Hibernate erro ao Gravar

WARNING: SQL Error: 0, SQLState: 23502
org.hibernate.exception.ConstraintViolationException: could not insert: [HN.Cliente]
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)

Caused by: org.postgresql.util.PSQLException: ERRO: valor nulo na coluna "cod" viola a restri??o n?o-nula
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:1548)
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1316)

Hibernate: insert into tb_usuario (NOME, FONE, EMAIL) values (?, ?, ?)
13/02/2008 16:05:09 org.hibernate.cfg.SettingsFactory buildSettings
INFO: Default entity-mode: pojo
13/02/2008 16:05:09 org.hibernate.cfg.SettingsFactory buildSettings
INFO: Named query checking : enabled
13/02/2008 16:05:09 org.hibernate.impl.SessionFactoryImpl <init>
INFO: building session factory
13/02/2008 16:05:09 org.hibernate.impl.SessionFactoryObjectFactory addInstance
INFO: Not binding factory to JNDI, no JNDI name configured

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
   <class name="HN.Cliente" table="tb_usuario">
       <id name="cod"  column="cod"  type="java.lang.Integer">
            <generator class="identity">
            </generator>
        </id>
        <property name="nome" column="NOME" type="string"/>
        <property name="fone" column="FONE" type="string"/>
        <property name="email" column="EMAIL" type="string"/>
    </class>
</hibernate-mapping>

<!DOCTYPE hibernate-configuration PUBLIC 
"-//Hibernate/Hibernate Configuration DTD 3.0//EN" 
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> 
<hibernate-configuration> 
    <session-factory> 
        <property name="hibernate.connection.driver_class">org.postgresql.Driver</property> 
        <property name="hibernate.connection.url">jdbc:postgresql://192.168.1.11:5432/CisterLabs</property> 
        <property name="hibernate.connection.username">postgres</property> 
        <property name="hibernate.connection.password"> </property> 
        <property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
        <property name="show_sql">true</property> 
        <!-- Hibernate XML mapping files --> 
        <mapping resource="HN/Cliente.hbm.xml"/>
        <!-- Hibernate Annotations (and package-info.java) 
        <mapping package="org.mypackage"/> 
        <mapping class="org.MyClass/>
        -->
    </session-factory> 
</hibernate-configuration><!DOCTYPE hibernate-configuration PUBLIC 
"-//Hibernate/Hibernate Configuration DTD 3.0//EN" 
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> 
<hibernate-configuration> 
    <session-factory> 
        <property name="hibernate.connection.driver_class">org.postgresql.Driver</property> 
        <property name="hibernate.connection.url">jdbc:postgresql://192.168.1.11:5432/CisterLabs</property> 
        <property name="hibernate.connection.username">postgres</property> 
        <property name="hibernate.connection.password"> </property> 
        <property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
        <property name="show_sql">true</property> 
        <!-- Hibernate XML mapping files --> 
        <mapping resource="HN/Cliente.hbm.xml"/>
        <!-- Hibernate Annotations (and package-info.java) 
        <mapping package="org.mypackage"/> 
        <mapping class="org.MyClass/>
        -->
    </session-factory> 
</hibernate-configuration>

public class Cliente 
{
    public int cod;
    public String nome;
    public String fone;
    public String email;
    
    /** Creates a new instance of Cliente */
    public Cliente() 
    {
    }

    public int getCod() {
        return cod;
    }

    public void setCod(int cod) {
        this.cod = cod;
    }

    public String getNome() {
        return nome;
    }

    public void setNome(String nome) {
        this.nome = nome;
    }

    public String getFone() {
        return fone;
    }

    public void setFone(String fone) {
        this.fone = fone;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

public class ClienteDAO {

    private SessionFactory factory;
    
    /** Creates a new instance of ClienteDAO */
    public ClienteDAO() 
    {
        factory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
    }
    public void inserir(Cliente cliente)
    {
        try
        {
            Session session = factory.openSession();
            session.save(cliente);
            session.flush();
            session.close();
        }catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
    public void alterar(Cliente cliente)
    {
        try
        {
            Session session = factory.openSession();
            session.update(cliente);
            session.flush();
            session.close();
        }catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
    public void excluir(Cliente cliente)
    {
        try
        {
            Session session = factory.openSession();
            session.delete(cliente);
            session.flush();
            session.close();
        }catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
    public Cliente buscar(Cliente cliente)
    {
        cliente = null;
        try
        {
            Session session = factory.openSession();
            cliente = (Cliente) session.get(Cliente.class, new Integer(1));
            session.flush();
            session.close();
        }catch(Exception ex)
        {
            ex.printStackTrace();
        }
        return cliente;
    }
}