How to pass data from Java to a JavaScript web application?

I have two applications: (1) a JavaFX application A and (2) a purely JavaScript web application B (no backend) accessed either as local files (for developers) or at a certain URL (for the users).

In A the user can save a file and then open B in the user's browser via Desktop.getDesktop().browse(). Then the user can load that file in B by clicking on an input element of type file, which allows the usage of FileReader.

That process is too cumbersome for the users, so I want to remove that requirement of the user to save and load a file.

My problem is: How can I transfer this data from A to B now?

• GET parameters only allow a few thousand characters, which is not enough for us
• I see no way to pass POST parameters from Java to the browser and also to access those parameters from within JavaScript
• FileReader cannot read files using path names because of security
• we can't use platform or browser dependent native code solutions because the users use different OSes and browsers
• the web application uses many libraries and advanced JavaScript features, which I don't expect a JavaFX HTML window to support

Is there any way to get one of the above methods to work or is there any alternative to pass data between Java and a purely client-based JavaScript application in a browser?

Update

As suggested in the accepted answer, I serve the data as using Jetty.

1. Java: Define JSON Server

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.eclipse.jetty.server.Request;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.handler.AbstractHandler;
import org.json.JSONArray;

public class JsonServer extends Server
{
    static final int PORT = 1234;  

    public JsonServer(JSONArray json)
    {
        super(PORT);
        setHandler(new JsonHandler(json));
    }

    public static class JsonHandler extends AbstractHandler
    {
        final JSONArray json;

        public JsonHandler(JSONArray json)
        {
            this.json = json;
        }

        @Override
        public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException
        {
            response.setContentType("application/json; charset=utf-8");
            response.setStatus(HttpServletResponse.SC_OK);

            try(PrintWriter out = response.getWriter();)
            {
                out.println(json);
            }
            baseRequest.setHandled(true);
        }

    }
}

2. Java: Start Server

            Server server = new JsonServer(json);
            server.start();
            browse("https://myapp.com?load=http://localhost:1234");

3. JavaScript: Fetch

    const load = url.searchParams.get("load");
    const json = await (await fetch(load)).json();