How to ping server in Vaadin

Here is a sample code on how to ping the server from a Vaadin 7 app

First we need some javascript to ping the server. The following is the code in ping.js

var host=location.host;
var path=location.pathname.split('/')[1]; 
var redPic=new Image(100,25); 
redPic.src='http://' + host + '/' + path + '/red.jpg';
var greenPic=new Image(100,25); 
greenPic.src='http://' + host + '/' + path + '/green.jpg';
var intervalTimer;
function pingHost(){
     var http = new XMLHttpRequest(); 
  http.open('GET', 'http://' + host + '/' + path + '/ping', false); 
  try {
   http.send('');
  } catch (e) { /*can't send over http....means server down*/
   var els = document.getElementsByTagName('img');
   for(i=0; i < els.length; i++) {
    if(els[i].src.indexOf('green.jpg') > -1) { 
     els[i].src=redPic.src;
     alert('serverDownMessage');
        break;
    } 
   }
   return;
  } 
  var els = document.getElementsByTagName('img');
  for(i=0; i < els.length; i++) {
   if(els[i].src.indexOf('red.jpg') > -1) { 
    els[i].src=greenPic.src;
    alert('serverUpMessage');
    location.reload();
    break;
   }
  }
  var timer=setTimeout(function(){alert('serverDownMessage')}, pingInterval);
  if (http.status == 200){  /*the server is up and running*/
   clearTimeout(timer); /*no need for a timer*/
  } 
};
intervalTimer=setInterval(function() {pingHost();}, pingInterval);

Then we need the Vaadin servlet that reads the javascript

@SuppressWarnings("serial")
@Title("My Monitor")
@Push
public class MyMonitorUI extends UI implements ManagerEventListener  {
 @VaadinServletConfiguration(productionMode = false, ui = MyMonitorUI.class)
 public static class Servlet extends VaadinServlet { }

 private static Logger logger = Logger.getLogger(MyMonitorUI.class);
 
 private int pingInterval = 10000; // 10 seconds
 private String serverDownMessage="Server Down!";
 private String serverUpMessage="Server Up!";
  
 @Override
 protected void init(VaadinRequest request) {
  
  ...
  // Read the JS File and Process it
  JavaScript.getCurrent().execute(readPingFile());
  
  logger.info("Started UI: "+hashCode());
 }
...
private String readPingFile() {
  String basepath = VaadinService.getCurrent().getBaseDirectory().getAbsolutePath();
  BufferedReader br=null;
  StringBuffer pingJavaScript= new StringBuffer();
  try {
   br= new BufferedReader(new FileReader(new File(basepath + "/ping.js")));
   String line="";
   while((line=br.readLine()) != null) {
    pingJavaScript.append(line);
   }
  } catch (Exception e) {
   logger.error("can't read js file", e);
  } finally {
   try {
    br.close();
   } catch (Exception e) {}
  }
  return pingJavaScript.toString().
    replaceAll("pingInterval", String.valueOf(pingInterval)).
    replaceAll("serverDownMessage", serverDownMessage).
    replaceAll("serverUpMessage", serverUpMessage);
 }

then we need to configure the servlet in web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>NtsTpsAsteriskMonitor</display-name> 
 
  <servlet>
    <servlet-name>Vaadin Application</servlet-name>
    <servlet-class>com.vaadin.server.VaadinServlet</servlet-class>
    <init-param>
        <description>Vaadin application class to start</description>
        <param-name>UI</param-name>
        <param-value>net.mydomain.MyMonitorUI</param-value>
   </init-param>
   <init-param>
     <param-name>pushmode</param-name>
  <param-value>automatic</param-value>
  </init-param>
  <async-supported>true</async-supported>
</servlet>

 <servlet-mapping>
    <servlet-name>Vaadin Application</servlet-name>
    <url-pattern>/ui/*</url-pattern>
</servlet-mapping>

<servlet-mapping>
    <servlet-name>Vaadin Application</servlet-name>
    <url-pattern>/VAADIN/*</url-pattern>
</servlet-mapping>
 
 <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
</web-app>