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HttpServletResponse、页面文件下载、验证码

summarize

在创建Servlet时，会覆盖service方法或doGet()/doPost(),它俩都是 HTTPServletresponse这些方法都有俩参数，一个是请求request，一个是响应response，HttpServletResponse是ServletResponset的子接口，

浏览器发出http请求，会直接传给Tomcat，Tomcat将请求的信息封装成request，和response 传给web，这时的response是空的，web调用write方法将内容存到缓冲区，此时，Tomcat从response里获取设置的内容组装成一个http响应（行、头、体）回写给客户端

响应行只能设置状态码

重定向特点：请求服务器两次，地址栏变化

location：

HttpServletResponse

response运行流程图

通过response设置响应行

设置响应行的状态码： setStatus(int sc)

package com.oracle.demo01;

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

/**
 * Servlet implementation class HeadServlet
 */
public class HeadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public HeadServlet() {
        super();
    }
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        设置响应头中的状态码
        response.setStatus(404);        
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

重定向：1.状态码：302 + 2 .响应头：Location
其实请求了两次但客户端只做了一次请求

原理图

package com.oracle.demo01;

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

public class Servlet01 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
/*//        设置响应状态码
        response.setStatus(302);
//        设置响应头中的Location  
        response.setHeader("Location","/WEB05/Servlet02");*/
        
//        重定向 sendRedirect方法  下面这一句话就等于上面两句话
        response.sendRedirect("/WEB05/Servlet02");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}
----------------------------------------------
package com.oracle.demo01;

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

public class Servlet02 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().write("Servlet02");
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

通过response设置响应头

package com.oracle.demo01;

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

public class RefreshServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        设置定时刷新的头
        response.setHeader("Refresh","5;url=https://www.baidu.com");
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

用JSP写一个重定向

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<script type="text/javascript">
    window.onload=function(){
        //获取span元素
        var second=document.getElementById("second");
        //定义秒数
        var time=5;
        //设置定时器
        var timer=setInterval(function(){
            //获得元素里面的文本节点
            second.innerHTML=time;
            time--;
            if(time<0){
                clearInterval(timer);
                location.href="http://www.baidu.com";
            }
        },1000);
    }    
</script>
</head>
<body>
    恭喜你，注册成功！
    <span id="second" style="color:red">5</span>
    秒后跳转，如没跳转，请点击
    <a href="http://www.baidu.com">这里</a>
</body>
</html>

通过response设置响应体

(1)响应体设置文本

PrintWriter getWriter()

获得字符流，通过字符流的write(String s)方法可以将字符串设置到response 缓冲区中，随后Tomcat会将response缓冲区中的内容组装成Http响应返回给浏览器端

package com.oracle.demo01;
//设置码表让页面显示中文
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class BodyServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
/*//        设置缓冲区编码表
        response.setCharacterEncoding("UTF-8");
//        设置客户端浏览器使用的码表
        response.setHeader("Content-Type","text/html;charset=utf-8");*/
//        解决响应的中文乱码问题 用一个犯法就可以 代替上面两句话
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().write("中国");
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

创建一个download.jsp （能解析的解析，不能解析的下载功能）

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
    <a href="/WEB05/download/a.png">a.png</a>
    <a href="/WEB05/download/a.txt">a.txt</a>
    <a href="/WEB05/download/a.zip">a.zip</a>
</body>
</html>

(1)响应头设置字节

ServletOutputStream getOutputStream()

获得字节流，通过该字节流的write(byte[] bytes)可以向response缓冲区中写入字节，在由Tomcat服务器将字节内容组成Http响应返回给浏览器。

2．案例-完成文件下载

文件下载的实质就是文件拷贝，将文件从服务器端拷贝到浏览器端。所以文件下载需要IO技术将服务器端的文件使用InputStream读取到，在使用 ServletOutputStream写到response缓冲区中

代码如下：

上述代码可以将图片从服务器端传输到浏览器，但浏览器直接解析图片显示在页面上，而不是提供下载，需要设置两个响应头，告知浏览器文件的类型和文件的打开方式。

1）告知浏览器文件的类型：response.setContentType(文件的MIME类型);

2）告示浏览器文件打开方式是下载：response.setHeader("Content-Disposition","attachment;filename=文件名称");

