This project will create and delete an entity record. I have chosen JPA for datastore in app engine. You can also look JDO for this purpose. But my choice to go for JPA is due to Sun standardization on it.
I am using App Engine SDK version 1.2.18
Project name: engineplay
URL: http://engineplay.appspot.com
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
Just read cheat sheets (
