NPE when trying to access a list from action class

[Dan Rasmussen]

I spent a couple of years with Struts 1 back in the day (2003 - 2005 or so). Just coming back to it again and have a maddening (but familiar feeling problem). Basically, I have a list that I am populating and mutating through a single jsp page that calls out separate actions depending on user actions (init() on entry, addFieldMapping() to add to the list, etc.). The init() properly initializes the list and it displays just fine when I get there but when I get to the addField, the list is null. I believe I have all the relevant code listed below. I suspect it has something to do with my jsp where I print out the list and ready it to be loaded back into the action class or something to do with validate() but I dumped the validate() method (not implemented).

My struts.xml has these entries:

        <!-- This works as planned-->
        <action name="beginMapFields" class="org.scmsolution.struts.action.MapFields" method="init">
          <result name="success">/mapFields.jsp</result>
          <result name="input">/mapFields.jsp</result>
        </action>
        
        <!-- This doesn't -->
        <action name="addFieldMapping" class="org.scmsolution.struts.action.MapFields" method="addField">
          <result name="success">/mapFields.jsp</result>
          <result name="input">/mapFields.jsp</result>
        </action>

I have a list in my action clss like so:

	List<FieldMapping> fmList;
	public List<FieldMapping> getFmList() {
		return fmList;
	}

	public void setFmList(List<FieldMapping> fmList) {
		this.fmList = fmList;
	} 

   //And here is the init with test data 
   public String init() throws Exception {
		fmList = new ArrayList<FieldMapping>();
		fmList.add(new FieldMapping("autoQCID", "id"));
		fmList.add(new FieldMapping("internalState", "BG_STATUS"));
		fmList.add(new FieldMapping("internalPriority", "BG_PRIORITY"));
		
		return SUCCESS;
	}

     // And here is the action method where it is crashing:
     public String addField() throws Exception {
		fmList.add(new FieldMapping("foo", "bar"));	//Crashes on this line	
		return SUCCESS;
	}

My jsp properly displays the test data populated in the init() but, when the addFieldMapping action is exercised, fmList is NULL in the addField() method.

I have been beating my head against this for hours, including changing approaches (started with a Map). Why is fmList not getting populated from my jsp? I have tried many flavors of the s:label line including:

<td><s:label name="%{'fmList[' + #stat.index + '].rtc'}" value="%{rtc}" /></td>

and

<s:label name="%{fmList['+#stat.index+'].rtc}" value="%{rtc}" />

Here is the last incantation of it:

  <s:form action="addFieldMapping"  theme="simple">
    <s:iterator value="fmList" status="stat" var="fmList">        
      <tr>
        <td><s:label name="%{fmList['+#stat.index+'].rtc}" value="%{rtc}" /></td>
        <td><s:label name="%{fmList['+#stat.index+'].target}" value="%{target}" /></td>
        <td><a href="deleteFieldMapping.action?rtcId="%{rtc}">delete</a></td>
      </tr>
    </s:iterator>
    <tr><td> </td></tr>
    <tr>
      <td><s:textfield name="rtcFieldName" size="40" /></td>
      <td><s:textfield name="targetFieldName" size="40"/></td>
      <td><s:submit/></td>
    </tr>
  </s:form>

Help! Thanks in advance.

[Dan Rasmussen]

OK. I have made a little bit of progress by looking at the page source in the browser and adding debug so that I could see when the setter was being called for fmList. The NPE is gone but the setter for the list is being called with an empty list. Please read on..

The first two problems I noticed was that the field name/id was not being properly generated. Second, I noticed that the setter was not being called for fmList. I figued that the setter wasn't being called because I was using an s:label, so I changed it to a text. Then I fooled around with the jsp to get the name/id properly populated. The resulting jsp for this field is:

        <td><s:textfield name="%{'fmList[' + #stat.index + '].rtc'}" value="%{rtc}" /></td>
        <td><s:textfield name="%{'fmList[' + #stat.index + '].target'}" value="%{target}" /></td>

which produces html like this:

<input type="text" name="fmList[1].rtc" value="internalState" id="addFieldMapping_fmList_1__rtc"/></td>

etc.

So now my setter is being called but the list being passed in is empty. Any suggestions?

Thanks,

Dan