rich：extendedDataTable selectionItems“not stored”

Question

我正在尝试使用extendedDataTable，这里是代码的一部分，xhtml部分：

<h:form>
             <rich:extendedDataTable id="network_table" value="#{NetworkBean.availableNetworkList()}" var="network" selection="#{NetworkBean.selection}" selectionMode="single" >
                  <a4j:ajax execute="@form" event="selectionchange" listener="#{NetworkBean.selectionListener}"/>



              <rich:column>
                                 <f:facet name="header">Available Networks</f:facet>
                                 <h:outputText value="#{network}"/>
             </rich:column>    
         </rich:extendedDataTable>
         <a4j:commandButton value="1" action="#{NetworkBean.availableMssList()}">

         </a4j:commandButton >
       </h:form> 

和支持bean的某些部分：

 private Collection<Object> selection;
 private List<String> selectionItems = new ArrayList<String>();
 public void selectionListener(AjaxBehaviorEvent event) {
        UIExtendedDataTable dataTable = (UIExtendedDataTable) event.getComponent();
        Object originalKey = dataTable.getRowKey();
        selectionItems.clear();
        for (Object selectionKey : selection) {
            dataTable.setRowKey(selectionKey);
            if (dataTable.isRowAvailable()) {
                System.out.print((String) dataTable.getRowData());
                selectionItems.add((String) dataTable.getRowData());
                System.out.print(selectionItems);
            }
        }
        dataTable.setRowKey(originalKey);
    }



public Collection<Object> getSelection() {
        return selection;
    }
/**
 * @param selection the selection to set
 */
public void setSelection(Collection<Object> selection) {
    this.selection = selection;
}

/**
 * @return the selectionItems
 */

public String getSelectionItem() {
    if (selectionItems == null || selectionItems.isEmpty()) {
        return null;
    }
    return selectionItems.get(0);
}

public List<String> getSelectionItems() {
    return selectionItems;
}

/**
 * @param selectionItems the selectionItems to set
 */
public void setSelectionItems(List<String> selectionItems) {
    this.selectionItems = selectionItems;
}

public void availableMssList(){
System.out.print("hithere");
    System.out.println(selectionItems);

}

所以我的问题在于selectionItems，如果我点击一行它工作正常并将选定的行打印到控制台（监听器中的两个打印输出）。但是当我尝试使用带有a4j命令按钮的availableMssList方法时，selectionItems似乎是空的。可能是什么问题？

感谢您的帮助！