为什么我在servlet里面用dom4j写的代码老是报错？

package json;





import java.io.IOException;

import java.io.PrintWriter;



import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;



import org.dom4j.Document;

import org.dom4j.DocumentHelper;

import org.dom4j.Element;

import org.dom4j.io.OutputFormat;

import org.dom4j.io.XMLWriter;



public class xmljson extends HttpServlet

{

	@Override

	protected void doGet(HttpServletRequest req, HttpServletResponse resp)

			throws ServletException, IOException

	{

		String name = req.getParameter("name");

		Gbean g = new Gbean();

		if(name.equals("zhangsan"))

		{

			g.setName("zhangsan");

			g.setAge(20);

		}

		else

		{

			g.setName("lishi");

			g.setAge(30);

		}

		

		Document document = DocumentHelper.createDocument();

		Element root = DocumentHelper.createElement("users");

		document.add(root);

		root.addComment("This is a xml!");

		Element n = DocumentHelper.createElement("name");

		root.add(n);

		Element a = DocumentHelper.createElement("age");

		root.add(a);

		n.setText(g.getName());

		a.setText(String.valueOf(g.getAge()));

		OutputFormat format = OutputFormat.createPrettyPrint();

		//传输xml时要设置类型

		resp.setContentType("text/xml,charset=utf-8");

		//阻止浏览器的缓存

		resp.setHeader("pragma", "no-cache");

		resp.setHeader("cache-control", "no-cache");

		

		PrintWriter out = resp.getWriter();

		XMLWriter xmlWriter = new XMLWriter(out,format);

		xmlWriter.write(document);

		out.flush();

	}

}

这是错误代码：
十二月 03, 2013 11:42:37 上午 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [xmljson] in context with path [/test] threw exception [Servlet execution threw an exception] with root cause
java.lang.NoClassDefFoundError: org/dom4j/DocumentHelper
at json.xmljson.doGet(xmljson.java:37)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardH