I have a strange Vaadin "Communication Problem" error when I try to do the following:
I have two interfaces, the first has a link that points to another interface, here is the code:
public class MenuUI extends UI {
@WebServlet(value = "/*", asyncSupported = true)
@VaadinServletConfiguration(productionMode = false, ui = MenuUI.class)
public static class Servlet extends VaadinServlet {
}
@Override
protected void init(VaadinRequest request) {
final VerticalLayout layout = new VerticalLayout();
layout.setMargin(true);
layout.setSpacing(true);
setContent(layout);
MenuBar menuBar = new MenuBar();
MenuItem submenu1 = menuBar.addItem("Submenu 1", null);
MenuItem submenu2 = menuBar.addItem("Submenu 2", null);
submenu1.addItem("Option 1", null);
submenu1.addItem("Option 2", null);
submenu2.addItem("Option 3", null);
submenu2.addItem("Option 4", new Command() {
@Override
public void menuSelected(MenuItem selectedItem) {
Notification.show("That was option 4");
}
});
layout.addComponent(menuBar);
Link link = new Link("Go to ShortcutKeys",
new ExternalResource(Page.getCurrent().getLocation().toString() + "Shortcut"));
link.setTargetName("_blank");
layout.addComponent(link);
}
}
This interface leads to this: 
When I click on the link, the new user interface is initialized, here is the code for the new user interface:
public class ShortcutUI extends UI {
@WebServlet(value = "/Shortcut", asyncSupported = true)
@VaadinServletConfiguration(productionMode = false, ui = ShortcutUI.class)
public static class Servlet extends VaadinServlet {
}
@Override
protected void init(VaadinRequest request) {
final VerticalLayout layout = new VerticalLayout();
layout.setMargin(true);
layout.setSpacing(true);
setContent(layout);
final TextField tf = new TextField("Your data:");
layout.addComponent(tf);
Button button = new Button("Send data (ENTER)");
button.setClickShortcut(KeyCode.ENTER, ModifierKey.SHIFT);
button.addClickListener(new Button.ClickListener() {
public void buttonClick(ClickEvent event) {
layout.addComponent(new Label(tf.getValue()));
tf.setValue("");
tf.focus();
}
});
layout.addComponent(button);
}
It leads to:
Here I type in some text, and then when I click on the "Send Data" button, I get the following:
? , , , ShortcutUI init() MenuUI, :
public class MenuUI extends UI {
@WebServlet(value = "/*", asyncSupported = true)
@VaadinServletConfiguration(productionMode = false, ui = MenuUI.class)
public static class Servlet extends VaadinServlet {
}
@Override
protected void init(VaadinRequest request) {
final VerticalLayout layout = new VerticalLayout();
layout.setMargin(true);
layout.setSpacing(true);
setContent(layout);
MenuBar menuBar = new MenuBar();
MenuItem submenu1 = menuBar.addItem("Submenu 1", null);
MenuItem submenu2 = menuBar.addItem("Submenu 2", null);
submenu1.addItem("Option 1", null);
submenu1.addItem("Option 2", null);
submenu2.addItem("Option 3", null);
submenu2.addItem("Option 4", new Command() {
@Override
public void menuSelected(MenuItem selectedItem) {
Notification.show("That was option 4");
}
});
layout.addComponent(menuBar);
Link link = new Link("Go to ShortcutKeys",
new ExternalResource(Page.getCurrent().getLocation().toString() + "Shortcut"));
link.setTargetName("_blank");
layout.addComponent(link);
final TextField tf = new TextField("Your data:");
layout.addComponent(tf);
Button button = new Button("Send data (ENTER)");
button.setClickShortcut(KeyCode.ENTER, ModifierKey.SHIFT);
button.addClickListener(new Button.ClickListener() {
public void buttonClick(ClickEvent event) {
layout.addComponent(new Label(tf.getValue()));
tf.setValue("");
tf.focus();
}
});
layout.addComponent(button);
}
}
:
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debug Vaadin:
Communication error: (SyntaxError) : Unexpected token < - Original JSON-text: html>
<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta
http-equiv="X-UA-Compatible" content="IE=11;chrome=1" /> <style type="text/css">html,
body {height:100%;margin:0;}</style> <link rel="shortcut icon"
type="image/vnd.microsoft.icon"
href="./../../VAADIN/themes/chapter_4_menubar_and_shortcutkeys/favicon.ico" /> <link
rel="icon" type="image/vnd.microsoft.icon"
href="./../../VAADIN/themes/chapter_4_menubar_and_shortcutkeys/favicon.ico" /> </head>
<body scroll="auto" class=" v-generated-body"> <div id="Chapter4MenuBarandShortcutKeys-
1103734620" class=" v-app chapter_4_menubar_and_shortcutkeys"> <div class=" v-app-
loading"></div> <noscript> You have to enable javascript in your browser to use an
application built with Vaadin. </noscript> </div> <script type="text/javascript"
src="./../../VAADIN/vaadinBootstrap.js"></script> <script type="text/javascript">//<!
[CDATA[ if (!window.vaadin) alert("Failed to load the bootstrap javascript:
./../../VAADIN/vaadinBootstrap.js"); if (typeof window.__gwtStatsEvent != 'function') {
vaadin.gwtStatsEvents = []; window.__gwtStatsEvent = function(event)
{vaadin.gwtStatsEvents.push(event); return true;}; }
vaadin.initApplication("Chapter4MenuBarandShortcutKeys-1103734620",{ "authErrMsg": {
"caption": "Authentication problem", "message": "Take note of any unsaved data, and
<u>click here<\/u> or press ESC to continue." }, "comErrMsg": { "caption": "Communication
problem", "message": "Take note of any unsaved data, and <u>click here<\/u> or press ESC
to continue." }, "debug": true, "heartbeatInterval": 300, "serviceUrl": "./../..",
"sessExpMsg": { "caption": "Session Expired", "message": "Take note of any unsaved data,
and <u>click here<\/u> or press ESC key to continue." }, "standalone": true, "theme":
"chapter_4_menubar_and_shortcutkeys", "vaadinDir": "./../../VAADIN/", "versionInfo":
{"vaadinVersion": "7.3.7"}, "widgetset": "com.vaadin.DefaultWidgetSet" }); //]]></script>
</body> </html
?