昨天和今天早上，用DataSource从数据库后台动态加载数据，我的业务是这样的：
我有两个SelectItem选择框，第一个选择框里面的数据是单位，第二个选择框中的数据是对应单位的人，因为人可能有重名，所以还需要加载对应的人的其他信息（如身份证等等），来个图：
（选择好之后的样子）
第一个选择框中的单位变了，第二的选择框中只加载对应单位的人的数据，但是关键问题出来了，我加载的数据每一次都不会改变，不管第一个选择框的选择的是什么，第二个选择框只能加载一次数据，就是初始化那一次，我尝试了将对应的DataSource重新初始化，甚至写一个匿名的DataSource，问题都没有解决，最后在各种查文档、各种实践之后，我才发现，原来问题在于DataSource内的一个方法：setID(String id),只要id不变，不管你怎么折腾，数据还是那些数据，所以我果断将DataSource的id换了，奇迹就出现了，数据重新加载了，下面附上关键代码：
DataSource类：
public class TobaLeafGridFarmerInfoXmlDs extends DataSource {
private String townId=null;
private String villageId=null;
private static TobaLeafGridFarmerInfoXmlDs instance=null;
public static TobaLeafGridFarmerInfoXmlDs getInstance(String townId,String villageId){
if(instance==null){
instance=new TobaLeafGridFarmerInfoXmlDs("TobaLeafGridFarmerData",townId,villageId);
}
return instance;
}
public TobaLeafGridFarmerInfoXmlDs(String id,String townId,String villageId){
setTownId(townId);
setVillageId(villageId);
setID(id);//此处的id很重要的，相同的id返回的数据是一样的
setTitleField("farmerName");
setRecordXPath("/List/TobaLeafGridFarmerData");
DataSourceTextField townNameField = new DataSourceTextField("townName","乡镇",70,true);
DataSourceTextField villageIdField = new DataSourceTextField("villageId","村委会Id",30,true);
villageIdField.setHidden(true);
DataSourceTextField villageField = new DataSourceTextField("village","村委会",100,true);
DataSourceTextField farmerNameField = new DataSourceTextField("farmerName","烟农",50,true);
DataSourceTextField computerCodeField = new DataSourceTextField("computerCode","合同号",80,true);
DataSourceTextField IdCardField = new DataSourceTextField("idCard","身份证",80,true);
setFields(townNameField,villageIdField,villageField,farmerNameField,computerCodeField,IdCardField);
setDataFormat(DSDataFormat.XML);
//下面这个URL是一个Servlet的URL，数据来源是从数据库中取数的
setDataURL(HOST + "analyse/farmerDataServlet"+"?townId="+townId+"&villageId="+(villageId==null?"":villageId));
setClientOnly(true);
setCacheAllData(false);
}
public String getTownId() {
return townId;
}
public void setTownId(String townId) {
this.townId = townId;
}
public String getVillageId() {
return villageId;
}
public void setVillageId(String villageId) {
this.villageId = villageId;
}
}
选择框部分代码（selectVillage为第一个选择框，selectFarmer为第二个选择框，都是SelectItem实例）：
DataSource farmerXmlDS=new TobaLeafGridFarmerInfoXmlDs(townId,townId,"");
final ListGrid pickListProperties = new ListGrid();
pickListProperties.setShowFilterEditor(true);
ListGridField townName = new ListGridField("townName",70);
ListGridField villageId = new ListGridField("villageId",30);
villageId.setHidden(true);
ListGridField village = new ListGridField("village",120);
ListGridField farmerName = new ListGridField("farmerName",50);
ListGridField computerCode = new ListGridField("computerCode",90);
ListGridField idCard = new ListGridField("idCard",120);
selectFarmer.setPickListFields(townName,villageId,village,farmerName,computerCode,idCard);
selectFarmer.setOptionDataSource(farmerXmlDS);
selectFarmer.setDisplayField("farmerName");
selectFarmer.setValueField("computerCode");
selectFarmer.setPickListWidth(460);
selectFarmer.setPickListProperties(pickListProperties);
selectVillage.addChangeHandler(new ChangeHandler() {
public void onChange(ChangeEvent event) {
String selectedVillageId = (String) event.getValue();
DataSource farmerXmlDS=new TobaLeafGridFarmerInfoXmlDs(
selectedVillageId==null?townId:selectedVillageId,townId,selectedVillageId);
selectFarmer.setOptionDataSource(farmerXmlDS);
selectFarmer.redraw();
}
});
servlet关键代码：
@Override
protected void service(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
String xmlstring = "";
User user = WebRalasafe.getCurrentUser(request);
String townId=request.getParameter("townId");
String villageId=request.getParameter("villageId");
try {
xmlstring = getFarmerInfoXml(townId,villageId);
} catch (Exception e) {
e.printStackTrace();
}
response.setContentType("text/xml;charset=utf-8");
response.getWriter().write(xmlstring.toString());
}
public String getFarmerInfoXml(String townId,String villageId) throws Exception{
String farmerInfoXml="";
String sql="select …"; //sql略
List<String[]> result = xxx.QuerySql(sql); //自己写的执行sql的方法
/*TobaLeafGridFarmerData是一个模型类（继承com.google.gwt.user.client.rpc.IsSerializable接口，可序列化），有一堆属性和对应getter和setter方法，没有其他多余的内容*/
List<TobaLeafGridFarmerData> datas=new ArrayList<TobaLeafGridFarmerData>();
for(int i=0;i<result.size();i++){
String tmp[]=result.get(i);
TobaLeafGridFarmerData data=new TobaLeafGridFarmerData();
data.setTownName(tmp[0]);
data.setVillageId(tmp[1]);
data.setVillage(tmp[2]);
data.setFarmerName(tmp[3]);
data.setComputerCode(tmp[4]);
data.setIdCard(tmp[5]);
datas.add(data);
}
String rootElement = "List";
boolean isOutputToFile = false;
String outFilePath = "";
farmerInfoXml = Xmloutput.object2Xml(rootElement, datas, isOutputToFile,
outFilePath);
return farmerInfoXml;
}
/*大概思路：首先DataSource（通过servlet）加载数据，同时要定义对应元数据（上面的是TobaLeafGridFarmerData），然后SelectItem再调用这个DataSource，就这样*/