Creating and invoking OpenWhisk actions

Actions are stateless code snippets that run on the OpenWhisk platform. An action can be a JavaScript function, a Swift function, or a custom executable program packaged in a Docker container. For example, an action can be used to detect the faces in an image, aggregate a set of API calls, or post a Tweet.

Actions can be explicitly invoked, or run in response to an event. In either case, a run of an action results in an activation record that is identified by a unique activation ID. The input to an action and the result of an action are a dictionary of key-value pairs, where the key is a string and the value a valid JSON value.

Actions can be composed of calls to other actions or a defined sequence of actions.

Creating Java actions

The process of creating Java actions is similar to that of JavaScript and Swift actions. The following sections guide you through creating and invoking a single Java action, and adding parameters to that action.

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

Creating and invoking an action

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

public static com.google.gson.JsonObject main(com.google.gson.JsonObject);

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

import com.google.gson.JsonObject;
public class Hello {
    public static JsonObject main(JsonObject args) {
        JsonObject response = new JsonObject();
        response.addProperty("greeting", "Hello World");
        return response;
    }
}

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

$ javac Hello.java
$ jar cvf hello.jar Hello.class

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

You can create a OpenWhisk action called helloJava from this JAR file as follows:

$ wsk action create helloJava hello.jar

When you use the command line and a .jar source file, you do not need to specify that you are creating a Java action; the tool determines that from the file extension.

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

$ wsk action invoke --blocking --result helloJava

  {
      "greeting": "Hello World"
  }

Note: If the JAR file has more than one class with a main method matching required signature, the CLI tool uses the first one reported by jar -tf.