For high demanding apps, its also very to good to keep their site ready to server quickly. Not a bad deal of paying $9 a month about it.

The enhancements and according to the announcement on their group:

• The Always On feature allows applications to pay and keep 3 instances of their application always running, which can significantly reduce application latency.
• Developers can now enable Warmup Requests. By specifying  a handler in an app’s appengine-web.xml, App Engine will attempt to to send a Warmup Request to initialize new instances before a user interacts with it. This can reduce the latency an end-user sees for initializing your application.
• The Channel API is now available for all users.
• Task Queue has been officially released, and is no longer an experimental feature. The API import paths that use ‘labs’ have been deprecated. Task queue storage will count towards an application’s overall storage quota, and will thus be charged for.
• The deadline for Task Queue and Cron requests has been raised to 10 minutes. Datastore and API deadlines within those requests remain unchanged.
• For the Task Queue, developers can specify task retry-parameters in their queue.xml.
• Metadata Queries on the datastore for datastore kinds, namespaces, and entity properties are available.
• URL Fetch allowed response size has been increased, up to 32 MB. Request size is still limited to 1 MB.
• The Admin Console Blacklist page lists the top blacklist rejected visitors.
• The automatic image thumbnailing service supports arbitrary crop sizes up to 1600px.
• Overall average instance latency in the Admin Console is now a weighted average over QPS per instance.
• Added a low-level AysncDatastoreService for making calls to the datastore asynchronously.
• Added a getBodyAsBytes() method to QueueStateInfo.TaskStateInfo, this returns the body of the task state as a pure byte-string.
• The whitelist has been updated to include all classes from javax.xml.soap.
• Fixed an issue sending email to multiple recipients.

You can download GAE 1.4.0 from Google Code.

Related posts:

1. How to avoid DeadlineExceededException in Google App Engine
2. Java in Google App Engine, what’s new for developers
3. Tutorial: JPA on Google App Engine

Now if user presses the refresh button or you want to get user information again from twitter any where in your application. You need to use the access token and this method twitter.setOAuthAccessToken(String1, String2). Because user has already been authenticated by Twitter. You can save this information in session.

HttpSession session = request.getSession();

twitter.setOAuthConsumer(Constants.CONSUMER_KEY,Constants.CONSUMER_SECRET);

if (session.getAttribute("aToken") == null){
	// request token
	String token = (String) session.getAttribute("token");
	String tokenSecret = (String)session.getAttribute("tokenSecret");
	AccessToken accessToken =
		twitter.getOAuthAccessToken(token, tokenSecret);
	twitter.setOAuthAccessToken(accessToken);

	// save the access token, that are different from request token
	session.setAttribute("aToken", accessToken.getToken());
	session.setAttribute("aTokenSecret", accessToken.getTokenSecret());

}else{
	// use the access token to authenticate user whenever you want
	twitter.setOAuthAccessToken((String)session.getAttribute("aToken"),
		(String)session.getAttribute("aTokenSecret"));
}
User user = twitter.verifyCredentials();

If you are also developing some app by using Twitter4J API on Google App Engine. Please give feedback on it.

Twitter4J Quick Tutorial: A quick tutorial on how to develop Twitter app by using Twitter4J on Google App Engine.

Related posts:

1. Tutorial: Java based Twitter App on Google App Engine
2. How to register an app with Twitter
3. How to avoid DeadlineExceededException in Google App Engine

Read them here:

• Cloud Computing
• Google App Engine for Java

[ad#co-2]

Related posts:

1. Java in Google App Engine, what’s new for developers
2. Got two books on Google App Engine for Java
3. Upgrading to Google App Engine Java SDK 1.2.8

You guys can share your feedback if you read any book or any material on it.

Related posts:

1. Useful cheat sheets on Cloud Computing and Google App Engine for Java
2. Java in Google App Engine, what’s new for developers
3. Upgrading to Google App Engine Java SDK 1.2.8

I have presented following slides there. Also coded a basic sample application to show how to create and deploy new Java project on GAE. Which can send emails, how to use authentication by Google, and how to do database interaction. It was a very interesting topic for me to present. Participants also liked it.

Comments and feedback is welcomed on the presentation material from you guys.

