Java webengine concurrency with spark web service

Question

I am trying to create a web service that can spawn a crawler using com.ui4j.api.browser.BrowserFactory (JavaFX webview) since getBrowser method can only return one instance, as is also documented.

I am trying to run this concurrently in separate threads so that I can spawn multiple browsers at once.

Have also followed : Thread safe and concurrent use, multiple instances?

But I don't understand how can I add criteria's dynamically / create callables dynamically since all this code is running behind a Webservice made from JavaSpark

This is my code :

import static spark.Spark.get;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Runcrawl {


    public static void main(String[] args) {

        get("/crawl/:keyword", (req, res) - > {
            ExecutorService executor = Executors.newWorkStealingPool(3);
            return executor.submit(() - > {
                String threadName = Thread.currentThread().getName();
                System.out.println("threadName : " + threadName);
                return launchBrowser();
            }).get();
        });

    }
}

Right now I am using executor.submit().get() , which runs fine but blocks the thread thus not letting me launch a separate browser.

Thanks, any help would be appreciated.