为什么primefaces数据表单选实体为null？

Question

我在我的项目中使用了primefaces 3.1.1和jsd 2.0，当我在表上选择一行时，我的实体对象返回null.where是错误还是丢失？

table.xhtml

<p:dataTable  value="#{dataBean.tmVlist}" var="a" >  


       <f:facet name="header">  
        Centre  
       </f:facet> 


    <p:column headerText="PSSE" style="width:24%">  
    #{a.psseCode}  
    </p:column> 


    <p:column headerText="Adı" style="width:24%">  
       #{a.name}
    </p:column>  

     <p:column headerText="İl"> 
     #{a.centre}
     </p:column>

     <p:column headerText="Bölge">
     #{a.area}
     </p:column>

    <p:column headerText="Durum">
    #{a.state}
    </p:column>

    <p:column>
      <p:commandButton id="selectButton"  update=":form2:display" oncomplete="carDialog.show()"  value="edit" title="edit"> 

            <f:setPropertyActionListener value="#{a}"  target="#{deleteBean.selectedTm}" />  
</p:commandButton>
    </p:column> 

    </p:dataTable>

java代码：

public tmView selectedTm=new tmView();


public tmView getSelectedTm() {
    return selectedTm;
}


public void setSelectedTm(tmView selectedTm) {
    this.selectedTm = selectedTm;
}

thx for help