How do I set up a Google AppEngine webapp with JSF

Question

A couple of weeks ago I was asked to create a web application to run on Google Cloud Platform (SDK v1.9.48 by the time this post was typed in). After configuring AppEngine settings (creating account, project, cloud database, source code repository, etc, etc, etc...), I was ready to develop my first webapp using GAE Eclipse Plugin.

It was huge my dissapointment when I found out that GAE only brings along by default support for JSP and servlets.

Then I said: "May God help me! Back to J2EE's stone age again? I'm used to JSF and (C)DI for the UI. How am I going to integrate in a GAE webapp these 3 J2EE standards and make it run smoothly (if such a thing is possible)?":

• JSF 2.x
• (C)DI 1.x
• JPA 2.x

Just keep reading this post and you'll know how!

Answer 1

Well, I decided not to give up so easily and get into the problem. After a couple of weeks of hard researching and trial-error coding, I found the solution to this mess.

Before beginning with the post, I'll give you some great resources that can help you to pull this together:

1. Configuring JSF 2.2 to run on the Google App Engine Using Eclipse
2. A workaround for a session data loss bug
3. List of libraries I used (at least half of them come bundled in GAE SDK)

Frameworks:

1. Datanucleus 3.1.1 (JPA 2.0)
2. Oracle Mojarra 2.2.4 (JSF 2.2).
3. Google Guice 4.0 (DI 1.0)

This is how I got it to work:

The most important configuration is in web.xml. The JSF initialization MUST RUN FIRST: I found out that com.sun.faces.config.ConfigureListener is in charge of that step and it always looks for the FacesServlet declaration. Since JSF requests MUST be served by Guice with a FacesHttpServlet wrapper (which I'll post later) in order to enable DI, then:

I declared the FacesServlet WITHOUT <servlet-mapping>s (I figured out that step by trial-error coding).

It's only declared to initialize the FacesContextFactory. This is the MUST-HAVE structure of the web.xml:

            <?xml version="1.0" encoding="utf-8"?>
            <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                version="2.5" 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_2_5.xsd">
                <display-name>BrochureForce</display-name>

                <description>Purchase orders generator configured to run on the Google AppEngine.</description>

                <context-param>
                    <description>Project stage (Development or Production)</description>
                    <param-name>javax.faces.PROJECT_STAGE</param-name>
                    <param-value>Development</param-value>
                </context-param>

                <context-param>
                    <description>
                                Designate client-side state saving, since GAE doesn't handle 
                                server side (JSF default) state management.
                    </description>
                    <param-name>javax.faces.STATE_SAVING_METHOD</param-name>
                    <param-value>client</param-value>
                </context-param>

                <context-param>
                    <description>Sets the default suffix for JSF pages to .xhtml</description>
                    <param-name>javax.faces.DEFAULT_SUFFIX</param-name>
                    <param-value>.xhtml</param-value>
                </context-param>

                <context-param>
                    <description>
                             When enabled, the runtime initialization and default ResourceHandler 
                             implementation will use threads to perform their functions. Set this 
                             value to false if threads aren't desired (as in the case of single-threaded
                             environments such as the Google AppEngine).
                             Note that when this option is disabled, the ResourceHandler will not 
                             pick up new versions of resources when ProjectStage is development.
                        </description>
                    <param-name>com.sun.faces.enableThreading</param-name>
                    <param-value>false</param-value>
                </context-param>

                <context-param>
                    <description>Allows dependency-injection into ManagedBeans</description>
                    <param-name>com.sun.faces.injectionProvider</param-name>
                    <param-value>mypackage.jsf.JsfInjectionProvider</param-value>
                </context-param>

                <context-param>
                    <description>Specify JBoss Expression Language Over Default</description>
                    <param-name>com.sun.faces.expressionFactory</param-name>
                    <param-value>org.jboss.el.ExpressionFactoryImpl</param-value>
                </context-param>

