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
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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

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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