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.