java.lang.IllegalStateException哎，闹不明白啊！

问题RT，求解！
描述：
一个查询列表的功能

1、在struts2的action里用response.getWriter().write()返回json格式的字符串
2、JSP里用ajax处理返回数据
3、抛出异常（数据正常显示）

org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet default threw exception
java.lang.IllegalStateException
at org.apache.catalina.connector.ResponseFacade.sendError(ResponseFacade.java:407)
at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:771)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:506)
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:852)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:619)


PS：1、数据返回正常，显示正常，就是后台抛异常。
2、我直接访问action没有异常。问题应该出在JSP上


JSP引入的js代码：

var pageNo = 1;

var pageSize = 10;



//加载数据，渲染到html

function loadList() {

	var startDate = $("#startDate").val();

	var endDate = $("#endDate").val();

	var username = $("#username").val();

	var userid = $("#userid").val();

	var url = rootPath+'/user/userMgr!queryList';

	var params = "pageSize=" + pageSize + "&pageNo=" + pageNo + "&username="

			+ username + "&userid=" + userid + "&startDate=" + startDate

			+ "&endDate=" + endDate;

	// alert(params);

	$

			.ajax({

				type : "POST",

				dataType : "text",

				data : params,

				url : url, // 请求action地址

				success : function(res) {

					var page = eval("(" + res + ")");

					var list = page.result; // 获取数据

					// 拼html

					var html = "<div class='tab-title'><h4><span class='icon-play'></span>數據展現：</h4></div>"

							+ "<table width='100%' cellspacing='0' cellpadding='0' border='0' class='table-form color-table'><thead>";



					// 表头

					var tableHead = [ '帳號', '用户名称', '密码', '性别', '註冊時間',

							'是否為管理員', '操作' ];

					html += "<tr>";

					for ( var i = 0; i < tableHead.length; i++) {

						if (i == tableHead.length - 1) {

							html += "<th align='center' style='border-right:none'>";

						} else {

							html += "<th align='center'>";

						}

						html += tableHead[i];

						html += "</th>";

					}

					html += "</tr></thead><tbody>";



					// 表身

					for ( var i = 0; i < list.length; i++) {

						var stat = [ "否", "是" ]; // 管理員狀態

						var sex = [ "男", "女" ]; // 性別

						var opt = [

								" <a href='javascript:updateToot(" + list[i].ID

										+ "," + list[i].IS_ROOT

										+ ")'>設為管理員</a> ",

								"<a href='javascript:updateToot(" + list[i].ID

										+ "," + list[i].IS_ROOT

										+ ")'>取消管理員</a>" ];



						var userid = '';

						var username = '';

						var pwd = '';

						var gender_flag = '';

						var in_time = '';

						var root = 0;



						if (list[i].USERID)

							userid = list[i].USERID;

						if (list[i].USERNAME)

							username = list[i].USERNAME;

						if (list[i].PWD)

							pwd = list[i].PWD;

						if (list[i].GENDER_FLAG)

							gender_flag = sex[list[i].GENDER_FLAG - 1];

						if (list[i].IN_TIME)

							in_time = list[i].IN_TIME;

						if (list[i].IS_ROOT)

							root = list[i].IS_ROOT;



						html += "<tr>";

						html += "<td>" + userid + "</td>";

						html += "<td>" + username + "</td>";

						html += "<td>" + pwd + "</td>";

						html += "<td>" + gender_flag + "</td>";

						html += "<td>" + in_time + "</td>";

						html += "<td>" + stat[root] + "</td>";

						html += "<td>" + opt[root] + "</td>";

						html += "</tr>";

					}

					html += "</tbody></table>";

					// alert(createPage(page));

					html += createPage(page);

					$("#item_info").html(html);

					$(".color-table tbody tr:odd").addClass("tr-color");// 增加行样式，

					// nextPage

					$(".page").click(nextPage);

					return false;

				}

			});

}

function updateToot(id, is_root) {

	var url = '<%=contextPath%>/user/userMgr!updateRoot?id=' + id + '&is_root='

			+ is_root;

	// alert(url);

	$.post(url, function(data) {

		alert(data);

		loadList();

	});

}



function getDate(day, format) {

	return day.format(format);

}



/** 分布组件* */

function prePage() {

	if (pageNo == 1) {

		pageNo = 1;

	} else {

		pageNo--;

	}

	loadList();

}

function nextPage() {

	pageNo++;

	loadList();

}

function firstPage() {

	pageNo = 1;

	loadList();

}

function lastPage(totalPages) {

	pageNo = totalPages;

	loadList();

}



jQuery(function() {

	// 加载数据

	loadList();



	var today = new Date();

	var format = "yyyy-mm-dd";

	var todayVal = today.valueOf();

	var tomorrowVal = todayVal + 1 * 24 * 60 * 60 * 1000;

	var tomorrow = new Date(tomorrowVal);

	today = new Date(todayVal - 1 * 24 * 60 * 60 * 1000 * 365);

	// $('#startDate').val(getDate(today, format));

	$('#endDate').val(getDate(tomorrow, format));



	$("#search_user").click(function() {

		// alert("click");

		$("#alert_mes").hide();

		loadList();

	});



});