最近遇到一个小问题，写了一个验证码生成的servlet，在HTML用<img>时无法显示，单独访问servlet时却可以看到验证码；然后发现把下面这一段代码挪到输出图片前面就没有出现这个问题了！想问问各位大佬这是什么原因。
//3.将验证码字符串存储到Session对象中
HttpSession session = request.getSession();
session.setAttribute("checkCode", checkCode);
这是验证码生成的Servlet代码
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 javax.servlet.http.HttpSession;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
@WebServlet("/checkCodeServlet")
public class checkCodeServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//创建checkCode变量存储验证码字符串
String checkCode = "";
//1.创建图片对象
int width = 80;
int height = 25;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
//2.美化图片
Graphics g = image.getGraphics();//创建画笔对象
//2.1填充背景色
g.setColor(Color.PINK);
g.fillRect(0, 0, width, height);
//2.2画边框
g.setColor(Color.BLUE);
g.drawRect(0, 0, width - 1, height - 1);
//2.3写验证码
String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
Random rm = new Random();
for (int i = 1; i <= 4; i++) {
int x = rm.nextInt(str.length());
String s = str.charAt(x) + "";
g.drawString(s, width / 5 * i, height / 3 * 2);
checkCode += s;
}
//2.4画干扰线
g.setColor(Color.green);
for (int i = 0; i < 30; i++) {
int x1 = rm.nextInt();
int x2 = rm.nextInt();
int y1 = rm.nextInt();
int y2 = rm.nextInt();
g.drawLine(x1, x2, y1, y2);
}
//3.将验证码字符串存储到Session对象中
HttpSession session = request.getSession();
session.setAttribute("checkCode", checkCode);
//4.将图片输出到页面上
ImageIO.write(image, "jpg", response.getOutputStream());
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doPost(request, response);
}
}
这是HTML页面的代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>login</title>
<script>
window.onload=function () {
var image = document.getElementById("checkcodeimage");
image.onclick=function () {
var time = new Date().getTime();
image.src="/day1/checkCodeServlet?"+time;
}
}
</script>
<style type="text/css">
input{
margin-top: 5px;
}
.check input{
width: 50px;
}
.check img{
margin-top: 10px;
margin-left: 5px;
}
</style>
</head>
<body>
<form action="" method="post">
用户名：<input type="text" name="username" id="username"><br>
<span>密 码：</span><input type="password" name="password" id="password"><br>
<div class="check">
验证码：<input type="text" name="checkcode">
<img src="/day1/checkCodeServlet" id="checkcodeimage">
</div>
<br>
<input type="submit" value="登录">
</form>
</body>
</html>
有图有真相
