Why does google app engine fail to find "org/datanucleus/jpa/exceptions/NotProviderException" ?? The jar file is in the classpath

[user198923 via StackOverflow]

I am building a google app engine supported application. When I am trying to read the data from the app engine datastore (via a call to the backend API), I get the following error:

(Note: That I currently have the datanucleus.jpa... jar file in the standard lib directory of the webinf folder. That is where the class that is being reported as "not found" exists.)

Error, as reported by google app engine logs

com.google.api.server.spi.SystemService invokeServiceMethod: org/datanucleus/jpa/exceptions/NotProviderException
java.lang.NoClassDefFoundError: org/datanucleus/jpa/exceptions/NotProviderException
    at com.google.appengine.runtime.Request.process-3104daaaf762dd6a(Request.java)
    at java.lang.Class.getDeclaredConstructors0(Native Method)

Error, as reported within my app.

10-11 18:22:59.415   W/System.err﹕ com.google.api.client.googleapis.json.GoogleJsonResponseException: 503 Service Unavailable
10-11 18:22:59.415  W/System.err﹕ {
10-11 18:22:59.415  W/System.err﹕ "errors" : [ {
10-11 18:22:59.415  W/System.err﹕ "domain" : "global",
10-11 18:22:59.415  W/System.err﹕ "reason" : "backendError",
10-11 18:22:59.415  W/System.err﹕ "message" : "java.lang.NoClassDefFoundError: org/datanucleus/jpa/exceptions/NotProviderException"
10-11 18:22:59.415  W/System.err﹕ } ],
10-11 18:22:59.415  W/System.err﹕ "code" : 503,
10-11 18:22:59.415  W/System.err﹕ "message" : "java.lang.NoClassDefFoundError: org/datanucleus/jpa/exceptions/NotProviderException"
10-11 18:22:59.415  W/System.err﹕ }
10-11 18:22:59.415  W/System.err﹕ at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(SourceFile:113)
10-11 18:22:59.415  W/System.err﹕ at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(SourceFile:40)
10-11 18:22:59.415  W/System.err﹕ at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.a(SourceFile:312)

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