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我有一个应用引擎连接的 android 应用程序。当我尝试进行后端 api 调用时，设备在日志 cat 中的 EMF 文件上收到 java.lang.NoClassDefFoundError。然后当我检查应用引擎仪表板上的日志时，我看到 java.lang.ExceptionInInitialzer 错误。

EMF.java

package com.theincrowd;

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

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

    private EMF() {
    }

    public static EntityManagerFactory get() {
        return emfInstance;
    }
}

这是来自应用引擎控制台的日志

com.google.api.server.spi.SystemService invokeServiceMethod: Could not initialize class com.theincrowd.EMF
java.lang.NoClassDefFoundError: Could not initialize class com.theincrowd.EMF
    at com.theincrowd.PromoterEndpoint.getEntityManager(PromoterEndpoint.java:171)
    at com.theincrowd.PromoterEndpoint.insertPromoter(PromoterEndpoint.java:102)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:45)
    at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:337)
    at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:162)
    at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:120)
    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)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:483)
    at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:490)
    at com.google.tracing.TraceContext.runInContext(TraceContext.java:777)
    at com.google.tracing.TraceContext$DoInTraceContext.runInContext(TraceContext.java:754)
    at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:345)
at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:337)
    at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:487)
    at java.lang.Thread.run(Thread.java:722)

我阅读了这篇文章http://javaeesupportpatterns.blogspot.com/2012/07/javalangnoclassdeffounderror-how-to.html中的解释 但是，我无法识别代码中的问题。

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