com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract void org.ERPPrototyp.client.RPC.LoginService.login(java.lang.String,java.lang.String) throws org.ERPPrototyp.client.exceptions.AuthenticationException' threw an unexpected exception: org.hibernate.HibernateException: identifier of an instance of org.ERPPrototyp.User was altered from 1 to 1
        at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:360)
        at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:546)
        at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:163)
        at com.google.gwt.user.server.rpc.RemoteServiceServlet.doPost(RemoteServiceServlet.java:85)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:390)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
[...and so on]

public class User {

    private Integer userId;
    private String firstName = "";
   
    //Getter and setter
}

public class UserTest {
    public User buildUser() {
        User user = new User();
        user.setFirstName("John");
        return user;
    }
   
    public void startTest() {  //entry Point
        User user = buildUser();
        SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();
       
        session.save(user);
        session.flush();
    }

User.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <!-- <class dynamic-insert="false" dynamic-update="false" mutable="true" name="org.ERPPrototyp.User" optimistic-lock="version" polymorphism="implicit" select-before-update="false" table="users"> -->

    <class name="org.ERPPrototyp.User" table="users">
        <id name="userId" type="java.lang.Integer" column="user_id" >
            <generator class="increment" />
        </id>
        <property name="firstName" type="java.lang.String" column="first_name" length="20" />
    </class>
</hibernate-mapping>



hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory name="session1">
    <property name="hibernate.connection.driver_class">org.postgresql.Driver</property>
    <property name="hibernate.connection.url">jdbc:postgresql://localhost:5432/erp</property>
    <property name="hibernate.connection.username">uname</property>
    <property name="hibernate.connection.password">pw</property>
    <property name="hibernate.show_sql">true</property>
    <property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>
    <!-- Currently I have no Idea what autocommit does, but it sounds freakin awsome -->
    <property name="connection.autocommit">true</property>
    <!-- Mapping files -->
    <!-- <mapping resource="com/visualbuilder/hibernate/User.hbm.xml" /> -->
    <mapping resource="User.hbm.xml"/>
    <!-- <mapping resource="hibernate.hbm.xml"/> -->
  </session-factory>
</hibernate-configuration>



CREATE TABLE users
(
  user_id integer NOT NULL,
  first_name character varying(20),
  CONSTRAINT users_pkey PRIMARY KEY (user_id)
)
WITH (OIDS=FALSE);
ALTER TABLE users OWNER TO erp;

got this from my tomcat log:
Hibernate: select max(user_id) from users
Hibernate: insert into users (first_name, user_id) values (?, ?)