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Apache OpenWhisk runtimes for java

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Quick Java Action

A Java action is a Java program with a method called main that has the exact signature as follows:

public static main(;

For example, create a Java file called with the following content:


public class Hello {
    public static JsonObject main(JsonObject args) {
        String name = "stranger";
        if (args.has("name"))
            name = args.getAsJsonPrimitive("name").getAsString();
        JsonObject response = new JsonObject();
        response.addProperty("greeting", "Hello " + name + "!");
        return response;

In order to compile, test and archive Java files, you must have a JDK 8 installed locally.

Then, compile into a JAR file hello.jar as follows:

jar cvf hello.jar Hello.class

Note: google-gson must exist in your Java CLASSPATH when compiling the Java file.

You need to specify the name of the main class using --main. An eligible main class is one that implements a static main method as described above. If the class is not in the default package, use the Java fully-qualified class name, e.g., --main com.example.MyMain.

If needed you can also customize the method name of your Java action. This can be done by specifying the Java fully-qualified method name of your action, e.q., --main com.example.MyMain#methodName

Create the Java Action

To use as a docker action:

wsk action update helloJava hello.jar --main Hello --docker openwhisk/java8action

This works on any deployment of Apache OpenWhisk

To use on a deployment of OpenWhisk that contains the runtime as a kind:

wsk action update helloJava hello.jar --main Hello --kind java:8

Invoke the Java Action

Action invocation is the same for Java actions as it is for Swift and JavaScript actions:

wsk action invoke --result helloJava --param name World
      "greeting": "Hello World!"

Local development


  • Gradle
  • Docker Desktop (local builds)

Build and Push image to a local Docker registry

  1. Start Docker Desktop (i.e., Docker daemon)

  2. Build the Docker runtime image locally using Gradle:

./gradlew core:java8:distDocker

This will produce the image whisk/java8action and push it to the local Docker Desktop registry with the latest tag.

  1. Verify the image was registered:
$ docker images whisk/*
REPOSITORY           TAG     IMAGE ID            CREATED             SIZE
whisk/java8action    latest  35f90453905a        7 minutes ago       521MB

Build and Push image to a remote Docker registry

Build the Docker runtime image locally using Gradle supplying the image Prefix and Registry domain (default port):

docker login
./gradlew core:java8:distDocker -PdockerImagePrefix=$prefix-user

Deploying the Java runtime image to OpenWhisk

Deploy OpenWhisk using ansible environment that contains the kind java:8 Assuming you have OpenWhisk already deployed locally and OPENWHISK_HOME pointing to root directory of OpenWhisk core repository.

Set ROOTDIR to the root directory of this repository.

Redeploy OpenWhisk

cd $OPENWHISK_HOME/ansible
ANSIBLE_CMD="ansible-playbook -i ${ROOTDIR}/ansible/environments/local"
$ANSIBLE_CMD setup.yml
$ANSIBLE_CMD couchdb.yml
$ANSIBLE_CMD initdb.yml
$ANSIBLE_CMD wipe.yml
$ANSIBLE_CMD openwhisk.yml

Or you can use wskdev and create a soft link to the target ansible environment, for example:

ln -s ${ROOTDIR}/ansible/environments/local ${OPENWHISK_HOME}/ansible/environments/local-java
wskdev fresh -t local-java


Install dependencies from the root directory on $OPENWHISK_HOME repository

./gradlew install

Using gradle to run all tests

./gradlew :tests:test

Using gradle to run some tests

./gradlew :tests:test --tests *ActionContainerTests*

Using IntelliJ:

  • Import project as gradle project.
  • Make sure working directory is root of the project/repo

Using container image to test

To use as docker action push to your own dockerhub account

docker tag whisk/java8action $user_prefix/java8action
docker push $user_prefix/java8action

Then create the action using your the image from dockerhub

wsk action update helloJava hello.jar --main Hello --docker $user_prefix/java8action

The $user_prefix is usually your dockerhub user id.


Gradle build fails with "Too many files open"

This may occur on MacOS as the default maximum # of file handles per session is 256. The gradle build requires many more and is unable to open more files (e.g., For example, you may see something like:

> /Users/XXX/.gradle/caches/4.6/scripts-remapped/build_4mpzm2wl8gipqoxzlms7n6ctq/7gdodk7z6t5iivcgfvflmhqsm/cp_projdf5583fde4f7f1f2f3f5ea117e2cdff1/ (Too many open files)

You can see this limit by issuing:

$ ulimit -a
open files                      (-n) 256

In order to increase the limit, open a new terminal session and issue the command (and verify):

$ ulimit -n 10000

$ ulimit -a
open files                      (-n) 10000

Gradle Task fails on :core:java8:tagImage

Docker daemon is not started and the Task is not able to push the image to your local registry.


Apache 2.0