<property
    name="username"
    column="username"
    unique="true"
/>

<hibernate-mapping>
    <class name="my.package.model.User" table="users">
        <id name="id">
            <generator class="increment"/>
        </id>
        <property name="username">
            <column name="username" length="16" not-null="true" unique="true"/>
        </property>
        <property name="password"/>
        <property name="firstName"/>
        <property name="lastName"/>
        <property name="email"/>
    </class>
</hibernate-mapping>

    public void save(User user) {
        getHibernateTemplate().saveOrUpdate(user);
    }

        try {
            User user = new User();
            user.setUsername("pbarry");
            dao.save(user);
        } catch(Exception ex) {
            log.error(ex);
            throw ex;
        }

11:27:38,595 - DEBUG - net.sf.hibernate.util.JDBCExceptionReporter - SQL Exception
java.sql.BatchUpdateException: failed batch
   at org.hsqldb.jdbc.jdbcStatement.executeBatch(Unknown Source)
   at org.hsqldb.jdbc.jdbcPreparedStatement.executeBatch(Unknown Source)
   at net.sf.hibernate.impl.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:54)
   at net.sf.hibernate.impl.BatcherImpl.executeBatch(BatcherImpl.java:126)
   at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2421)
   at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2371)
   at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2240)
   at org.springframework.orm.hibernate.HibernateAccessor.flushIfNecessary(HibernateAccessor.java:214)
   at org.springframework.orm.hibernate.HibernateTemplate.execute(HibernateTemplate.java:177)
   at org.springframework.orm.hibernate.HibernateTemplate.saveOrUpdate(HibernateTemplate.java:317)
   at my.package.dao.impl.UserDAOImpl.save(UserDAOImpl.java:21)
   at my.package.dao.UserDAOTest.testSaveNewDup(UserDAOTest.java:91)
    ...

        try {
           DataSource ds = (DataSource)context.getBean("dataSource");
           Connection con = ds.getConnection();
           PreparedStatement ps = con.prepareStatement(
               "INSERT INTO users (id,username) VALUES (?,?)");
           ps.setInt(1,8);
           ps.setString(2,"pbarry");
           ps.executeUpdate();
        } catch(Exception ex) {
            log.error(ex);
            throw ex;
        }

java.sql.SQLException: Unique constraint violation: SYS_CT_127 in statement [INSERT INTO users (id,username) VALUES (?,?)]

try {
   session.save(user)
}
catch (Hibernate or sql Exception) {
   user.setName("an other name");
   session = HibernateUtil.getSession();
   session.save(user);
   ....
}