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Creating and invoking Java actions

The process of creating Java actions is similar to that of other actions. The following sections guide you through creating and invoking a single Java action, and demonstrate how to bundle multiple files and third party dependencies.

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

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

An action supports not only a JSON object but also a JSON array as a return value.

It would be a simple example that uses an array as a return value:

import com.google.gson.JsonArray;
import com.google.gson.JsonObject;
public class HelloArray {
    public static JsonArray main(JsonObject args) {
        JsonArray jsonArray = new JsonArray();
        jsonArray.add("a");
        jsonArray.add("b");
        return jsonArray;
    }
}

You can also create a sequence action with actions accepting an array param and returning an array result.

You can easily figure out the parameters with the following example:

import com.google.gson.JsonArray;
public class Sort {
    public static JsonArray main(JsonArray args) {
        return args;
    }
}

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 --main Hello

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.

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

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!"
  }

Find out more about parameters in the Working with parameters section.