java前后台数据请求返回问题

Question

将后台list数据转化成jsonarray返回前台，前台访问list中元素的属性显示在下拉框中，执行只输出System.out.println("getDataset.java中2");哪里有问题？

后台.java代码
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
String test_name = request.getParameter("testname");
System.out.println("test_name: "+test_name);
DBController con = new DBController(test_name);
//查找list
System.out.println("进入前");
List list = con.select(test_name);
System.out.println("getDataset.java中");
System.out.println("list:"+list.get(0).getDatasetPath());
System.out.println("getDataset.java中2");
JSONArray list1=JSONArray.fromObject(list);
System.out.println("getDataset.java中3");
response.setCharacterEncoding("UTF-8");//解决显示中文乱码问题
PrintWriter writer = response.getWriter();
writer.write(list1.toString());
writer.flush();
System.out.println("list:"+list.get(0).getId());
}
前台.jsp代码

 <div class="form-group" id="select_now">
        <label class="col-xs-3 control-label">选择已有数据集</label>
        <div class="col-xs-3">  
            <select class="form-control" name="select_dataset" id="select_dataset" onclick="getdatasets();">
            </select>
        </div>
      </div>

function getdatasets(){
$.ajax({
    type : "POST",
    url:"<%=contextPath%>/getDatasets",
    data : {
        type : "0",
        testname : "自动分类",
    },
    dataType : "text",
    async : false,
    context : null,
    success : function(list) {
        alert(typeof list);
         alert("成功1");
        //下拉菜单选项显示数据库名，值为数据库的真实地址
        $("#select_dataset").html('');
        //alert("成功2");
        for(var i = 0; i<list.length; i++){
            var inner = "";
            //alert("成功3");
            inner += "<option value='" + list.get(i).getDatasetPath() +"'>" + list.get(i).getDataset() + "</option>";
            $("#select_dataset").append(inner);
        }
        list=[];
        alert("成功5");
    }

})

}

Answer 1

改了一下你的代码。至于java3没有执行到，你看原来的代码是不是在java3之前抛异常了protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
String test_name = request.getParameter("testname");
System.out.println("test_name: " + test_name);
DBController con = new DBController(test_name);
//查找list
System.out.println("进入前");
List list = con.select(test_name);
if (list != null && !list.isEmpty()) {//非空处理，否则list为null时报空指针，或者长度为0时，下面的list.get(0)就会报下标越界
System.out.println("getDataset.java中");
System.out.println("list:" + list.get(0).getDatasetPath());
System.out.println("getDataset.java中2");
String jsonStr = JSON.toString(list);//一次转完，不用再把JSONArray toString
System.out.println("getDataset.java中3");
//response.setCharacterEncoding("UTF-8");//解决显示中文乱码问题
PrintWriter writer = response.getWriter();
writer.write(jsonStr);
writer.flush();
System.out.println("list:" + list.get(0).getId());
}

}

function getdatasets() {
$.ajax({
type: "POST",
url: "<%=contextPath%>/getDatasets",
data: {
type: "0",
testname: "自动分类",
},
dataType: "json",//返回的是json格式的数据，用text貌似接收不到吧
async: false,
success: function (jsonData) {
alert(typeof jsonData);
alert("成功1");

            if (jsonData && jsonData != '[]') { //返回的数据有效。list不为null,undefined,'','[]'等时才说明有数据返回
                var dataList = JSON.parse(jsonData);
                //下拉菜单选项显示数据库名，值为数据库的真实地址
                var select_dataset = $("#select_dataset");//对象获取一遍就行，没必要重复去解析获取对象，浪费性能
                select_dataset.html('');
                //alert("成功2");

                var inner = "";
                for (var i = 0; i < dataList.length; i++) {
                    //alert("成功3");
                    inner += "<option value='" + dataList.get(i).getDatasetPath() + "'>" + dataList.get(i).getDataset() + "</option>";
                }
                select_dataset.append(inner);//在循环里面一次拼接好，在这里给select一次赋值就行，没必要每个option都添加一次
                //list = [];
                alert("成功5");
            }

        }

    });
}