Related posts:

1. Java in Google App Engine, what’s new for developers
2. How to avoid DeadlineExceededException in Google App Engine
3. Useful cheat sheets on Cloud Computing and Google App Engine for Java

I am using App Engine SDK version 1.2.18

Project name: engineplay
URL: http://engineplay.appspot.com
[ad#co-2]

Project structure:

UserPrefs.java: The entity class

package com.engineplay.datastore.pojos;
import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.Id;
import com.engineplay.datastore.EMFService;
import com.google.appengine.api.users.User;

@Entity(name = "UserPrefs")
public class UserPrefs {
 @Id
 private String userId;
 private int donuts;
 @Basic
 private User user;

 public UserPrefs(String userId) {
 this.userId = userId;
 }

 /* TODO create getter and setters for above properties yourself */

 public static UserPrefs getPrefsForUser(User user) {
 UserPrefs userPrefs = null;
 EntityManager em = EMFService.get().createEntityManager();
 try {
 if (em.find(UserPrefs.class, user.getEmail()) == null) {

 userPrefs = new UserPrefs(user.getEmail());
 userPrefs.setUser(user);

 } else {
 userPrefs = em.find(UserPrefs.class, user.getEmail());
 }
 } finally {
 em.close();
 }
 return userPrefs;
 }

 public void save() {
 EntityManager em = EMFService.get().createEntityManager();
 try {
 em.persist(this);
 } finally {
 em.close();
 }
 }

 public void remove() {
 EntityManager em = EMFService.get().createEntityManager();
 try {
 UserPrefs userPrefs = em.find(UserPrefs.class, user.getEmail());
 em.remove(userPrefs);
 } finally {
 em.close();
 }
 }
}

EMFService.java: The class to create EntityManagerFactory instance.

package com.engineplay.datastore;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class EMFService {
 private static final EntityManagerFactory emfInstance = Persistence
 .createEntityManagerFactory("transactions-optional");

 private EMFService() {
 }

 public static EntityManagerFactory get() {
 return emfInstance;
 }
}

LoginServlet.java

package com.engineplay.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.engineplay.datastore.pojos.UserPrefs;
import com.google.appengine.api.users.User;
import com.google.appengine.api.users.UserService;
import com.google.appengine.api.users.UserServiceFactory;

public class LoginServlet extends HttpServlet {

 public void doGet(HttpServletRequest req, HttpServletResponse resp)
 throws IOException {

 UserService userService = UserServiceFactory.getUserService();
 User user = userService.getCurrentUser();
 String navBar, form = null;
 int donuts = 0;
 if (user == null) {
 navBar = "<p>Welcome! <a href=\"" + userService.createLoginURL("/") +
 "\">Sign in or register</a> to customize.</p>";
 form = "";
 } else {
 UserPrefs userPrefs = UserPrefs.getPrefsForUser(user);

 if (userPrefs != null) {
 donuts = userPrefs.getDonuts();
 }
 navBar = "<p>Welcome, " + user.getEmail() + "! You can <a href=\"" +
 userService.createLogoutURL("/") +
 "\">sign out</a>.</p>";
 form = "<form action=\"/donuts\" method=\"post\">" +
 "<label for=\"donuts\">" +
 "Need more donuts?:" +
 "</label>" +
 "<input name=\"donuts\" id=\"donuts\" type=\"text\" size=\"4\" />" +
 " <input name=\"olddonuts\"  type=\"hidden\" value=\"" + donuts + "\" /> " +
 " <input name=\"userId\"  type=\"hidden\" value=\"" +user.getEmail()+ "\" /> " +
 "   <input type=\"submit\" value=\"  ADD  \" /><br><input type=\"submit\"  name =\"deleteBtn\" value=\"  DELETE ME!  \" />" +
 "</form>";

 }
 resp.setContentType("text/html");
 PrintWriter out = resp.getWriter();
 out.print("<html><head><title>Engine Play - Donuts</title</head><body>");
 out.println(navBar);
 if (donuts != 0)
 out.println("<p>Donuts you have: " + donuts + "</p>");
 else
 out.println("<p> No donuts you have   </p>");
 out.println(form);
 out.print("</body></html>");
 }
}

