Root cause:
org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed
Are you using Google App Engine for java and JDO?
If you ask for a list of something you might have to call the method size(). (studip i know)
Thecode below would throw org.datanucleus.exceptions.NucleusUserException:"Object Manager has been closed" if we removed the call to size()
public List<Customer>getAllCustomers()
{
List<Customer> customers = new ArrayList();
PersistenceManager pm = pmfInstance.getPersistenceManager();
String query = "select from " + Customer.class.getName();
customers = (List<Customer> pm.newQuery(query).execute();
customers.size();
pm.close();
return customers;
}