Servlet

Question

我的servlet出错了

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloServlet extends HttpServlet
{
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException
 {
  response.setContentType("text/html");
  PrintWriter out = response.getWriter();

   out.println("<H1>Hello Servlet</H1>");
    out.close();
   }
  }

浏览器中显示的错误

Error: 500

Location: /P1/servlet/HelloServlet

  Internal Servlet Error:
  java.lang.NullPointerException
     at java.lang.ClassLoader.resolveClass0(Native Method)
at java.lang.ClassLoader.resolveClass(ClassLoader.java:588)
at org.apache.tomcat.loader.AdaptiveClassLoader.loadClass(AdaptiveClassLoader.java:518)
at org.apache.tomcat.loader.AdaptiveServletLoader.loadClass(AdaptiveServletLoader.java:174)
at org.apache.tomcat.core.ServletWrapper.loadServlet(ServletWrapper.java:265)
at org.apache.tomcat.core.ServletWrapper.init(ServletWrapper.java:289)
at org.apache.tomcat.core.Handler.service(Handler.java:254)
at org.apache.tomcat.core.ServletWrapper.service(ServletWrapper.java:372)
at org.apache.tomcat.core.ContextManager.internalService(ContextManager.java:797)
at org.apache.tomcat.core.ContextManager.service(ContextManager.java:743)
at org.apache.tomcat.service.http.HttpConnectionHandler.processConnection(HttpConnectionHandler.java:210)
at org.apache.tomcat.service.TcpWorkerThread.runIt(PoolTcpEndpoint.java:416)
at org.apache.tomcat.util.ThreadPool$ControlRunnable.run(ThreadPool.java:498)
at java.lang.Thread.run(Thread.java:484)

代码似乎编译得很好。我没有遇到问题。请帮我解决一下。感谢