                <!-- JSF INITIALIZATION GOES FIRST!! -->
                <servlet>
                    <description>
                            JSF 2 Servlet. There's NO servlet-mapping defined for this servlet because
                            it's declared here in order to enforce the FacesFactory to load properly
                            so that an instance of this servlet can be injected in the FacesHttpServlet
                            used by Guice to serve JSF requests and as injection provider at the same time.
                            Furthermore, the "load-on-startup" property is set to "0" to tell Jetty
                            that this servlet MUST be loaded first.
                    </description>
                    <servlet-name>JSF Servlet</servlet-name>
                    <servlet-class>javax.faces.webapp.FacesServlet</servlet-class>
                    <load-on-startup>0</load-on-startup>
                </servlet>
                <listener>
                    <description>JSF Initialization.</description>
                    <listener-class>com.sun.faces.config.ConfigureListener</listener-class>
                </listener>
                <!-- JSF INITIALIZATION GOES FIRST!! -->

                <listener>
                    <description>PERSISTENCE ENGINE INITIALIZATION AND SHUTDOWN.</description>
                    <listener-class>mypackage.listener.PersistenceManagerSetupListener</listener-class>
                </listener>

                <!-- ***** Specify session timeout of thirty (30) minutes. ***** -->
                <session-config>
                    <session-timeout>30</session-timeout>
                </session-config>

                <welcome-file-list>
                    <welcome-file>index.jsf</welcome-file>
                    <welcome-file>index.xhtml</welcome-file>
                </welcome-file-list>
                <!-- **************************************************** -->
                <!-- DI API initialization (Google Guice Implementation). -->
                <!-- **************************************************** -->
                <filter>
                    <description>Google Guice filter which enables DI.</description>
                    <filter-name>GuiceFilter</filter-name>
                    <filter-class>com.google.inject.servlet.GuiceFilter</filter-class>
                </filter>
                <filter-mapping>
                    <filter-name>GuiceFilter</filter-name>
                    <url-pattern>/*</url-pattern>
                </filter-mapping>
                <listener>
                    <description>
                                This listener initializes the Guice injector and wraps the JSF Servlet
                                into a HttpServlet in order to serve JSF requests via Guice Filter.
                        </description>
                    <listener-class>mypackage.listener.GuiceListener</listener-class>
                </listener>
                <!-- **************************************************** -->
            </web-app>

Second, I'm not trying to inject a managed bean instance into another anymore. Instead, I'm injecting a bound business logic instance into the beans (in other words, emulating EJB behavior). This is what I did:

1. I defined a @BindingAnnotation for the business logic implementation:

           import static java.lang.annotation.ElementType.TYPE;
           import static java.lang.annotation.RetentionPolicy.RUNTIME;
           import java.lang.annotation.Documented;
           import java.lang.annotation.Retention;
           import java.lang.annotation.Target;
           import com.google.inject.BindingAnnotation;
   
           @Documented
           @BindingAnnotation
           @Retention(RUNTIME)
           @Target({ TYPE })
           public @interface BusinessLogic {}
   

2. I defined a business logic interface with its implementation and annotated both with the @BusinessLogic annotation (This is an example that registers a visit made to the page. The fields are: the visit number, the source IP and the timestamp):

           import java.util.List;
           import mypackage.annotation.BusinessLogic;
           import mypackage.dataaccess.entity.Visit;
   
           @BusinessLogic
           public interface VisitsHandler {
               public void insertVisit();
               public List<Visit> getPageVisits();
   
               // Propiedades
               public String getCurrentVisit();
               public void setCurrentVisit(String currentVisit);
           }
   

and its implementation:

            import java.util.ArrayList;
            import java.util.Date;
            import java.util.List;

            import mypackage.annotation.BusinessLogic;
            import mypackage.jsf.logic.VisitsHandler;
            import mypackage.dataaccess.PersistenceManager;
            import mypackage.dataaccess.Queries;
            import mypackage.dataaccess.entity.Visit;

            @BusinessLogic
            public class VisitsHandlerImpl implements VisitsHandler {
                private String currentVisit;

                public void insertVisit() {
                    PersistenceManager pMgr = PersistenceManager.getInstance();
                    Visit newVisit = new Visit();
                    newVisit.setUserIp("127.0.0.1");
                    newVisit.setTimestamp(new Date(System.currentTimeMillis()));
                    pMgr.insert(newVisit);
                    pMgr = null; // Dereference the singleton instance.
                    this.currentVisit = newVisit.toString();
                }

