01

01 : Building a Simple GraalVM Polyglot Application

Estimated time: 20 minutes

Content

• Introduction

Introduction

Creating polyglot Apps is simple from GraalVM Perspective. The purpose of this lab is to build a simple JAX-RS Controller that will run some simple Javascript code. To do so, You will have to create an Helidon project using the Helidon CLI first.

My first Polyglot REST Endpoint

Create your Helidon covid19-trends Microprofile REST Application from your terminal and follow the diaglog

 
$  helidon init
Using Helidon version 2.3.2
Helidon flavor
  (1) SE
  (2) MP
Enter selection (Default: 1): 2
Select archetype
  (1) bare | Minimal Helidon MP project suitable to start from scratch
  (2) quickstart | Sample Helidon MP project that includes multiple REST operations
  (3) database | Helidon MP application that uses JPA with an in-memory H2 database
Enter selection (Default: 1): 2
Project name (Default: quickstart-mp): covid19-trends
Project groupId (Default: me.nono-helidon): com.oracle.graalvm.demos
Project artifactId (Default: quickstart-mp): covid19-trends
Project version (Default: 1.0-SNAPSHOT):
Java package name (Default: me.nono.mp.quickstart): com.oracle.graalvm.demos

Start the Helidon Dev loop to automatically preview the changes

$ helidon dev
helidon dev starting ...
| building
| build completed (9,0 seconds)
| covid19-trends starting

We now have a basic Helidon java project, in order to use GraalVM Polyglot features and API, the project required the Graal-Sk dependency.

Add GraalVM SDK dependency

Add the Graal SDK https://mvnrepository.com/artifact/org.graalvm.sdk/graal-sdk/21.2.0 to your maven pom.xml or Gradle Build file accordingly. Once dependencies are OK, you now have to create your covid-19 controller.

Create the Covid19Resource Controller

By using the quickstart artifact, we have had a default Controller present after generating the project,

Rename the the src/main/java/com/oracle/graalvm/demos/GreetingController.java file to src/main/java/com/oracle/graalvm/demos/Covid19Controller.java and edit it with the following content

package com.oracle.graalvm.demos;

import org.graalvm.polyglot.Context;

import javax.enterprise.context.RequestScoped;
import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;


@Path("/covid19/fr/")
@RequestScoped
public class CovidResource {

    private Context polyglot;

    @Inject
    public CovidResource() {
        try {
          // First create a polyglot contexte to run our code
          // Context provides an execution environment for guest languages. 
          // you can pass a list of expected language for this context in the newBuilder Method 
          // R language requires the allowAllAccess flag to be set to true to run .

            this.polyglot = Context.newBuilder().allowAllAccess(true).build();
      } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Path("/help")
    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public Response help() {
       // Evaluate a polyglott expression
       // language = js
       // Code to execute as the second parameter.
       // convert the Value returned asString
        String welcome = polyglot.eval("js", "print('{\"Welcome to GraalVM Polyglot EMEA HOL!\"}');").asString();
        return Response.ok(welcome).build();

    }

}

Update the Main Unit test file src/test/java/com/oracle/graalvm/demos/MainTest.java

//Ensure the /covid19/fr/help return an HTTP 200
@Test
    void testHelloWorld() {
        Client client = ClientBuilder.newClient();


        Response r = client
                .target(serverUrl)
                .path("/covid19/fr/help")
                .request()
                .get();
       Assertions.assertEquals(200, r.getStatus(), "GET health status code");


    }

Run the application from your browser/Terminal if the helidon Dev loop is not enabled, Build and start the application using

# build and run 
$ mvn clean install 
$ java -jar target/covid19-trends.jar

Testing from Curl

$ curl -i  http://localhost:8080/covid19/fr/help
HTTP/1.1 200 OK
Content-Length: 0
Date: Thu, 19 Aug 2021 16:02:44 +0200
connection: keep-alive

From the application logs you can see the printed message

2021.08.11 11:55:11 INFO io.helidon.common.HelidonFeatures Thread[features-thread,5,main]: Helidon MP 2.3.2 features: [CDI, Config, Fault Tolerance, Health, JAX-RS, Metrics, Open API, REST Client, Security, Server, Tracing]
{"Welcome to GraalVM Polyglot EMEA HOL!"}

Quiz

From the previous curl command, we have no output in the console/Browser. Why? Ho to fix that?

Solution

Change the Javascript instruction to return the expected string instead of printing it to the console

  String welcome = polyglot.eval("js", "'Welcome to GraalVM Polyglot EMEA HOL!\\n';").asString();

Testing from curl again

$ curl -i  http://localhost:8080/covid19/fr/help
HTTP/1.1 200 OK
Content-Type: application/json
Date: Thu, 19 Aug 2021 16:03:29 +0200
connection: keep-alive
content-length: 38

Welcome to GraalVM Polyglot EMEA HOL!

Summary

In this labs, you built and run a simple Polyglot Application Mixing Java and Javascript. ✅ Building Polyglot applications require the same tooling as those required for traditionnal Apps.