve decided to build a small application based on a presentation I’ve made about 2 years ago at JavaZone. The application does the following:

  • Start a WebSocket client to collect in realtime the unconfirmed bitcoin transactions.
  • Connect to Postgres with the winner of Techempower Benchmarks #16 pg-reactive-client.
  • On unconfirmed transaction save it to postgres.
  • On save publish the UTX map-reduced value to vert.x eventbus
  • Start a HTTP server serve a single page application
  • On server start bridge the eventbus from the server to the webbrowser using sockjs
  • On the browser, render the UTX values in realtime as they arrive from the server websocket.

As you can see there is already a lot happening here, servers, clients, websockets, eventbus. I could keep adding more stuff but it’s a PoC right!

Show me the code

Now that we have the idea, we need to implement it, as I said, this was a remake of an old presentation so the code was almost all done for me. This is how it turned out to be:

final Vertx vertx = Vertx.vertx();
final EventBus eb = vertx.eventBus();
final PgPool postgres = PgClient.pool(...);

  .connect(self -> {
    self.subscribeUnconfirmed(json -> {
        "INSERT INTO UTX (data) VALUES ($1)",
        Tuple.of(Json.create(json)), ar -> {
          int utxValue = 0;
          if (json.containsKey("out")) {
            // reduce step
            for (Object o : json.getJsonArray("out")) {
              utxValue += ((JsonObject) o).getInteger("value", 0);
          eb.publish("data.updates", utxValue);

final Router app = Router.router(vertx);


  .listen(Integer.getInteger("port", 8080));

Error handling and configuration ommited for brevity.

This is a very simple application and in fact this is exactly how you would code it with Vert.x, I did not do any hack to get it to run on Graal SVM.

Build it

Building the code is trivial, but building the image turned out to have some issues. Luckyly the issues are simple and can all be addressed with a simple substitution java file.

So building the native image was as simple as:

native-image \
  -Djava.net.preferIPv4Stack=true \
  -Dvertx.disableDnsResolver=true \
  -H:IncludeResources="(META-INF/services|webroot)/.*" \
  -H:+ReportUnsupportedElementsAtRuntime \
  -H:ReflectionConfigurationFiles=./src/main/svm/reflection.json \
  -jar "target/bitcoin-viewer.jar"

So what is important to see here, first I specify that I prefer the IPV4 stack from the VM (but this is not really a requirement). Second, I disable Vert.x DNS resolver for the reason that it is not working correctly with SVM and vert.x will fallback to the VM default (blocking) DNS resolver which works fine. I then specify what resources I’d like to keep from my fatjar and specify the substitution file.

Once it completes you can see:

-rwxrwxr-x.  1 plopes plopes 24603024 10 aug 11:45 bitcoin-viewer

Your application is now just 24,6MB!!!

Dockerize it

Graal SVM relies on Glibc so this means you can run it on many images such as ubuntu, fedora, centos… but these images are quite big, so why not alpine?

Alpine images are build against musl-libc and this is an issue. Luckily the alpine project has documented how to get glibc working on alpine and there are already some images on docker hub like frolvlad/alpine-glibc.

So lets convert to docker shall we?

# GraalVM docker image used for AoT compilation
FROM panga/graalvm-ce:latest AS build-aot
# Add maven wrapper
ADD mvnw app/
ADD mvnw.cmd app/
ADD .mvn app/.mvn/
# Add settings.xml to allow snapshots
ADD settings.xml root/.m2/
# Add pom
ADD pom.xml app/
# Add sources
ADD src app/src/
# Set working dir
# Build (java side)
RUN ./mvnw -Pnative-image package
# Build image
RUN native-image \
    --no-server \
    -Djava.net.preferIPv4Stack=true \
    -Dvertx.disableDnsResolver=true \
    -H:IncludeResources="(META-INF/services|webroot)/.*" \
    -H:+ReportUnsupportedElementsAtRuntime \
    -H:ReflectionConfigurationFiles=./src/main/svm/reflection.json \
    -jar "target/bitcoin-viewer.jar"
# Create new image from alpine
FROM frolvlad/alpine-glibc:alpine-3.8
RUN apk add --no-cache ca-certificates
# Copy generated native executable from build-aot
COPY --from=build-aot /app/bitcoin-viewer /bitcoin-viewer
# Set the entrypoint
ENTRYPOINT [ "/bitcoin-viewer" ]


So when we start our application and postgres and run docker stats we can see:

c7f5e7af56fe  vertx-bitcoin   0.61%  5.133MiB / 15.54GiB  0.03%
a3536684f175  postgres        1.26%  11.27MiB / 15.54GiB  0.07%

I’d say that it amazing a complex application running in 5MB of RAM!!! with instant startup!

And about the image sizes, running docker images we see:

REPOSITORY      TAG      IMAGE ID       CREATED             SIZE
vertx-bitcoin   latest   3b8b487b8ad8   43 minutes ago      37.9MB

So the total size is 38MB!!!

Java is now a viable Serveless technology and a it we can agree that java ain’t slow and bloated right?!

Paulo Lopes

I'm Paulo and I've used my 10+ years of software development experience writing, rewriting, banging my head against the wall, editing and re-editing high-performance web application to make Vert.x an even more awesome framework.

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