在Android studio中创建JPA模型Google App Engine（GAE）时出错

Question

按照本教程http://www.appsrox.com/android/tutorials/instachat/7/#20，我尝试为我的项目创建一个GCM聊天应用程序。客户项目运作良好。但是，在Server项目中他们使用GAE并且还使用了JPA / JPO，但是他们在Eclipse中工作过，所以让我混淆了JPA如何在build.gradle中设置必要的Jars文件。

我的服务器和客户端项目没有显示错误，但我得到的结果与教程不同。此外，在appengine.google.com日志中，有以下错误行：

Error for /send
 java.lang.NoClassDefFoundError: Could not initialize class    com.myapp.chat.backend.model.EMFService
 at   com.myapp.chat.backend.SendServlet.doPost(SendServlet.java:41)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
 at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
 ...

此错误涉及以下几行：

SendServlet.java

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    String msg = req.getParameter(ServerConstants.MSG);
    String from = req.getParameter(ServerConstants.FROM);
    String to = req.getParameter(ServerConstants.TO);

    Contact contact = null;
    EntityManager em = EMFService.get().createEntityManager();
    try {
        contact = Contact.find(to, em);
        if (contact == null) return;
    } finally {
        em.close();
    }

    String regId = contact.getRegId();
    Sender sender = new Sender(ServerConstants.API_KEY);
    Message message = new Message.Builder()
        .addData(ServerConstants.TO, to).addData(ServerConstants.FROM, from).addData(ServerConstants.MSG, msg)
        .build();

    try {
        Result result = sender.send(message, regId, 5);     
        logger.log(Level.WARNING, "Result: " + result.toString());
    } catch (IOException e) {
        logger.log(Level.SEVERE, e.getMessage());
    }
}

我的Contact.java文件：

public class Contact {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String email;
private String regId;

public Contact() {}

public Contact(String email, String regId) {
    this.email = email;
    this.regId = regId;
}

public static Contact find(String email, EntityManager em) {
    Query q = em.createQuery("select c from Contact c where c.email = :email");
    q.setParameter("email", email);
    List<Contact> result = q.getResultList();

    if (!result.isEmpty()) {
        return result.get(0);
    }
    return null;
}

public Long getId() {
    return id;
}
public String getEmail() {
    return email;
}
public void setEmail(String email) {
    this.email = email;
}
public String getRegId() {
    return regId;
}
public void setRegId(String regId) {
    this.regId = regId;
}

}

我的EMFService.java有以下几行：

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

    private EMFService() {
    }

    public static EntityManagerFactory get() {
        return emfInstance;
    }

    private static EntityManager getEntityManager() {
        return EMFService.get().createEntityManager();
    }
}

我的build.gradle：

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.google.appengine:gradle-appengine-plugin:1.9.18'
    }
}

repositories {
    jcenter();
}

apply plugin: 'java'
apply plugin: 'war'
apply plugin: 'appengine'

sourceCompatibility = JavaVersion.VERSION_1_7
targetCompatibility = JavaVersion.VERSION_1_7

dependencies {
    appengineSdk 'com.google.appengine:appengine-java-sdk:1.9.18'
    compile 'com.google.appengine:appengine-endpoints:1.9.18'
    compile 'com.google.appengine:appengine-endpoints-deps:1.9.18'
    compile 'javax.servlet:servlet-api:2.5'
    compile files('libs/javax.persistence-2.0.0.jar')
    compile files('libs/gcm-server.jar')
    compile 'jta:jta:1.0.1b'
    compile 'com.googlecode.json-simple:json-simple:1.1.1'
    compile 'com.google.appengine.orm:datanucleus-appengine:2.1.2'
    compile 'org.datanucleus:datanucleus-enhancer:3.1.1'
    compile 'javax.jdo:jdo2-api:2.3-eb'
    compile 'javax.transaction:jta:1.1'
    compile 'org.datanucleus:datanucleus-api-jdo:3.1.3' // WE NEED THIS
    compile 'org.ow2.asm:asm:4.0'
    compile 'org.datanucleus:datanucleus-api-jpa:3.1.3'
}

appengine {
enhancer {
    version = "v2"
    api="jdo"
    enhanceOnBuild = true
}
downloadSdk = true
appcfg {
    oauth2 = true
}
endpoints {
    getClientLibsOnBuild = true
    getDiscoveryDocsOnBuild = true
}
}

Web.xml中

<?xml version="1.0" encoding="utf-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="2.5">
<servlet>
    <servlet-name>Register</servlet-name>
    <servlet-class>com.myapp.chat.backend.RegisterServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Register</servlet-name>
    <url-pattern>/register</url-pattern>
</servlet-mapping>

<servlet>
    <servlet-name>Unregister</servlet-name>
    <servlet-class>com.myapp.chat.backend.UnregisterServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Unregister</servlet-name>
    <url-pattern>/unregister</url-pattern>
</servlet-mapping>

<servlet>
    <servlet-name>Send</servlet-name>
    <servlet-class>com.myapp.chat.backend.SendServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Send</servlet-name>
    <url-pattern>/send</url-pattern>
</servlet-mapping>


<servlet>
    <servlet-name>SystemServiceServlet</servlet-name>
    <servlet-class>com.google.api.server.spi.SystemServiceServlet</servlet-class>
    <init-param>
        <param-name>services</param-name>
        <param-value>com.myapp.chat.backend.MyEndpoint</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>SystemServiceServlet</servlet-name>
    <url-pattern>/_ah/spi/*</url-pattern>
</servlet-mapping>

<welcome-file-list>
    <welcome-file>index.html</welcome-file>
</welcome-file-list>
</web-app>

的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="transactions-optional">
    <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
    <properties>
        <property name="datanucleus.NontransactionalRead" value="true"/>
        <property name="datanucleus.NontransactionalWrite" value="true"/>
        <property name="datanucleus.ConnectionURL" value="appengine"/>
        <property name="datanucleus.appengine.datastoreEnableXGTransactions" value="true" />
        <property name="datanucleus.appengine.autoCreateDatastoreTxns" value="true" />
    </properties>
</persistence-unit>

因为我使用的是不同的IDE，所以我不确定每个文件的放置位置。但是，当我点击＆＃34;部署App Engine＆＃34;一切正常。请有人帮我解决调试问题。谢谢！