//            告知浏览器文件的类型
            response.setContentType(getServletContext().getMimeType(filenameEncoder));
//            告知浏览器以附件的方式提供下载功能 而不是解析
//            Content-Disposition：设置用什么方式打开  attachment:代表附件
            response.setHeader("Content-Disposition","attachment;filename="+filenameEncoder);

3)解决中文名显示乱码问题

//        获取文件名
        String filename=request.getParameter("file");//??.png
//        解决文件名中文乱码问题
        filename=new String(filename.getBytes("ISO-8859-1"),"UTF-8");//乱码.png
//        获取User-Agent获取客户端浏览器是哪个浏览器 
        String agent=request.getHeader("User-Agent");
        String filenameEncoder="";
        if (agent.contains("MSIE")) {
                // IE浏览器
                filenameEncoder= URLEncoder.encode(filename, "utf-8");
                filenameEncoder= filenameEncoder.replace("+", " ");
        } else if (agent.contains("Firefox")) {
                // 火狐浏览器
        BASE64Encoder base64Encoder = new BASE64Encoder();
                filenameEncoder= "=?utf-8?B?"
                        + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
                // 其它浏览器
                filenameEncoder= URLEncoder.encode(filename, "utf-8");                
        }
//            告知浏览器文件的类型
            response.setContentType(getServletContext().getMimeType(filenameEncoder));
//            告知浏览器以附件的方式提供下载功能 而不是解析 
//            Content-Disposition：设置用什么方式打开  attachment:代表附件
            response.setHeader("Content-Disposition","attachment;filename="+filenameEncoder);

全部代码

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
    <a href="/WEB05/DownLoadServlet?file=a.png">a.png</a>
    <a href="/WEB05/DownLoadServlet?file=a.txt">a.txt</a>
    <a href="/WEB05/DownLoadServlet?file=a.zip">a.zip</a>
</body>
</html>
---------------------------------------
package com.oracle.demo02;
//页面文件下载 显示中文
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;

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

import sun.misc.BASE64Encoder;

public class DownLoadServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        获取文件名
        String filename=request.getParameter("file");//??.png
//        解决文件名中文乱码问题
        filename=new String(filename.getBytes("ISO-8859-1"),"UTF-8");//乱码.png
//        获取User-Agent获取客户端浏览器是哪个浏览器 
        String agent=request.getHeader("User-Agent");
        String filenameEncoder="";
        if (agent.contains("MSIE")) {
                // IE浏览器
                filenameEncoder= URLEncoder.encode(filename, "utf-8");
                filenameEncoder= filenameEncoder.replace("+", " ");
        } else if (agent.contains("Firefox")) {
                // 火狐浏览器
        BASE64Encoder base64Encoder = new BASE64Encoder();
                filenameEncoder= "=?utf-8?B?"
                        + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
                // 其它浏览器
                filenameEncoder= URLEncoder.encode(filename, "utf-8");                
        }
//            告知浏览器文件的类型
            response.setContentType(getServletContext().getMimeType(filenameEncoder));
//            告知浏览器以附件的方式提供下载功能 而不是解析
            response.setHeader("Content-Disposition","attachment;filename="+filenameEncoder);
//        获取字节输出流
        ServletOutputStream sos=response.getOutputStream();
//        获取数据源绝对路径
        String realpath=getServletContext().getRealPath("download/"+filename);
//        获取字节输入流
        FileInputStream fis=new FileInputStream(realpath);
//        开始复制
        byte[] bytes=new byte[1024];
        int len=0;
        while((len=fis.read(bytes))!=-1){
            sos.write(bytes,0,len);
        }
//        释放资源
        fis.close();
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

生成验证码

这是一个写好的验证码代码

package com.oracle.demo02;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

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

/**
 * 验证码生成程序
 * 
 * 
 * 
 */
public class CheckImgServlet extends HttpServlet {

    // 集合中保存所有成语
    private List<String> words = new ArrayList<String>();

