java.lang.ClassNotFoundException: net.sf.json.JSONObject
昨天学习json，遇到一个很奇怪的问题：
1.我已经在工程的lib目录下添加了json-lib-2.1-jdk15.jar
2.同一工程下的PeopleBean.java类可以正确运行：

Java code

import java.util.HashMap;import java.util.Map;import net.sf.json.JSONObject;import net.sf.json.JSONSerializer;import net.sf.json.xml.XMLSerializer;public class PeopleBean {    private String name;        private int age;        private boolean gendar;        //setter/getter    public PeopleBean() {    }    public PeopleBean(String name, int age, boolean gendar) {        this.name = name;        this.age = age;        this.gendar = gendar;    }    public static void main(String[] args) throws Exception{        /**         * 创建JSONObject对象         */        //Create a JSONObject from scratch        JSONObject object1=new JSONObject();        object1.element("name", "张三");        object1.element("age", 24);        object1.element("gendar", true);                    //Create a JSONObject from formatted String        String jsonString1="{name:\"李四\",age:25,gendar:false}";        JSONObject object2=JSONObject.fromObject(jsonString1);                        String jsonString2="{'name':'王五','age':22,'gendar':true}";        JSONObject object3=(JSONObject)JSONSerializer.toJSON(jsonString2);                //Create a JSONObject from a Map        Map map=new HashMap();        map.put("name", "刘六");        map.put("age", 23);        map.put("gendar", true);        JSONObject object4=(JSONObject)JSONSerializer.toJSON(map);                //Create a JSONObject from a bean        PeopleBean WangNan=new PeopleBean("王l",26,true);        JSONObject object5=(JSONObject)JSONSerializer.toJSON(WangNan);                /**         * 把JSONObject对象转化为Bean或Object         */        PeopleBean people=(PeopleBean)object5.toBean(object5, PeopleBean.class);        System.out.println("name===="+people.getName()+" age======"+people.getAge()+"  gendar======"+people.isGendar());            /**         * From JSON to XML         */                JSONObject object6=new JSONObject(true);        XMLSerializer xmlSerializer=new XMLSerializer();        String xml1=xmlSerializer.write(object6);        System.out.println(xml1);                xmlSerializer.setRootName("people");        xmlSerializer.setTypeHintsEnabled(false);        String xml2=xmlSerializer.write(object5);        System.out.println(xml2);            /**         * From XML to JSON         */        JSONObject object7=(JSONObject)xmlSerializer.read(xml2);        System.out.println(object7);    }}

3.同一工程下的UserController.java在tomcat启动后，报错java.lang.ClassNotFoundException: net.sf.json.JSONObject:

Java code

import net.sf.json.JSONObject;import net.sf.json.JSONSerializer;import org.springframework.web.servlet.ModelAndView;import org.springframework.web.servlet.mvc.multiaction1b0004:0:3;402881c42d8

请问是什么原因？

[解决办法]
.jar包冲突了吧