public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        /**
         * 通过response对象改变相应信息
         */
            //1、响应行
            //response.setStatus(404);    //修改状态码
            //response.sendError(404);        //发送404状态码+404错误页面

        //2、改变响应头
        response.setHeader("server", "JBoss");

        //3、实体内容
        //    response.getWriter().write("1、hello world");    //字符内容
        response.getOutputStream().write("2、hello world".getBytes());     //字节内容
    }

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Demo2 extends HttpServlet {

    /**
     * 案例一：请求重定向
     * 相当于超链接跳转页面
     */
    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        /**
         *  需求：跳转到另一个页面
         *  使用重定向，发送一个302状态码+location的响应头
         */
//        response.setStatus(302);
//        response.setHeader("location", "/HttpTest/index.jsp");    //location是响应头

        //请求重定向的简化写法
        response.sendRedirect( "/HttpTest/index.jsp");
    }
}

public class Demo3 extends HttpServlet {

    /**
     * 案例  定时刷新
     */
    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        /**
         * 页面定时刷新
         * 原理：浏览器认识refresh头，得到refresh头后重新申请资源
         */
        //response.setHeader("refresh", "1");     //每隔一秒刷新本页面
        /**
         * 隔n秒之后转到另外的资源
         */
        response.setHeader("refresh", "3;url=/HttpTest/index.jsp");    //隔三秒之后跳到index.jsp压面
    }

}

public class Demo4 extends HttpServlet {

    /**
     * 将图片写出到浏览器
     */
    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("image/jpg");
        /**
         * 设置已下载的方式打开文件
         */
        File file=new File("F:/4.jpg");
        response.setHeader("Content-Disposition", "attachment;filename="+file.getName());
        FileInputStream in=new FileInputStream(file);
        byte[] buf=new byte[1024];
        int len=0;
        //开始写
        while ((len=in.read(buf))!=-1) {
            response.getOutputStream().write(buf,0,len);
        }

    }

}