                @SuppressWarnings("rawtypes")
                public List<Visit> getPageVisits() {
                    PersistenceManager pMgr = PersistenceManager.getInstance();
                    List<Visit> visitsList = new ArrayList<Visit>(); 
                    List visits = pMgr.executeJpqlQuery(Queries.JPQL_VISITS);
                    for (Object v : visits) {
                        visitsList.add((Visit) v);
                    }
                    pMgr = null; // Dereference the singleton instance.
                    return visitsList;
                }

                /**
                 * @return the currentVisit
                 */
                public String getCurrentVisit() {
                    return currentVisit;
                }

                /**
                 * @param currentVisit
                 *            the currentVisit to set
                 */
                public void setCurrentVisit(String currentVisit) {
                    this.currentVisit = currentVisit;
                }   
            }

To avoid reinstantiation of the business logic objects, I defined a single instance for the DI binding:

            import mypackage.jsf.logic.VisitsHandler;
            import mypackage.jsf.logic.impl.VisitsHandlerImpl;
            interface InjectorConstants {

                // Url patterns for FacesServlet, as it would be defined in web.xml
                static String[] JSF_SERVLET_URL_PATTERNS = new String[] { "*.jsf", "*.xhtml" };

                // BUSINESS LOGIC OBJECTS.
                static Class<VisitsHandler> VISITS_HANDLER = VisitsHandler.class;
                static VisitsHandler VISITS_HANDLER_IMPL = new VisitsHandlerImpl();
            }

Now, the Guice module with the object bindings:

            import javax.faces.webapp.FacesServlet;
            import javax.inject.Singleton;

            import mypackage.cdi.annotation.ViewScoped;
            import mypackage.cdi.annotation.ViewScopedImpl;
            import mypackage.cdi.listener.PostConstructTypeListener;
            import mypackage.jsf.FacesHttpServlet;
            import com.google.inject.matcher.Matchers;
            import com.google.inject.servlet.ServletModule;

            public class JSFModule extends ServletModule {
                private void businessLogicBindings() {
                    bind(InjectorConstants.VISITS_HANDLER).toInstance(InjectorConstants.VISITS_HANDLER_IMPL);
                }

                private void systemBindings() {
                    // Add support for the @PostConstruct annotation for Guice-injected
                    // objects.
                    bindListener(Matchers.any(), new PostConstructTypeListener(null));

                    // Binding a custom implementation of "@ViewScoped" scope.
                    bindScope(ViewScoped.class, new ViewScopedImpl());
                }

                private void jsfBindings() {
                    // Define and bind FacesServlet as singleton object
                    // so it can be injected in FacesHttpServlet's constructor.
                    bind(FacesServlet.class).in(Singleton.class);

                    // JSF patterns to be served by FacesHttpServlet.
                    for (String urlPattern : InjectorConstants.JSF_SERVLET_URL_PATTERNS) {
                        serve(urlPattern).with(FacesHttpServlet.class);
                    }
                }

                @Override
                protected void configureServlets() {
                    // Guice injector bindings.
                    this.systemBindings();
                    this.businessLogicBindings();
                    this.jsfBindings();
                }
            }

The businessLogicBindings() method associates the business logic interface with the implementation instance. On the other hand, you can see on this line: serve(urlPattern).with(FacesHttpServlet.class);, Guice will reroute JSF requests to a HttpServlet wrapper with an injected FacesServlet instance:

            import java.io.IOException;
            import javax.faces.webapp.FacesServlet;
            import javax.inject.Inject;
            import javax.inject.Singleton;
            import javax.servlet.Servlet;
            import javax.servlet.ServletConfig;
            import javax.servlet.ServletException;
            import javax.servlet.ServletRequest;
            import javax.servlet.ServletResponse;
            import javax.servlet.http.HttpServlet;
            import javax.servlet.http.HttpServletRequest;
            import javax.servlet.http.HttpServletResponse;

            @Singleton
            public class FacesHttpServlet extends HttpServlet {

                private static final long serialVersionUID = 1L;

                private final Servlet facesServlet;

                @Inject
                public FacesHttpServlet(FacesServlet facesServlet) {
                    this.facesServlet = facesServlet;
                }

                @Override
                public void init(ServletConfig config) throws ServletException {
                    this.facesServlet.init(config);
                }

                @Override
                public ServletConfig getServletConfig() {
                    return this.facesServlet.getServletConfig();
                }

                @Override
                public String getServletInfo() {
                    return this.facesServlet.getServletInfo();
                }

