记账本开发记录——第十四天（2020.2.1）

今天主要学习了response对象，也就是http响应。学习了几个该对象下的方法和操作。关于其运行流程，如图：

 简单解释的话，就是：客户端发送http请求，tomcat(服务器)将请求封装成request对象和response对象，发送给web应用,web应用里对response进行操作，写到response的缓冲区内，之后被tomcat获取。tomcat获取好后将其组装成一个http响应，交给浏览器解析。

从以上的解释可以看到，response下的操作大概有三类：设置响应行，设置响应头，设置响应体，

对于响应行来说，可操作内容并不多，不再详细叙述。

关于设置响应头，进行了一个简单的demo测试，如下：

package header;

import java.io.IOException;
import java.sql.Date;

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

/**
 * Servlet implementation class HeaderServlet
 */
@WebServlet("/HeaderServlet")
public class HeaderServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public HeaderServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Date date = new Date(0);
        //设置响应头
        response.addHeader("name", "zhangsan");
        response.addIntHeader("age", 28);
        response.addDateHeader("birthday", date.getTime());
        response.setHeader("age", "50");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

在doGet中写了几种方法的演示，其中setHeader会替代旧的内容，而addHeader则会添加到后面。

设置响应体，也就是设置文本。主要用来在界面上写入文字或者其他内容。对于文字：

package content;

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

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

/**
 * Servlet implementation class TextServlet
 */
@WebServlet("/TextServlet")
public class TextServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public TextServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        //response.setCharacterEncoding("UTF-8");
        response.setHeader("Content-Type", "text/html;charset=UTF-8");
        PrintWriter writer = response.getWriter();
        //writer.write("hello response!");
        writer.write("你好世界!");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

可以看到，response.setHeader("Content-Type", "text/html;charset=UTF-8");  这一行是用来防止中文乱码的，因为tomcat默认的解码表是ISO8895-1,其中并不含有汉字，我们要将其设置为UTF-8才可以读取到中文内容。

那么对于非文字内容，就需要使用文件流来进行读取了，如下：

package content;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

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

/**
 * Servlet implementation class ByteServlet
 */
@WebServlet("/ByteServlet")
public class ByteServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ByteServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        
        //使用response获得字节输出流
        ServletOutputStream out = response.getOutputStream();
        
        //获得服务器上的图片
        String realPath = this.getServletContext().getRealPath("a.jpg");
        InputStream in = new FileInputStream(realPath);
        int len=0;
        byte[] buffer = new byte[1024];
        while((len=in.read(buffer))>0) {
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

可以看到，其操作和java内文件的读取代码基本一致。通过这些学习我们就可以完成一个小案例——进行文件的下载。

html界面：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
  <h1>使用a标签直接指向服务器上的资源</h1>
  <a href="/Day14/download/a.flv">a.flv</a><br>
  <a href="/Day14/download/a.jpg">a.jpg</a><br>
  <a href="/Day14/download/a.mp3">a.mp3</a><br>
  <a href="/Day14/download/a.mp4">a.mp4</a><br>
  <a href="/Day14/download/a.txt">a.txt</a><br>
  <a href="/Day14/download/a.zip">a.zip</a><br>
  <h1>使用服务器端编码的方式实现文件下载</h1>
  <a href="/Day14/DownloadServlet?filename=a.flv">a.flv</a><br>
  <a href="/Day14/DownloadServlet?filename=a.jpg">a.jpg</a><br>
  <a href="/Day14/DownloadServlet?filename=a.mp3">a.mp3</a><br>
  <a href="/Day14/DownloadServlet?filename=a.mp4">a.mp4</a><br>
  <a href="/Day14/DownloadServlet?filename=a.txt">a.txt</a><br>
  <a href="/Day14/DownloadServlet?filename=a.zip">a.zip</a><br>
  <a href="/Day14/DownloadServlet2?filename=美女.jpg">美女.jpg</a><br>
</body>
</html>

servlet层：

package content;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

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

/**
 * Servlet implementation class DownloadServlet
 */
@WebServlet("/DownloadServlet")
public class DownloadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DownloadServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        //获得要下载的文件的名称
        String filename = request.getParameter("filename");
        //要下载这个文件的类型-----客户端通过文件的MIME类型去区分类型
        response.setContentType(this.getServletContext().getMimeType(filename));
        
        //告诉客户端该文件不是直接解释,而是以附件打开
        response.setHeader("Content-Disposition", "attachment;filename="+filename);
        //获取文件的绝对路径
        String path = this.getServletContext().getRealPath("download/"+filename);
        //获得该文件的输入流
        InputStream in = new FileInputStream(path);
        //获得输出流---通过response获得的输出流用于向客户端写内容
        ServletOutputStream out = response.getOutputStream();
        //文件拷贝模版代码
        int len = 0;
        byte[] buffer = new byte[1024];
        while((len=in.read(buffer))>0) {
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

可以看到，操作和上面的写法基本一致。

接下来是一个拓展操作，关于验证码的显示。验证码的生成代码并不需要我们自己来写，可以直接从网上copy一份，我们直接从前端获取就好，如下：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<script>
  function changeImg(obj){
      obj.src="/Day14/CheckImg?time="+new Date().getTime();
  }
</script>
</head>
<body>
  <form action="LoginServlet" method="post">
      用户名:<input type="text" name="username"><br>
      密码:<input type="password" name="password"><br>
     验证码<input type="text" name="password"><img onclick="changeImg(this)" src="/Day14/CheckImg">
      <input type="submit" value="登录">
  </form>
</body>
</html>

通过查看验证码生成的servlet，就可以很简单的做好验证操作了。

贴一下生成验证码的servlet:

package cn.itcast.estore.web.servlet;

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 {
            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);
    }

}