在JAVA项目中验证码肯定是不可缺少的，首先给展示的是最简单的直接写在JSP中的验证码，可以通过session验证,也可以通过这段直接改写在controller中。

code.jsp

<%@ page language="java" pageEncoding="UTF-8"%>
<%@ page contentType="image/jpeg"  import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" %>
<%!
/** 
    * 生成随机着色 
    */
public Color getColor(){
        Random random = new Random();
int r = random.nextInt(256);//0-255  
int g = random.nextInt(256);
int b = random.nextInt(256);
return new Color(r,g,b);
    }
//定义验证码的随机字符串  
    String randomStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";
/** 
    * 生成随机字符 
    */
public String getStr(){
        StringBuffer sb = new StringBuffer();
        Random random = new Random();
for(int i=0;i<6;i++){
            sb.append(randomStr.charAt(random.nextInt(randomStr.length())));//每次提取一个随机字符  
        }
return sb.toString();
    }
%>
<%
//设置响应头部信息告诉浏览器不缓存图片  
    response.setHeader("pragma", "mo-cache");
    response.setHeader("cache-control", "no-cache");
    response.setDateHeader("expires", 0);
    BufferedImage image = new BufferedImage(100,30,BufferedImage.TYPE_INT_RGB);
    Graphics g = image.getGraphics();
    g.setColor(new Color(200,200,200));
    g.fillRect(0,0,100,30);
    Random random = new Random();
//绘制40条干扰线   
for (int i = 0; i < 40; i++) {
int x = random.nextInt(100);
int y = random.nextInt(30);
int xl = random.nextInt(x+10);
int yl = random.nextInt(y+10);
        g.setColor(getColor());
        g.drawLine(x, y, x + xl, y + yl);
    }
    g.setFont(new Font("宋体", Font.ROMAN_BASELINE,24));
    g.setColor(Color.BLACK);
    String randomCode = getStr();
    StringBuffer sb = new StringBuffer();
for(int i=0;i<randomCode.length();i++){
        g.drawString(randomCode.charAt(i)+"",random.nextInt(10),20+random.nextInt(8));//sb.append(randomCode.charAt(i));  
        g.translate(15,0);
    }
//将验证码放入Session中  
    session.setAttribute("RANDOMCODE",randomCode);
//通过流将验证码图片输出   
    ImageIO.write(image,"jpeg",response.getOutputStream());
    out.clear();
    out = pageContext.pushBody();
%>

前端显示

<img src="code.jsp" width="48" height="15">

下面还有个例子。

这例子是一样的效果有些不同，前端还是跟前面一样使用，还是作为参考。

code.jsp

<%@ page contentType="image/jpeg" language="java" import="java.util.*,java.awt.*,java.awt.image.*,javax.imageio.*" pageEncoding="utf-8"%>
<%!
    Color getRandColor(int fc,int bc){
        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);
    }
%>
<%
//设置页面不缓存  
    response.setHeader("Pragma","no-cache");
    response.setHeader("Cache-Control","no-catch");
    response.setDateHeader("Expires",0);
//在内存中创建图象  
int width = 60;
int height = 20;
    BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
//创建图象  
    Graphics g = image.getGraphics();
//生成随机对象  
    Random random = new Random();
//设置背景色  
    g.setColor(getRandColor(200,250));
    g.fillRect(0,0,width,height);
//设置字体  
    g.setFont(new Font("Tines Nev Roman",Font.PLAIN,18));
//随机产生干扰线  
    g.setColor(getRandColor(160,200));
for(int i = 0; i < 255; i++){
int x = random.nextInt(width);
int y = random.nextInt(height);
int xl = random.nextInt(12);
int yl = random.nextInt(12);
    }
//随机产生认证码,4位数字  
    String sRand = "";
for(int i = 0; i < 4; i++){
        String rand = String.valueOf(random.nextInt(10));
        sRand  += rand;
//将认证码显示到图象中  
        g.setColor(new Color(20 + random.nextInt(110),20 + random.nextInt(110),20 + random.nextInt(110)));
        g.drawString(rand,13*i+6,16);
    }
    session.setAttribute("Code",sRand);
//图像生效  
    g.dispose();
//输出图像到页面  
    ImageIO.write(image,"JPEG",response.getOutputStream());
    out.clear();
    out = pageContext.pushBody();
%>

到最后这里提下

解决getOutputStream() has already been called for this response

其实就是因为没有添加以下代码，在尾部添加即可，非常关键！

• out.clear();
• out = pageContext.pushBody();