                @Override
                public void destroy() {
                    super.destroy();
                    this.facesServlet.destroy();
                }

                @Override
                public void service(ServletRequest req, ServletResponse resp) throws ServletException, IOException {
                    HttpServletRequest httpReq = (HttpServletRequest) req;
                    String reqUrl = httpReq.getRequestURL().toString();
                    // A hack to redirect the index page. It's been throwing an error if the
                    // "/index.[xhtml|jsf]" is not explicitly specified in the request URL.
                    if(reqUrl.toLowerCase().endsWith("index.xhtml")) {
                        ((HttpServletResponse) resp).sendRedirect(reqUrl.replace("index.xhtml", "index.jsf"));
                    } else {
                        this.facesServlet.service(req, resp);
                    }
                }
            }

Now, the listener that initializes the injector:

            import java.util.HashMap;
            import mypackage.cdi.JSFModule;
            import mypackage.cdi.JsfInjectionProvider;
            import com.google.inject.AbstractModule;
            import com.google.inject.Guice;
            import com.google.inject.Injector;
            import com.google.inject.servlet.GuiceServletContextListener;

            public class GuiceListener extends GuiceServletContextListener {
                protected AbstractModule module;
                protected static Injector injector;
                private static HashMap<String, Object> instancesMap;

                public GuiceListener() {
                    // Bean instance list to ensure that we inject a unique bean instance.
                    instancesMap = new HashMap<>();

                    // Create the injector.
                    injector = Guice.createInjector(new JSFModule());
                }

                @Override
                public Injector getInjector() {
                    return injector;
                }

                /**
                 * given a class, generates an injected instance. Useful when an API call is
                 * needed internally.
                 */
                public static <T> T getInstance(Class<T> type) {
                    return injector.getInstance(type);
                }

                /**
                 * given an injectable instance, injects its dependencies and make sure to
                 * only inject one.
                 */
                public static void injectMembers(Object instance) {
                    Object obj = null;
                    if (JsfInjectionProvider.isBusinessLogicObject(obj)) {
                        String instanceClassName = instance.getClass().getName();
                        Object mappedInstance = instancesMap.get(instanceClassName);
                        if (mappedInstance == null) {
                            // It's a new bean instance. It's stored in the bean map
                            // to be able to reuse it.
                            instancesMap.put(instanceClassName, instance);
                            obj = instance;
                        } else {
                            // There's already a bean instance. Let's reuse it!.
                            obj = mappedInstance;
                        }
                    } else { // it should be a managed bean.
                        obj = instance;
                    }
                    injector.injectMembers(obj);
                }
            }

Last, but not least, Mojarra must register our DI implementation as its DI provider (see the <context-param> com.sun.faces.injectionProvider value):

            import javax.faces.bean.ManagedBean;
            import mypackage.cdi.annotation.BusinessLogic;
            import mypackage.cdi.listener.GuiceListener;
            import com.sun.faces.spi.InjectionProviderException;
            import com.sun.faces.vendor.WebContainerInjectionProvider;

            public class JsfInjectionProvider extends WebContainerInjectionProvider {
                @Override
                public void inject(Object obj) throws InjectionProviderException {
                    if (isManagedBean(obj) || isBusinessLogicObject(obj)) {
                        GuiceListener.injectMembers(obj);
                    }
                }

                /**
                 * As an arbitrary choice, the choice here is to inject only into
                 * {@code @ManagedBean} instances, so that other classes - not written by us
                 * - wouldn't be injected too. This choice could be altered.
                 * 
                 * @param obj
                 *            A JSF bean instance (annotated with @ManagedBean).
                 * @return
                 */
                private boolean isManagedBean(Object obj) {
                    return obj != null && obj.getClass().getAnnotation(ManagedBean.class) != null;
                }

                public static boolean isBusinessLogicObject(Object obj) {
                    return obj != null && obj.getClass().getAnnotation(BusinessLogic.class) != null;
                }
            }

All of this working altogether (yet ommitting the JPA part, which is not relevant at this point): ExampleBean:

            import java.io.Serializable;
            import java.util.List;

            import javax.annotation.PostConstruct;
            import javax.faces.bean.ManagedBean;
            import javax.inject.Inject;

            import mypackage.jsf.logic.VisitsHandler;
            import mypackage.dataaccess.entity.Visit;

