How to get JSON data via http get request

Question

I am new to Vaadin. As in topic I would like to make http get reaquest in order to retieve some JSON data. How could I do this ? I have been trying to make this by com.google.gwt.http.client.RequestBuilder, but I have obtained java.lang.UnsatisfiedLinkError: com.google.gwt.xhr.client.XMLHttpRequest.create(). I think the error is associated to GWT client - side nature. So how could I make http get request in Vaadin 7 server - side ? Here is my code:

package com.example.soaclient;

import javax.servlet.annotation.WebServlet; 
import com.google.gwt.http.client.Request;
import com.google.gwt.http.client.RequestBuilder;
import com.google.gwt.http.client.RequestCallback;
import com.google.gwt.http.client.RequestException;
import com.google.gwt.http.client.Response;
import com.google.gwt.json.client.JSONObject;
import com.google.gwt.json.client.JSONParser;
import com.google.gwt.json.client.JSONValue;
import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Button;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.Label;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;

@SuppressWarnings("serial")
@Theme("soaclient")
public class SoaclientUI extends UI {

    @WebServlet(value = "/*", asyncSupported = true)
    @VaadinServletConfiguration(productionMode = false, ui = SoaclientUI.class)
    public static class Servlet extends VaadinServlet {

    }

    @Override
    protected void init(VaadinRequest request) {

        final VerticalLayout layout = new VerticalLayout();

        layout.setMargin(true);
        setContent(layout);
        Button button = new Button("Click Me");

        button.addClickListener(new Button.ClickListener() {

            public void buttonClick(ClickEvent event) {

                layout.addComponent(new Label("Thank you for clicking"));

                String url = "some URL goes here";
                RequestBuilder builder = new RequestBuilder(RequestBuilder.GET, url);

                try {

                    Request request = builder.sendRequest(null, new RequestCallback() {

                        public void onError(Request request, Throwable exception) {
                            // Couldn't connect to server (could be timeout, SOP violation, etc.)
                        }

                        public void onResponseReceived(Request request, Response response) {

                            if (200 == response.getStatusCode()) {
                                // Process the response in response.getText()
                            } else {
                                // Handle the error.  Can get the status text from response.getStatusText()

                            }
                        }
                    });
                } catch (RequestException e) {
                  // Couldn't connect to server
                }
            }
        });

        layout.addComponent(button);
    }
}

Answer 1

With Vaadin you should not use anything from com.google.gwt.http.client package. That is only for client side development, e.g. when you make components that need client side counterparts.

Instead of GWT classes you should just stick to generic JDK libraries. E.g. you could simply use java.net.URL.openStream(). But if you are consuming some REST services, you could refer to my recent JAX-RS 2.0 Client article.