请问：servlet的异常问题

用户登陆的验证servlet的部分代码：
HttpSession session =request.getSession(true);
dbcon con=new dbcon();
try{
String usertype=(request.getParameter("type")).trim();
String username=(request.getParameter("username")).trim();
String pass_word=request.getParameter("password");

String query_login="select * from login_teacher";
ResultSet rs_login=con.executeQuery(query_login);
String teacher_login;
String teacher_id;
String teacher_psd;
String teacher_permit;
while(rs_login.next())
{
teacher_login=(rs_login.getString("teacher_login")).trim();
teacher_id=(rs_login.getString("teacher_id")).trim();
teacher_psd=rs_login.getString("teacher_psd");
teacher_permit=String.valueOf(rs_login.getByte("teacher_permit")).trim();
if(username.equals(teacher_login)||username.equals(teacher_id))
{
if(pass_word.trim().equals(teacher_psd.trim())&&teacher_permit.equals("1")&&usertype.equals("3"))
{
session.setAttribute("s_username",teacher_id);

getServletConfig().getServletContext().getRequestDispatcher("/base.jsp").forward(request,response);

}
else
{

getServletConfig().getServletContext().getRequestDispatcher("/login.jsp").forward(request,response);
}
}
TOMCAT抛出异常为：
[INFO,STDERR] java.lang.IllegalStateException: Cannot forward as OutputStream or
Writer has already been obtained
[INFO,STDERR] at org.apache.tomcat.facade.RequestDispatcherImpl.doForward(Requ
estDispatcherImpl.java:178)
[INFO,STDERR] at org.apache.tomcat.facade.RequestDispatcherImpl.forward(Reques
tDispatcherImpl.java:162)
[INFO,STDERR] at bean.login.doPost(login.java:54)
[INFO,STDERR] at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
[INFO,STDERR] at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)