GAE Java- Null pointer exception when trying to retrieve an object - google-app-engine

Question

I am new to GAE development.
for me insertion using jpa to the datastore is working fine, but retrieval is not happening.
Below is a method which authenticates the user when he try to login.
public boolean loginUser(User user) {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("SOAApp");
EntityManager mgr = emf.createEntityManager();
boolean login=false;
Query query=mgr.createQuery("select u from User u where u.userName=:uid and u.password=:pwd");
query.setParameter("uid", user.getUserName());
query.setParameter("pwd",user.getPassword());
query.setMaxResults(1);
try {
user=(User) query.getSingleResult();
System.out.println("user : "+user.getName());
if(user!=null) {
System.out.println("logged in...");
login=true;
}
}
catch(Exception e) {
e.printStackTrace();
}
finally {
mgr.close();
}
return login;
}
but I am getting the following errors when trying to execute the method by passing a user object with valid username and password.
java.lang.NullPointerException
at com.google.appengine.datanucleus.query.QueryEntityPKFetchFieldManager.fetchIntField(QueryEntityPKFetchFieldManager.java:70)
at org.datanucleus.identity.IdentityUtils.getApplicationIdentityForResultSetRow(IdentityUtils.java:93)
at com.google.appengine.datanucleus.EntityUtils.entityToPojo(EntityUtils.java:1009)
at com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:228)
at com.google.appengine.datanucleus.query.DatastoreQuery$2.apply(DatastoreQuery.java:225)
at com.google.appengine.datanucleus.query.LazyResult.resolveNext(LazyResult.java:96)
at com.google.appengine.datanucleus.query.LazyResult.resolveAll(LazyResult.java:121)
at com.google.appengine.datanucleus.query.LazyResult.size(LazyResult.java:115)
at com.google.appengine.datanucleus.query.StreamingQueryResult.size(StreamingQueryResult.java:151)
at org.datanucleus.store.query.Query.executeQuery(Query.java:1808)
at org.datanucleus.store.query.Query.executeWithMap(Query.java:1693)
at org.datanucleus.api.jpa.JPAQuery.getSingleResult(JPAQuery.java:231)
at com.soa.managers.UserManager.loginUser(UserManager.java:63)
at com.soa.servlets.LoginServlet.doPost(LoginServlet.java:48)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:123)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:61)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:94)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:383)
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$RequestHandler.content(HttpConnection.java:938)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Nov 03, 2012 3:25:02 PM com.google.appengine.tools.development.LocalResourceFileServlet doGet
line number-63
[at com.soa.managers.UserManager.loginUser(UserManager.java:63)]
is "user=(User) query.getSingleResult();"
Please help me

Answer 1

Still searching?
I had the same problem and my entity User had a primaryKey named userId that was of type 'int'. Now I changed it to 'Long' and all of my problems and warnings are all gone! Maybe this helps you out!
I look forward hearing from you!
Regards,
Stephan

Answer 2

The reason why u get nullpointerexception is that getSingleResult method returns null. Because u used (User)query.getSingleResult(), the typecasting of null value throws exception.
I think there is no related user in datastore, so you are getting exception.

Answer 3

<?xml version="1.0" encoding="UTF-8"?>
<datastore-indexes autoGenerate="true">
<datastore-index kind="HABERLER" ancestor="false" source="manual">
<property name="__key__" direction="desc"/>
</datastore-index>
HABERLER is table name. Write your table name instead of HABERLER. Then create a datastore-indexes.xml file in WEB-INF directory. Then copy above code into this file.

Answer 4

Make sure you that the keys for your entities are of valid types as detailed here:
https://developers.google.com/appengine/docs/java/datastore/jdo/creatinggettinganddeletingdata#Keys
The exception you are getting is most likely related to a key issue.