Response 写验证码：

package com.demo.response;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.awt.image.RenderedImage;
import java.io.IOException;
import java.nio.IntBuffer;
import java.util.Random;

public class Image extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //浏览器5s刷新
        resp.setHeader("refresh","5");

        //在内存中创建一个图片，设置宽高颜色
        BufferedImage bufferedImage = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);

        //得到图片 Graphics 是一支笔
        Graphics2D graphics = (Graphics2D) bufferedImage.getGraphics();

        //设置图片的背景颜色
        graphics.setColor(Color.black);
        graphics.fillRect(0,0,80,20); //填充

        //给图片写数据
        graphics.setColor(Color.white);
        graphics.setFont(new Font(null,Font.ITALIC,20));
        graphics.drawString(num(),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("expores",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");

        //把图片写给浏览器
        ImageIO.write(bufferedImage,"jpg",resp.getOutputStream());

    }

    //生成随机数
    private String num(){
        Random random = new Random();
        String num = random.nextInt(9999)+"";
        StringBuffer stringBuffer = new StringBuffer();
        for (int i = 0; i < 4-num.length(); i++) {
            stringBuffer.append("0"); //不足用0填充数字
        }
        return stringBuffer.toString() + num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

记得先注册后运行

    <servlet>
        <servlet-name>img</servlet-name>
        <servlet-class>com.demo.response.Image</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>img</servlet-name>
        <url-pattern>/img</url-pattern>
    </servlet-mapping>

Response 重定向：

一个 web 资源接收到客户端的请求后，它会通知客户端去访问另一个 web 资源，这个过程叫做重定向

sendRedirect 重定向

public class Redirect extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //resp.sendRedirect("/response/index.jsp");
        resp.sendRedirect("/response/img"); //重定向
    }

注意路径前要加自己命名的路径

web.xml 注册：

    <servlet>
        <servlet-name>re</servlet-name>
        <servlet-class>com.demo.response.Redirect</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>re</servlet-name>
        <url-pattern>/re</url-pattern>
    </servlet-mapping>

浏览器输入后缀，将会跳转到另一个页面，地址栏改变，实现重定向

实现原理：

resp.setHeader("Location","/response/img");
resp.setStatus(302); //302重定向

一个响应头 Location、一个状态码 302

重定向和转发的区别：

相同点：页面都会实现跳转

不同点：

1.请求转发的时候，url 不会变化

2.重定向时，url 地址栏会发生变化

写个简单的提交重定向

修改 index.jsp

<%-- ${pageContext.request.contextPath}代表当前的项目 --%>
<form action = "${pageContext.request.contextPath}/login" method = "get">
    username: <input type = "text" name = "username"> <br>
    password: <input type = "password" name = "password"> <br>
    <input type = "submit">
</form>

代码如下：

package com.demo.response;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class Request 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");

        resp.sendRedirect("/response/success.jsp");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

再写个跳转的 success.jsp

web.xml 注册：

    <servlet>
        <servlet-name>login</servlet-name>
        <servlet-class>com.demo.response.Request</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>login</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

注意 <url-pattern> 的路径名与 index.jsp 里的 ${pageContext.request.contextPath} 保持一致

运行，提交后进入 success 页面