DonutsServlet.java

package com.engineplay.servlet;

import java.io.IOException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.engineplay.datastore.pojos.UserPrefs;
import com.google.appengine.api.users.User;
import com.google.appengine.api.users.UserService;
import com.google.appengine.api.users.UserServiceFactory;

public class DonutsServlet extends HttpServlet {

 public void doPost(HttpServletRequest req, HttpServletResponse resp)
 throws IOException {
 UserService userService = UserServiceFactory.getUserService();
 User user = userService.getCurrentUser();
 UserPrefs userPrefs = UserPrefs.getPrefsForUser(user);
 try {

 String deleteOpt = req.getParameter("deleteBtn");
 if (deleteOpt == null){
 int donuts = new Integer(req.getParameter("donuts")).intValue();
 int oldDonuts = new Integer(req.getParameter("olddonuts")).intValue();

 userPrefs.setDonuts(donuts + oldDonuts);
 userPrefs.save();
 }else{
 String userId = req.getParameter("userId");
 userPrefs.setUserId(userId);
 userPrefs.remove();
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
 resp.sendRedirect("/Login");
 }
}

persistence.xml: Configuration file required for JPA

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://java.sun.com/xml/ns/persistence

http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"

 version="1.0">
 <persistence-unit name="transactions-optional">
 <provider>
 org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider
 </provider>
 <properties>
 <property name="datanucleus.NontransactionalRead"
 value="true" />
 <property name="datanucleus.NontransactionalWrite"
 value="true" />
 <property name="datanucleus.ConnectionURL"
 value="appengine" />
 </properties>
 </persistence-unit>
</persistence>

web.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="2.5">

 <servlet>
 <servlet-name>LoginServlet</servlet-name>
 <servlet-class>com.engineplay.servlet.LoginServlet</servlet-class>
 </servlet>
 <servlet>
 <servlet-name>DonutsServlet</servlet-name>
 <servlet-class>com.engineplay.servlet.DonutsServlet</servlet-class>
 </servlet>

 <servlet-mapping>
 <servlet-name>LoginServlet</servlet-name>
 <url-pattern>/Login</url-pattern>
 </servlet-mapping>
 <servlet-mapping>
 <servlet-name>DonutsServlet</servlet-name>
 <url-pattern>/donuts</url-pattern>
 </servlet-mapping>

 <welcome-file-list>
 <welcome-file>Login</welcome-file>
 </welcome-file-list>
</web-app>

Access the project here: http://engineplay.appspot.com

Related posts:

1. Upgrading to Google App Engine Java SDK 1.2.8
2. Determine whether application is running on local server or App Engine
3. Tutorial: Java based Twitter App on Google App Engine

org.datanucleus.metadata.InvalidMetaDataException:
Error in MetaData for field "user" in class "com.engineplay.datastore.pojos.UserPrefs" : this is declared as com.google.appengine.api.users.User with "persistence-modifier=none" yet has either "default-fetch-group=true" or "primary-key=true" specified! These should be false.

After some searching I came to know that I should upgrade my App Engine SDK. To get datanucleus-enhancer-1.1.4.jar. But when I upgraded my SDK from 1.2.0 to 1.2.8. I got following exception.

java.lang.RuntimeException: Unable to locate the App Engine agent. Please use dev_appserver, KickStart,  or set the jvm flag: "-javaagent:<sdk_root>/lib/agent/appengine-agent.jar"
at com.google.appengine.tools.development.DevAppServerFactory.testAgentIsInstalled(DevAppServerFactory.java:102)
at com.google.appengine.tools.development.DevAppServerFactory.createDevAppServer(DevAppServerFactory.java:77)
at com.google.appengine.tools.development.DevAppServerFactory.createDevAppServer(DevAppServerFactory.java:38)
Caused by: java.lang.NoClassDefFoundError: com/google/appengine/tools/development/agent/AppEngineDevAgent
at com.google.appengine.tools.development.DevAppServerFactory.testAgentIsInstalled(DevAppServerFactory.java:98)
... 6 more
Caused by: java.lang.ClassNotFoundException: com.google.appengine.tools.development.agent.AppEngineDevAgent
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
... 7 more

I got two problems to fix. First the JPA enhancer was not working and second I was unable to start my server through Eclipse.

Running the server was looking straight forward from the exception that I need to add extra VM arguments for all of my applications. Remember this, your previous application will not work unless you add these VM arguments seprately in their launcher configurations.

