Servlet 简单问题请教!!!

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
public class TestJ extends HttpServlet 
{
 
  public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
  {
    
     RequestDispatcher rd = getServletContext().getRequestDispatcher("/a.jsp");
rd.forward(request, response);                //response.sendRedirect("/a.jsp");
         //使用response.sendRedirect()就能跳到相应的页面，但是使用 上面的forward，老是报错：  /*  java.lang.IllegalArgumentException: Request cannot be null
at javax.servlet.ServletRequestWrapper.<init>(ServletRequestWrapper.java:50)
at javax.servlet.http.HttpServletRequestWrapper.<init>(HttpServletRequestWrapper.java:42)
at com.caucho.server.connection.RequestAdapter.<init>(RequestAdapter.java:96)
at com.caucho.server.webapp.DispatchRequest.<init>(DispatchRequest.java:97)
at com.caucho.server.webapp.DispatchRequest.createDispatch(DispatchRequest.java:107)
at com.caucho.server.webapp.RequestDispatcherImpl.forward(RequestDispatcherImpl.java:164)
at com.caucho.server.webapp.RequestDispatcherImpl.forward(RequestDispatcherImpl.java:112)
at TestJ.doPost(Testj.java:11)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChain.java:113)
at com.caucho.server.cache.CacheFilterChain.doFilter(CacheFilterChain.java:190)
at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:177)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:221)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:263)
at com.caucho.server.port.TcpConnection.run(TcpConnection.java:331)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:464)
at com.caucho.util.ThreadPool.run(ThreadPool.java:408)
at java.lang.Thread.run(Thread.java:534)
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Resin-3.0.8 (built Tue, 08 Jun 2004 02:25:40 PDT)  */
  }  
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
  {
    doPost(request, response); 
    
  }
  
  
}请问什么原因？？