Help "RPC Fault Server.Processing"

[Fang]

I'm trying integrate Java + Flex by using blazeDS.

Here, one of method in my class that flex remote to:

public List<Contact> findAll() {
        Session session = sessionFactory.openSession(); // this line
        //Query query = session.createQuery("from Contact");
        List<Contact> contacts = new ArrayList<Contact>();
        Contact ct1 = new Contact(1, "aaa", "bbb", 234897234, "dasds");
        Contact ct2 = new Contact(2, "asd", "cxv", 32423543, "vcbv");
        contacts.add(ct1);
        contacts.add(ct2);
        return contacts; 
} 

If I create example data by hand, my app run ok.
If I try to get data from database, my app got an error:

[RPC Fault faultString="java.lang.NullPointerException : null" faultCode="Server.Processing" faultDetail="null"]

Please help!!!

Sorry for my bad English

[Fang]

I tested the database connection by servlet, then I think I config hibernate right.