Following are the steps to do that.

1. Right click the project and select Run As –> Open Run Dialog.

2. Copy the message which you are getting in exception. This is exactly the same that we need to add in VM arguments section. The only addition we need to do is to add our SDK path.

3. Now if you run or debug your project. You server will run perfectly.

Related posts:

1. Java in Google App Engine, what’s new for developers
2. How to avoid DeadlineExceededException in Google App Engine
3. Useful cheat sheets on Cloud Computing and Google App Engine for Java

Note: You must read App Engine docs about how they take load-on-startup.

App Engine supports the <load-on-startup> element for servlet declarations. However, the load actually occurs during the first request handled by the web server instance, not prior to it.

Following is my code snippet.

StartupServlet.java: A generic servlet. Which will be initialized when server starts.

public class StartupServlet extends GenericServlet {

public void init() {
String serverInfo = getServletContext().getServerInfo();
/* ServletContext.getServerInfo() will return "Google App Engine Development/x.x.x"
* if will run locally, and "Google App Engine/x.x.x" if run on production envoirnment */
if (serverInfo.contains("Development")) {
Constants.DEV_MODE = true;
}else {
Constants.DEV_MODE = false;
}
}

@Override
public void service(ServletRequest arg0, ServletResponse arg1)
throws ServletException, IOException {
// TODO Auto-generated method stub

}
}

web.xml: Add following contents in it

  <servlet>
    <servlet-name>StartupServlet</servlet-name>
    <servlet-class>com.servlet.StartupServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
  </servlet>

    <servlet-mapping>
    <servlet-name>StartupServlet</servlet-name>
    <url-pattern>/Startup</url-pattern>
  </servlet-mapping>

Now you can use Constants.DEV_MODE anywhere in your application. It will be true if you are on local server. And false if you are on GAE. For this constant you can make a class with name Constant and declare a static boolean with name DEV_MODE.

If you have anyother idea to implement this solution or you can help me to improve my one. Please drop your comments.

Related posts:

1. Tutorial: JPA on Google App Engine
2. Java in Google App Engine, what’s new for developers
3. Useful cheat sheets on Cloud Computing and Google App Engine for Java

According to Google support:

I’d like to map my app to http://myurl.com (also known as a naked domain).

Due to recent changes, Google App Engine no longer supports mapping your app to a naked domain. If your domain registrar supports URL redirects, you can redirect from http://yourdomain.com to your app, which can be served from domains like http://www.yourdomain.com or http://appid.yourdomain.com.

For instructions on how to configure a redirect for your Google Apps domain, please see the article on URL forwarding.

Issue can be resolved if Google provides us IP address and we set a A record pointing to that IP address. But they dont.

If you guys have idea how naked domain can be configured with this URL Forwarding thing please feel free to share. I will also update if I got this problem resolved.

Helpful resources:

• Forwarding to a website while using GApps
• How to use Google app engine with my own domain (not subdomain)?

Related posts:

1. Useful cheat sheets on Cloud Computing and Google App Engine for Java
2. Java in Google App Engine, what’s new for developers
3. Got two books on Google App Engine for Java

Case

I am developing a Twitter application and I got this exception when I login my Twitter user name. I have some 120 friends and 170 followers. So I need to fill two lists of Twitter users with their name and image URL. It was a bad practice from my side that I started populating lists in a loop which contain external service lookup overhead (Twitter) and also big in iteration. So I got following exception.
This request (670f658c2bf64b44) started at 2009/10/02 23:18:23.902 UTC and was still executing at 2009/10/02 23:18:52.605 UTC.

Possible solution

As for those users who have friends and followers in thousands, this problem will persist. So what I am thinking is to put lists in session. Populate the page with first 30 users. And give a link to next 30 (paginate). I will keep updating two variables in session which will tell start and end to pick the users from the list. I will use DWR for this. By this my every request will be responded by container with in 30 seconds.

If you guys have some more optimum solution for it, please share me. Another solution has been discussed here too.

Related posts:

1. Upgrading to Google App Engine Java SDK 1.2.8
2. Tutorial: JPA on Google App Engine
3. New Google App Engine SDK 1.4.0 released with major improvements