alert(json)并不能输出？？？？？

严重: Servlet.service() for servlet [SearchServlet] in context with path [/ajax_autocomplete] threw exception [Servlet execution threw an exception] with root cause

java.lang.ClassNotFoundException: org.apache.commons.lang.exception.NestableRuntimeException

at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1711)

at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1556)

at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClass(ClassLoader.java:621)

.......

package imooc.ajaxautocomplete.search;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import net.sf.json.JSONArray;

public class SearchServlet extends HttpServlet {

	static List<String> datas=new ArrayList<String>();
	static{
		datas.add("ajax");
		datas.add("ajax操作");
		datas.add("ajax基础教程");
		datas.add("boolean");
		datas.add("CoCo");
	}
	
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		request.setCharacterEncoding("utf-8");
		response.setCharacterEncoding("utf-8");
		
		//首先获得客户端发送来的数据keyword
		String keyword=request.getParameter("keyword");
		//获得关键字之后进行处理，得到关联数据
		List<String> listData=getData(keyword);
		//返回json格式
		response.getWriter().write(JSONArray.fromObject(listData).toString());
	}
	
	//获得关联数据的方法
	public List<String> getData(String keyword){
		List<String> list=new ArrayList<String>();
		for(String data:datas){
			if(data.contains(keyword)){
				list.add(data);
			}
		}
		return list;
	}

}

------------------------------------

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
	xmlns="http://java.sun.com/xml/ns/javaee" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
	http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <display-name></display-name>
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>SearchServlet</servlet-name>
    <servlet-class>imooc.ajaxautocomplete.search.SearchServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>SearchServlet</servlet-name>
    <url-pattern>/search</url-pattern>
  </servlet-mapping>	
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>

</web-app>