-----------------------JS代码
function checkCardNo(obj){
    shuzi(obj);
    if(obj.value.length > 0){
        $.ajax({
          type: "POST",
          url: "checkJson/checkCardNo.action",
          data: {"searchWord":obj.value},
          dataType:"json",
          success: function(json){
              var nulldTag = json.nullTag;
              if(nulldTag=="N"){
                  alert("卡号已存在！");
                  $("#ticNo").val("");
                  $("#ticNo").focus();
                  return false;
              }
          },
          error:function(a,b,c){
              alert(a.status + " " + c);
          }
      });
    }
}

---------------------------struts2代码
@ParentPackage(value="json-default")
@Namespace("/checkJson")
@Results({
        @Result(name = "success",type="json")
})

@Action(value="checkCardNo")
    public String checkCardNo(){
        this.list = this.infCardService.findCard(null, this.searchWord,this.getPageNo(), -1);
        if (this.list != null && this.list.size() > 0) {
            this.nullTag = "N";
        }else{
            this.nullTag = "Y";
        }
        return SUCCESS;//走到这里nullTag是有值的。我抓了一张调试的图，但是发不上来。。。
    }

private String nullTag;
    @JSON(name="nullTag")
    public String getNullTag() {
        return nullTag;
    }
    public void setNullTag(String nullTag) {
        this.nullTag = nullTag;
    }