Servlet 获取IllegelStateException

package com.stono.servlet.synchronize;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class TestServletD extends HttpServlet {
    private static final long serialVersionUID = 1L;
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // ServletOutputStream os = resp.getOutputStream();
        // os.print(0);
        // os.flush();
        PrintWriter writer = resp.getWriter();
        writer.println("this is from printwriter");
        writer.flush();
        RequestDispatcher dispatcher = req.getRequestDispatcher("a.do");
        dispatcher.forward(req, resp);
    }
}