            @ManagedBean(name="jsfbExample")
            public class ExampleBean implements Serializable {

                private static final long serialVersionUID = 1L;

                @Inject
                private VisitsHandler visitsHandler;

                @PostConstruct
                public void init() {
                    System.out.println("ExampleBean - Injection works! visitsHandler = " + visitsHandler); // It works.
                }

                /**
                 * Method to test EL engine processing with parameters. 
                 * @param param
                 * @return
                 */
                public void insertVisit() {
                    this.visitsHandler.insertVisit();
                }

                public List<Visit> getPageVisits() {
                    return this.visitsHandler.getPageVisits();
                }


                /**
                 * @return the currentVisit
                 */
                public String getCurrentVisit() {
                    return this.visitsHandler.getCurrentVisit();
                }

                /**
                 * @param currentVisit
                 *            the currentVisit to set
                 */
                public void setCurrentVisit(String currentVisit) {
                    this.visitsHandler.setCurrentVisit(currentVisit);
                }
            }

Now, you can create a *.xhtml file as your inxdex and put this testing code on it:

            <!DOCTYPE html 
                     PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
                     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
            <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"
                xmlns:f="http://java.sun.com/jsf/core"
                xmlns:h="http://java.sun.com/jsf/html"
                xmlns:ui="http://java.sun.com/jsf/facelets">
            <h:head id="head">
                <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
                <title>Welcome to JSF 2.1 on the Google AppEngine!</title>
            </h:head>
            <h:body>
                    <h:form>
                        <h:outputText id="lastVisit" value="#{jsfbExample.currentVisit}" /><br/>
                        <h:commandButton value="New visit!"
                            actionListener="#{jsfbExample.insertVisit()}">
                            <f:ajax execute="@this" render="pageVisitsList" />
                        </h:commandButton>
                        <h:commandButton value="Last inserted visit!">
                            <f:ajax execute="@this" render="lastVisit" />
                        </h:commandButton>
                        <h:panelGrid id="pageVisitsList">
                            <c:forEach var="visit" items="#{jsfbExample.pageVisits}">
                                <h:outputText value="#{visit.toString()}" />
                            </c:forEach>
                        </h:panelGrid>
                    </h:form>
            </h:body>
            </html>

JPA feature is easier since its configuration neither depends on JSF nor DI. PersistenceManagerSetupListener:

            package mypackage.listener;

            import javax.servlet.ServletContextEvent;
            import javax.servlet.ServletContextListener;
            import mypackage.dataaccess.PersistenceManager;
            import mypackage.utils.StringMap;

            public class PersistenceManagerSetupListener implements ServletContextListener {

                @Override
                public void contextInitialized(ServletContextEvent servletContextInitEvt) {

                  // This is only a wrapper over HashMap<String, String>
                    StringMap initProperties = new StringMap();

                    // Check the System properties to determine if we are running on cloud
                    // or not, and set up the JDBC driver accordingly.
                    String platform = System.getProperty("com.google.appengine.runtime.version").toLowerCase()
                            .contains("google app engine") ? "cloud" : "dev";
                    initProperties.put("datanucleus.ConnectionURL", System.getProperty(platform + ".db.url"));
                    initProperties.put("datanucleus.ConnectionDriverName", System.getProperty(platform + ".db.driver"));
                    initProperties.put("datanucleus.ConnectionUserName", System.getProperty(platform + ".db.user"));
                    initProperties.put("datanucleus.ConnectionPassword", System.getProperty(platform + ".db.password"));
                    // I implemented password encryption. See Datanucleus' "ConnectionEncryptionProvider" interface documentation.
                  initProperties.put("datanucleus.ConnectionPasswordDecrypter",
                            System.getProperty(platform + ".db.encryptionProvider"));

