为什么response.setHeader和response.addHeader()不起作用？

在servlet中是这样写的：

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

public class sv1 extends HttpServlet
{
private static final String CONTENT_TYPE = "text/html; charset=GBK";
/**Initialize global variables*/
public void init() throws ServletException
{
}
/**Process the HTTP Post request*/
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
response.setContentType(CONTENT_TYPE);
PrintWriter out = response.getWriter();
response.addHeader( "aaa", "abcde" );
response.sendRedirect( "http://localhost:7001/cu.jsp" );
}
/**Clean up resources*/
public void destroy()
{
}
}
在cu.jsp中显示请求头中没有我加入的aaa。