"USERS0_"."U_NAME": 标识符无效

"USERS0_"."U_NAME": 标识符无效   是什么原因？下面是Users.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">
<!-- 
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="com.hr.entity.Users" table="users" lazy="true">
        <id name="id" type="java.lang.Integer">
            <column name="u_id" />
            <generator class="sequence">
<param name="sequence">users</param>
</generator>
        </id>
        <many-to-one name="position" class="com.hr.entity.Position" fetch="select">
            <column name="u_p_id" not-null="false" />
        </many-to-one>
        <property name="name" type="java.lang.String">
            <column name="u_name" length="60" not-null="false" />
        </property>
        <property name="trueName" type="java.lang.String">
            <column name="u_true_name" length="60" not-null="false" />
        </property>
        <property name="password" type="java.lang.String">
            <column name="u_password" length="60" not-null="false" />
        </property>
        <property name="quanxian" type="com.hr.usertype.CatalogList">
            <column name="u_quanxian" length="300" not-null="false" />
        </property>
        <property name="time" type="java.sql.Date">
            <column name="u_time" length="23" not-null="false" />
        </property>
        <property name="state" type="java.lang.Integer">
            <column name="u_state" not-null="false" />
        </property>
        <set name="toMessages" inverse="true">
            <key>
                <column name="mes_to_id" />
            </key>
            <one-to-many class="com.hr.entity.ToMessage" />
        </set>
        <set name="messages" inverse="true">
            <key>
                <column name="mes_from_id" />
            </key>
            <one-to-many class="com.hr.entity.Message" />
        </set>
    </class>
</hibernate-mapping>