                    // ***********************************************************************************************************
                    // THESE 2 ARE A MUST-HAVE!!!
                    // ***********************************************************************************************************
                    initProperties.put("datanucleus.identifier.case", System.getProperty("persistencemanager.identifier.case"));
                    initProperties.put("datanucleus.storeManagerType", System.getProperty("persistencemanager.storeManagerType"));
                    // ***********************************************************************************************************

                    initProperties.put("datanucleus.NontransactionalRead",
                            System.getProperty("persistencemanager.NontransactionalRead"));
                    initProperties.put("datanucleus.NontransactionalRead",
                            System.getProperty("persistencemanager.NontransactionalRead"));
                    initProperties.put("datanucleus.NontransactionalWrite",
                            System.getProperty("persistencemanager.NontransactionalWrite"));
                    initProperties.put("datanucleus.singletonEMFForName",
                            System.getProperty("persistencemanager.singletonEMFForName"));
                    initProperties.put("javax.persistence.query.timeout", System.getProperty("persistencemanager.query.timeout"));
                    initProperties.put("datanucleus.datastoreWriteTimeout",
                            System.getProperty("persistencemanager.datastoreWriteTimeout"));


                    // Initialize persistence engine.
                    PersistenceManager.initialize(initProperties);
                }

                @Override
                public void contextDestroyed(ServletContextEvent servletContextDestroyedEvt) {
                    PersistenceManager.shutdown();
                }
            }

All the persistence init properties are defined in app-engine.xml. Its basic structure:

            <appengine-web-app ...>
                <application>cloud-project-id</application>
                <version>1</version>
                <threadsafe>true</threadsafe>
                <system-properties>
                    <!-- Cloud platform properties (their name starts with "cloud") -->
                    <property name="cloud.db.url"
                        value="jdbc:google:mysql://(cloud-connection-name)/(db-name)" />
                    <property name="cloud.db.driver"
                        value="com.google.appengine.api.rdbms.AppEngineDriver" />
                        <!--  ...  -->
                    <!-- Dev platform properties (their name starts with "dev") -->
                    <property name="dev.db.url" value="jdbc:mysql://(db-server):(db-port)/(db-name)" />
                    <property name="dev.db.driver" value="com.mysql.jdbc.Driver" />
                        <!--  ...  -->
                    <!-- Datanucleus properties --> 
                    <!-- *********************************************** -->
                    <!-- THESE 2 ARE A MUST-HAVE!!! Others are optional -->
                    <!-- *********************************************** -->
                    <property name="persistencemanager.storeManagerType" value="rdbms" />

                    <!-- This means that all DB identifiers MUST be defined in lowercase. -->
                    <property name="persistencemanager.identifier.case" value="LowerCase" /> 
                    <!-- *********************************************** -->
                        <!--  ...  -->
                </system-properties>
                <sessions-enabled>true</sessions-enabled>
                <async-session-persistence enabled="false" />
                <static-files>
                    <exclude path="/**.xhtml" />
                </static-files>
            </appengine-web-app>

You must define at least one persistence unit (in "persistence.xml"):

            <?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="MyPersistenceUnit">
                    <!-- DATANUCLEUS' JPA 2.0 PERSISTENCE PROVIDER CLASS -->
                    <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>

                    <!-- ENTITY CLASSES -->
                    <class>mypackage.dataaccess.entity.Visit</class>

                    <!-- DON'T PROCESS UNLISTED CLASSES AS ENTITY CLASSES. -->
                    <exclude-unlisted-classes>true</exclude-unlisted-classes>
                </persistence-unit>
            </persistence>

and some initialize and shutdown methods in your persistence manager object(s) to create and destroy the EntityManagerFactory and the EntityManager(s). Something like this:

                public static void initialize(Map properties) {
                    if (!isInitialized) {
                        if (properties == null) {
                            emfInstance = Persistence.createEntityManagerFactory("MyPersistenceUnit");
                        } else {
                            emfInstance = Persistence.createEntityManagerFactory("MyPersistenceUnit", properties);
                        }
                        emInstance = emfInstance.createEntityManager();
                        isInitialized = true;
                    }
                }

                public static void shutdown() {
                    try {
                        emInstance.close();
                    } catch (Exception e) {}
                    try {
                        emfInstance.close();
                    } catch (Exception e) {}
                }

The "Visit" class is just an Entity class which maps the 3 fields (Number of visit, source IP and timestamp) and it's registered in the "persistence.xml" file.

I wrote this post as a tutorial that shows, step-by-step, how I managed to run these technologies on GAE (SDK 1.9.48 by the time I typed these lines in). It's taken me weeks of researching and trial-error coding, and I expect with this guide to help other Java programmers not to go through that mess as I did.

Hope this guide can help others to create great J2EE apps in GAE.