java - 如何设置具有 JSF 2.2、JPA 2.0 和依赖注入(inject)功能的 Google AppEngine Web 应用程序？

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几周前，我被要求创建一个在 Google Cloud Platform 上运行的 Web 应用程序(在撰写本文时 SDK v1.9.48)。配置 AppEngine 设置(创建帐户、项目、云数据库、源代码存储库等)后，我准备使用 GAE Eclipse 插件开发我的第一个 Web 应用程序。

当我发现 GAE 仅默认支持 JSP 和 servlet 时，我感到非常失望。

然后我说:“愿上帝帮助我!又回到 J2EE 的石器时代了吗？我习惯了 JSF 和 (C)DI 的 UI。我如何将这 3 个 J2EE 标准和 (C)DI 集成到 GAE Web 应用程序中？让它顺利运行(如果可能的话)？”:

• JSF 2.x
• (C)​​DI 1.x
• JPA 2.x

只要继续阅读这篇文章，您就会知道如何做!

Answer 1

好吧，我决定不那么轻易放弃并陷入这个问题。经过几周的努力研究和试错编码，我找到了解决这个困惑问题的方法。

在开始这篇文章之前，我将为您提供一些很棒的资源，可以帮助您将其整合在一起:

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)

框架:

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)

这就是我让它工作的方法:

最重要的配置位于 web.xml 中。 JSF 初始化必须首先运行:我发现 com.sun.faces.config.ConfigureListener负责这一步，它总是寻找 FacesServlet声明。由于 JSF 请求必须由 Guice 提供 FacesHttpServlet包装器(我稍后会发布)以启用 DI，然后:

我声明了FacesServlet 没有 <servlet-mapping> s(我通过试错编码找出了该步骤)。

它只是声明来初始化 FacesContextFactory 。这是 web.xml 必须具备的结构:

            <?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>

其次，我不再尝试将托管 Bean 实例注入(inject)另一个实例。相反，我将绑定(bind)的业务逻辑实例注入(inject)到 Bean 中(换句话说，模拟 EJB 行为)。这就是我所做的:

1. 我定义了 @BindingAnnotation业务逻辑实现:

           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. 我定义了一个业务逻辑接口(interface)及其实现，并用 @BusinessLogic 进行了注释。注解(这是一个注册页面访问的例子，字段为:访问次数、源IP、时间戳):

           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);
           }
   

及其实现:

            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;
                }   
            }

为了避免重新实例化业务逻辑对象，我为 DI 绑定(bind)定义了一个实例:

            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();
            }

现在，带有对象绑定(bind)的 Guice 模块:

            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();
                }
            }

businessLogicBindings()方法将业务逻辑接口(interface)与实现实例相关联。另一方面，您可以在这一行看到:serve(urlPattern).with(FacesHttpServlet.class); ，Guice 将使用注入(inject)的 FacesServlet 将 JSF 请求重新路由到 HttpServlet 包装器。实例:

            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);
                    }
                }
            }

现在，初始化注入(inject)器的监听器:

            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);
                }
            }

最后但并非最不重要的一点是，Mojarra 必须将我们的 DI 实现注册为其 DI 提供程序(请参阅 <context-param> com.sun.faces.injectionProvider 值):

            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;
                }
            }

所有这些都可以正常工作(但省略了 JPA 部分，此时该部分不相关): 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);
                }
            }

现在，您可以创建一个 *.xhtml 文件作为索引，并将此测试代码放在上面:

            <!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 功能更容易，因为它的配置既不依赖于 JSF 也不依赖于 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();
                }
            }

所有持久化初始化属性均在 app-engine.xml 中定义。 。其基本结构:

            <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>

您必须至少定义一个持久性单元(在“persistence.xml”中):

            <?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>

以及持久性管理器对象中的一些初始化和关闭方法，用于创建和销毁 EntityManagerFactory 和 EntityManager。像这样的事情:

                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) {}
                }

“Visit”类只是一个实体类，它映射 3 个字段(访问次数、源 IP 和时间戳)，并注册在“persistence.xml”文件中。

我将这篇文章作为教程编写，逐步展示了我如何设法在 GAE 上运行这些技术(当我输入这些行时，SDK 1.9.48)。我花了数周的时间进行研究和试错编码，我希望本指南能够帮助其他 Java 程序员避免像我一样经历这种困惑。

希望本指南可以帮助其他人在 GAE 中创建出色的 J2EE 应用程序。