我想不明白的问题

 1 package com.response;
 2 
 3 import java.awt.Color;
 4 import java.awt.Font;
 5 import java.awt.Graphics;
 6 import java.awt.Graphics2D;
 7 import java.awt.image.BufferedImage;
 8 import java.io.IOException;
 9 import java.io.PrintWriter;
10 import java.util.Random;
11 
12 import javax.imageio.ImageIO;
13 import javax.servlet.ServletException;
14 import javax.servlet.http.HttpServlet;
15 import javax.servlet.http.HttpServletRequest;
16 import javax.servlet.http.HttpServletResponse;
17 
18 public class Demo2 extends HttpServlet {
19 
20         public static final int WIDTH=120;//设置图像的宽
21         public static final int HEIGHT=25;//设置图像的高
22         public void doGet(HttpServletRequest request, HttpServletResponse response)
23                         throws ServletException, IOException {
24         this.doPost(request, response);
25         }
26   //输出注册时候的随机图片
27         public void doPost(HttpServletRequest request, HttpServletResponse response)
28                         throws ServletException, IOException {
29                 //BufferedImage 可以自动生成图像，BufferedImage.TYPE_INT_RGB是图片的类型
30                 BufferedImage image=new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
31                 //得到这个图形
32                 Graphics g=image.getGraphics();
33                 
34                 //1.设置图像的背景色
35                 setBackaground(g);
36                 //2.设置图像的边框
37                 setBorder(g);
38                 //3.在图像上画干扰线
39                 drawRandomLine(g);
40                 //4.在图像上写随机数
41                 drawRanderNum((Graphics2D)g);
42                 //5.把图像写给浏览器,设置浏览器打开文件的格式
43                 response.setContentType("image/jpeg");
44                 //发响应头控制浏览器不要缓存
45                 response.setDateHeader("expries", -1);
46                 response.setHeader("Cache-Control","no-cache");
47                 response.setHeader("Pragma","no-cache");
48                 ImageIO.write(image, "jpg", response.getOutputStream());
49 
50         }
51 private void drawRanderNum(Graphics2D g) {
52         g.setColor(Color.RED);//设置字的颜色为红色
53         g.setFont(new Font("宋体",Font.BOLD,20));//设置字体的样式，Font.BOLD为加粗，20字体的高度
54         int x=10;
55         for(int i=0;i<4;i++)
56         {
57                 int degree=new Random().nextInt()%30;//定义字符旋转多少度，%30是表示度数为-30到30度
58                 String shu=new Random().nextInt(10)+"";
59                 g.rotate(degree*Math.PI/180, x, 20);//设置旋转的弧度，后两个参数是基于那个点旋转
60                 g.drawString(shu , x, 20);//这是往图片上写入数字，参数是写入数字的坐标
61                 g.rotate(-degree*Math.PI/180, x, 20);//清掉上次的旋转，确保下次的旋转正确 
62                 x=x+30; 
63         }
64         
65 }
66 private void drawRandomLine(Graphics g) {
67         g.setColor(Color.GREEN);//设置干扰线的颜色
68         for(int i=0;i<5;i++)//i为干扰线的条数
69         {
70                 int x1=new Random().nextInt(WIDTH);//x1,y1为干扰线的起点的坐标，x坐标必须在图像的宽内，y坐标在高之内
71                 int y1=new Random().nextInt(HEIGHT);
72                 
73                 int x2=new Random().nextInt(WIDTH);
74                 int y2=new Random().nextInt(HEIGHT);
75                 g.drawLine(x1, y1, x2, y2);//这个方法是画出干扰线
76         }
77         
78 }
79 private void setBorder(Graphics g) {
80         g.setColor(Color.BLUE);//？这里为什么只变的是边框 
81         g.drawRect(1, 1, WIDTH-2, HEIGHT-2);
82                 
83         }
84 private void setBackaground(Graphics g) {
85         g.setColor(Color.WHITE);//设置图形的背景色为白色
86         g.fillRect(0, 0, WIDTH, HEIGHT);//填充矩形
87         
88 }
89 
90 }