    @Override
    public void init() throws ServletException {
        // 初始化阶段，读取new_words.txt
        // web工程中读取 文件，必须使用绝对磁盘路径
        String path = getServletContext().getRealPath("/WEB-INF/new_words.txt");
        try {
//            读取这个文件
            BufferedReader reader = new BufferedReader(new FileReader(path));
            String line;
            while ((line = reader.readLine()) != null) {
                words.add(line);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 禁止缓存
        // response.setHeader("Cache-Control", "no-cache");
        // response.setHeader("Pragma", "no-cache");
        // response.setDateHeader("Expires", -1);

        int width = 120;
        int height = 30;

        // 步骤一 绘制一张内存中图片
        BufferedImage bufferedImage = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);

        // 步骤二 图片绘制背景颜色 ---通过绘图对象
        Graphics graphics = bufferedImage.getGraphics();// 得到画图对象 --- 画笔
        // 绘制任何图形之前 都必须指定一个颜色
        graphics.setColor(getRandColor(200, 250));
        graphics.fillRect(0, 0, width, height);

        // 步骤三 绘制边框
        graphics.setColor(Color.WHITE);
        graphics.drawRect(0, 0, width - 1, height - 1);

        // 步骤四 四个随机数字
        Graphics2D graphics2d = (Graphics2D) graphics;
        // 设置输出字体
        graphics2d.setFont(new Font("宋体", Font.BOLD, 18));

        Random random = new Random();// 生成随机数
        int index = random.nextInt(words.size());
        String word = words.get(index);// 获得成语

        // 定义x坐标
        int x = 10;
        for (int i = 0; i < word.length(); i++) {
            // 随机颜色
            graphics2d.setColor(new Color(20 + random.nextInt(110), 20 + random
                    .nextInt(110), 20 + random.nextInt(110)));
            // 旋转 -30 --- 30度
            int jiaodu = random.nextInt(60) - 30;
            // 换算弧度
            double theta = jiaodu * Math.PI / 180;

            // 获得字母数字
            char c = word.charAt(i);

            // 将c 输出到图片
            graphics2d.rotate(theta, x, 20);
            graphics2d.drawString(String.valueOf(c), x, 20);
            graphics2d.rotate(-theta, x, 20);
            x += 30;
        }

        // 将验证码内容保存session
        request.getSession().setAttribute("checkcode_session", word);

        // 步骤五 绘制干扰线
        graphics.setColor(getRandColor(160, 200));
        int x1;
        int x2;
        int y1;
        int y2;
        for (int i = 0; i < 30; i++) {
            x1 = random.nextInt(width);
            x2 = random.nextInt(12);
            y1 = random.nextInt(height);
            y2 = random.nextInt(12);
            graphics.drawLine(x1, y1, x1 + x2, x2 + y2);
        }

        // 将上面图片输出到浏览器 ImageIO
        graphics.dispose();// 释放资源        
        //将图片写到response.getOutputStream()中
        ImageIO.write(bufferedImage, "jpg", response.getOutputStream());
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

    /**
     * 取其某一范围的color
     * 
     * @param fc
     *            int 范围参数1
     * @param bc
     *            int 范围参数2
     * @return Color
     */
    private Color getRandColor(int fc, int bc) {
        // 取其随机颜色
        Random random = new Random();
        if (fc > 255) {
            fc = 255;
        }
        if (bc > 255) {
            bc = 255;
        }
        int r = fc + random.nextInt(bc - fc);
        int g = fc + random.nextInt(bc - fc);
        int b = fc + random.nextInt(bc - fc);
        return new Color(r, g, b);
    }
}

在配置一下 web.xml，因为这个验证码代码没有配置过Servlet，所以从新配置，更改一下 name 和class就可以

<servlet>
    <description></description>
    <display-name>CheckImgServlet</display-name>
    <servlet-name>CheckImgServlet</servlet-name>
    <servlet-class>com.oracle.demo02.CheckImgServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>CheckImgServlet</servlet-name>
    <url-pattern>/CheckImgServlet</url-pattern>
  </servlet-mapping>

在导入已经写好的验证码成语

验证码生成程序实现点击事件

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
<script type="text/javascript">
    function code(node){
        /* 这样请求每次都不一样 */
        node.src="/WEB05/CheckImgServlet?time="+new Date().getTime();
    }
</script>
</head>
<body>
    <img alt="" src="/WEB05/CheckImgServlet" onClick="code(this)">
</body>
</html>

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原文地址：https://www.cnblogs.com/zs0322/p/11119782.html