hibernate 异常处理

异常：not-null property references a null or transient value

用户与角色之间的关系是多对一，现在添加一用户，出现此异常的原因是
1.表User中外键角色编号不为NULL，并且在User.hbm.xml 中 用户于角色之间的关系是多对一  
  <many-to-one
        name="role"
        class="com.cn.bireport.domain.Role"
    >
        <column name="role_id" not-null="true" length="50"/>
   </many-to-one>

而在Action中没有对角色编号赋值（为null）才导致下面的异常。

javax.servlet.ServletException: not-null property references a null or transient value: com.cn.bireport.domain.User.role; nested exception is org.hibernate.PropertyValueException: not-null property references a null or transient value: com.cn.bireport.domain.User.role
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
com.cn.bireport.common.CharacterEncodeFilter.doFilter(CharacterEncodeFilter.java:41)