制作一个验证码的切换功能。（具体过程）
1.首先，在包下new一个servlet，比如：VertifyCodeServlet
2.给urlPatterns一个名字，如："/code"
3.在doGet（）中先需要一张图片。
这个图片可以通过BufferedImage来定义：BufferedImage image=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB)
而width要事先定义好，int width=100;int heigh30;
4.这张图片上的默认颜色是黑色的，故需要更改颜色。这里用到Graphics
Graphics graphics=image.getGraphics();
graphics.setColor(Color.gray),而后填充上面的矩形。graphics.fillRect(x:0,y:0,width,height)
5在图片中输出随机的字符。
往图片中写字符用graphics.drawString(str,x,y);
而要随机生成，事先要先设置一个字符集
String data="asdfg1233";
Random random=new Random();
在随机得到字符串的位置：int position=random.netInt(data.length());
拿到字符：data.subString(position,position+1).
这里subString(beginIndex,endIndex)方法是拿到一个字符串的子字符串，长度为endIndex-beginIndex.
这样再结合需要四个这样的字符，就有：
for(int i=0;i<4;i++)
{
int posttion=random.nextINT（data.length()）；
String randomstr=data.subString(position,position+1)
graphics.drawString(randomstr,x:width/5*(i+1),y:15)
}
而后要将这幅图片写入浏览器中，用ImageIO,他可以将其转化成outputstream类型。
ImageIO.write(image,formatName:"jpg",response.getoutputStream)
附上代码：
vertifyCode.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<h2>切换验证码</h2>
<script>
function changeImageCode() {
document.getElementById("btn").isDisabled=true;
document.getElementById("identify").src="code?ts="+new Date().getTime();
// document.getElementById("identify").src="code";
}
</script>
<img src="code" id="identify" onload="btn.disabled=false" style="cursor: pointer; vertical-align: auto">
<input type="button" value="看不清，切换代码" id="btn" onclick="changeImageCode()" style="vertical-align: auto">
</body>
</html>VertifyCodeServlet
package net.zixue.servlet;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
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.io.IOException;
import java.util.Random;
@WebServlet(name = "VertifyCodeServlet",urlPatterns = "/code")
public class VertifyCodeServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
int width=100;
int height=30;
String data="abcdefg1234560";
Random random=new Random();
BufferedImage image=new BufferedImage(width,height,BufferedImage.TYPE_INT_BGR);
Graphics graphics = image.getGraphics();
graphics.setColor(Color.gray);
graphics.fillRect(0,0,width,height);
graphics.setColor(Color.black);
for (int i=0;i<4;i++)
{
int position = random.nextInt(data.length());
String randomstr = data.substring(position, position + 1);
graphics.drawString(randomstr,width/5*(i+1),15);
}
ImageIO.write(image,"jpg",response.getOutputStream());
}
}