Controller部分

 /**
     * 修改公告
     */
    @GetMapping("/edit/{sid}")
    public String edit(@PathVariable("sid") Long sid, ModelMap mmap)
    {
        mmap.put("classes", classesService.selectClassesAll());
        mmap.put("student", studentService.selectStudentById(sid));
        //二级联动默认选中
        mmap.put("grade", gradeService.selectGradeList(new SysGrade()));
        //通过id进行判断并且取值，通过id进行逐层查询获取id，先向上准确查询，在向下模糊查询，默认选中在下面。
        int classesId1= studentService.selectStudentById(sid).getCid();
        int gradeId1 = classesService.selectClassesById(Long.parseLong(String.valueOf(classesId1))).getGid();
        //
        mmap.put("abc",classesId1);
        mmap.put("cba",gradeId1);
        //
        mmap.put("classes", classesService.selectClassesById1(Long.parseLong(String.valueOf(gradeId1))));
        return prefix + "/edit";
    }

Edit.html

 $(document).ready(function () {
        //获取controller层的函数，进行默认选中
        var gradeId = [[${cba}]];
        var classesId = [[${abc}]];
        $("#grade").val(gradeId);
        $("#classes").val(classesId);
        $("#grade").change(function () {//同上面一样
            $.ajax({
                url: ctx + "system/classes/select",
                type: 'GET',
                data: {
                    gid: $("#grade").val(),
                },
                success:function (data) {
                    $("#classes").empty();
                    $("#classes").append("<option value=''>请选择班级</option>");
                    for (var i = 0; i < data.length; i++){
                        $("#classes").append("<option value='" + data[i].cid + "'>" + data[i].cname + "</option>");
                    }
                }
            })
        });
        $("#classes").change(function () {//赋值给input输入框添加到数据库
            $("#class").val($("#classes").val());
        })
    })

<div class="form-group">
            <label class="col-sm-2 control-label is-required">学生年级：</label>
            <div class="col-sm-10">
                <select id="grade" class="form-control">
                    <option value="">请选择年级</option>
                    <option th:each="sc:${grade}" th:value="${sc.gid}" th:text="${sc.gname}" ></option>
                </select>
            </div>
        </div>
        <div class="form-group">
            <label class="col-sm-2 control-label is-required">学生班级：</label>
            <div class="col-sm-10">
                <select id="classes"  class="form-control">
                    <option value="">请选择班级</option>
                    <option th:each="cl:${classes}" th:value="${cl.cid}" th:text="${cl.cname}" th:field="*{cid}"></option>
                </select>
            </div>
        </div>
        <div class="form-group">
            <div class="col-sm-10">
                <input id="class" name="class" th:field="*{cid}" class="form-control" type="hidden" required>
            </div>
        </div>

效果图