jQuery.noConflict();   
jQuery(document).ready(function () {
     /*
     * 加载名单返回类型json—list
     * @param {Object} data
     */
    var url = "aa.action";
    var params;    
    jQuery.getJSON(url,params,function callback(data){
        alert("2");//返回不了
      jQuery.each(data.items,function(i,item){
        jQuery("#maindiv").append(item.cname);
    });//这句有没有问题，我参照jquery+ajax帮助文档写的
    })
});

<div id="maindiv"></div>       

package com.tyut.actions;
import java.util.List;
import net.sf.json.JSONArray;
import com.opensymphony.xwork2.Action;
import com.tyut.domain.Candiate;
import com.tyut.service.CandiateService;

public class CandiateAction {

    private Candiate candiate;
    
    public Candiate getCandiate() {
        return candiate;
    }
    
    private JSONArray resulttype;

    
    public JSONArray getResulttype() {
        return resulttype;
    }

    public void setResulttype(JSONArray resulttype) {
        this.resulttype = resulttype;
    }

    public void setCandiate(Candiate candiate) {
        this.candiate = candiate;
    }
        
    public String aa(){
        CandiateService candiateService = new CandiateService();
        List<Candiate> candiateList = candiateService.getList();

        /*for (Iterator iter = candiateList.iterator(); iter.hasNext();) {  

           Candiate candiate = (Candiate) iter.next();  
           System.out.println(candiate.getCname());

          }  */
        JSONArray jsarry=new JSONArray();  
        jsarry=JSONArray.fromObject(candiateList);  
        
        resulttype=jsarry;     
        return Action.SUCCESS;
    }
}

        <package name="aa" namespace="/" extends="json-default">
        <action name="aa" class="com.tyut.actions.CandiateAction" method="aa">
           <result type="json">
              <param name="root">resulttype</param>
           </result>
        </action>
    </package>