HttpServletResponse

• web服务器接收到客户端的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse；

  • 如果要获取客户端请求过来的参数：找HttpServletRequest
  • 如果要给客户端响应一些信息：找HttpServletResponse

• 负责向浏览器发送数据的方法↓

      public PrintWriter getWriter() throws IOException;
      public ServletOutputStream getOutputStream() throws IOException;
  

• 负责向浏览器发送响应头的方法↓

      public void setCharacterEncoding(String charset);
      public void setContentLength(int len);
      public void setContentLengthLong(long len);
      public void setContentType(String type);
      public void setDateHeader(String name, long date);
      public void addDateHeader(String name, long date);
      public void setHeader(String name, String value);
      public void addHeader(String name, String value);
      public void setIntHeader(String name, int value);
      public void addIntHeader(String name, int value);
  

• 响应的状态码↓

      public static final int SC_CONTINUE = 100;
      public static final int SC_SWITCHING_PROTOCOLS = 101;
      public static final int SC_OK = 200;
      public static final int SC_CREATED = 201;
      public static final int SC_ACCEPTED = 202;
      public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
      public static final int SC_NO_CONTENT = 204;
      public static final int SC_RESET_CONTENT = 205;
      public static final int SC_PARTIAL_CONTENT = 206;
      public static final int SC_MULTIPLE_CHOICES = 300;
      public static final int SC_MOVED_PERMANENTLY = 301;
      public static final int SC_MOVED_TEMPORARILY = 302;
      public static final int SC_FOUND = 302;
      public static final int SC_SEE_OTHER = 303;
      public static final int SC_NOT_MODIFIED = 304;
      public static final int SC_USE_PROXY = 305;
      public static final int SC_TEMPORARY_REDIRECT = 307;
      public static final int SC_BAD_REQUEST = 400;
      public static final int SC_UNAUTHORIZED = 401;
      public static final int SC_PAYMENT_REQUIRED = 402;
      public static final int SC_FORBIDDEN = 403;
      public static final int SC_NOT_FOUND = 404;
      public static final int SC_METHOD_NOT_ALLOWED = 405;
      public static final int SC_NOT_ACCEPTABLE = 406;
      public static final int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
      public static final int SC_REQUEST_TIMEOUT = 408;
      public static final int SC_CONFLICT = 409;
      public static final int SC_GONE = 410;
      public static final int SC_LENGTH_REQUIRED = 411;
      public static final int SC_PRECONDITION_FAILED = 412;
      public static final int SC_REQUEST_ENTITY_TOO_LARGE = 413;
      public static final int SC_REQUEST_URI_TOO_LONG = 414;
      public static final int SC_UNSUPPORTED_MEDIA_TYPE = 415;
      public static final int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
      public static final int SC_EXPECTATION_FAILED = 417;
      public static final int SC_INTERNAL_SERVER_ERROR = 500;
      public static final int SC_NOT_IMPLEMENTED = 501;
      public static final int SC_BAD_GATEWAY = 502;
      public static final int SC_SERVICE_UNAVAILABLE = 503;
      public static final int SC_GATEWAY_TIMEOUT = 504;
      public static final int SC_HTTP_VERSION_NOT_SUPPORTED = 505;
  

  常见应用

  向浏览器输出消息

  下载文件

  ①获取下载文件的路径

  ②命名下载文件名

  ③设置浏览器能够支持下载

  ④获取下载文件的输入流

  ⑤创建缓冲区

  ⑥获取OutputStream对象

  ⑦将FileOutputStream流写入Buffer缓冲区

  ⑧使用OutputStream将缓冲区中的数据输出到客户端

@WebServlet("/file")
public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1.获取下载文件的路径
        String realPath = this.getServletContext().getRealPath("/WEB-INF/classes/a.jpg");// ‘/‘表示web根目录
        System.out.println("下载的文件路径：" + realPath);
        //2.获取要下载的文件名
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        //3.设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名URLEncoder.encode编码，否则有可能乱码
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
        //4.获取下载文件的输入流
        FileInputStream in = new FileInputStream(new File(realPath));
        //5.创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        //6.获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();
        //7. 将FileOutputStream流写入到buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端！
        while ((len=in.read(buffer)) != -1) {
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }
}

验证码功能

• 前端实现
• 后端实现，需要用到Java的图片类，生产一个图片

@WebServlet("/yzm")
public class yzmServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器3秒自动刷新一次;
        // resp.setHeader("refresh","3");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics(); //笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");
        //把图片写给浏览器
        ImageIO.write(image,"jpg", resp.getOutputStream());
    }

    //生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length() ; i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }
}

实现重定向

B一个web资源收到客户端A请求后，B会通知A去访问另外一个web资源C，这个过程叫重定向。

@WebServlet("/redir")
public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         /*
        resp.setHeader("Location","/r/img");
        resp.setStatus(302);  /重定向
        responseDemo_war：项目名
         */
        resp.sendRedirect("/responseDemo_war/yzm");
    }
}

常见场景：

• 用户登录

重定向和转发的区别？

• 请求转发url不会产生变化，重定向会发生变化
• 请求转发可以携带参数，重定向不能携带参数
• 请求转发只能发送一个请求，重定向至少发送两次请求

简单实现登录重定向

• @WebServlet("/login")
  public class LoginServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          //处理请求
          String username = req.getParameter("username");
          String password = req.getParameter("password");
          System.out.println(username+":"+password);
          //重定向时候一定要注意，路径问题，否则404；
          resp.sendRedirect("/responseDemo_war/success.jsp");
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req,resp);
      }
  }
  

• index.jsp

• <%@ page contentType="text/html;charset=UTF-8" language="java" %>
  
  
  Hello World!
  <%--这里提交的路径，需要寻找到项目的路径--%>
  <%--${pageContext.request.contextPath}代表当前的项目--%>
  
      用户名： 
  
      密码： 
  
      
  
  
  
  

• success.jsp

  <%@ page contentType="text/html;charset=UTF-8" language="java" %>
  
  
      
  
  
  Success
  
  
  

原文：https://www.cnblogs.com/saxonsong/p/14808033.html