综合getOutputStream() has already been called for this response 出错示例

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JAVA基础
AJAX
Jsp
java

AJAX验证jsp页面验证码

在做登录的时候往往会加上验证码，然后提交form表单到action验证，然而，传统的这种做法往往验证不通过，返回INPUT页面时，出现验证码图片不显示，并抛出空指针异常，遇到这种情况，通常的解决方法是将action页面跳转到input页面的时候定义成重定向，这样则可以解决图片不显示的问题，并不会抛异常，但验证错误信息和文本框输入信息却不会显示在返回页面，这只是我在实际中遇到的问题，当然，传统做法很可能还会带来诸多各种各样问题，AJAX的使用则彻底解决上述种种问题，下面是一个简易登录页面验证流程：images.jsp

<%@ page language="java" contentType="image/jpeg ; charset=UTF-8"
    pageEncoding="UTF-8" import="java.awt.*,
java.awt.image.*,javax.imageio.*,java.io.*"%>
<%@ page import ="com.CreatImage"%>
<%@ taglib uri="/webwork" prefix="ww" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<%
    response.setHeader("Pragma","No-cache");
    response.setHeader("Cache-Control","no-cache");
    response.setDateHeader("Expires", 0);

    CreatImage c = new CreatImage();
    BufferedImage  image = c.creatImage();
    OutputStream b = response.getOutputStream();
%>
<%ImageIO.write(image, "JPEG", response.getOutputStream());

    //必须添加以下两行否则tomcat下jsp出现getOutputStream() has already been called for this response异常
    out.clear();
    out = pageContext.pushBody();
%>
</body>
</html>

上面页面没有验证图片生成代码，验证图片生成代码写在com.CreatImage中，然后再jsp页面<%@ page import ="com.CreatImage"%>导入；
CreatImage.java:

package com;

import java.awt.*;
import java.awt.image.*;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.*;

import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.opensymphony.webwork.ServletActionContext;
import com.opensymphony.xwork.ActionSupport;

public class CreatImage extends ActionSupport{
   /**
*
*/
    private static final long serialVersionUID = 1L;
    public String sRand="";
   Image image = null;
   public Color getRandColor(int fc,int bc){//给定范围获得随机颜色
    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);
    }
   public BufferedImage creatImage(){

// 在内存中创建图象
int width=60, height=20;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

// 获取图形上下文
Graphics g = image.getGraphics();

//生成随机类
Random random = new Random();

     // 设定背景色
     g.setColor(getRandColor(200,250));
     g.fillRect(0, 0, width, height);

//设定字体
g.setFont(new Font("Times New Roman",Font.PLAIN,18));

          //画边框
         //g.setColor(new Color());
         //g.drawRect(0,0,width-1,height-1);

         // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
         g.setColor(getRandColor(160,200));
         for (int i=0;i<155;i++)
         {
       int x = random.nextInt(width);
       int y = random.nextInt(height);
       int xl = random.nextInt(12);
       int yl = random.nextInt(12);
       g.drawLine(x,y,x+xl,y+yl);
         }
         // 取随机产生的认证码(4位数字)
         //String rand = request.getParameter("rand");
         //rand = rand.substring(0,rand.indexOf("."));

         String str1=randomStr(4);// 得到随机字符
         HttpSession session = ServletActionContext.getRequest().getSession();
      session.setAttribute("validatenumber",str1);
         for (int i=0;i<4;i++){
       String rand=str1.substring(i,i+1);
      // 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)));
//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
        g.drawString(rand,13*i+6,16);
         }
         // 图象生效
         g.dispose();

         return image;
       }
// 得到随机字符

public String randomStr(int n) {
      String str1 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";
      String str2 = "";
      int len = str1.length() - 1;
      double r;
      for (int i = 0; i < n; i++) {
    r = (Math.random()) * len;
    str2 = str2 + str1.charAt((int) r);
      }
      return str2;
}

   public void creatvalidateimage(){
    HttpSession session = ServletActionContext.getRequest().getSession();
          String valnumber = (String)session.getAttribute("validatenumber");
          HttpServletResponse response = ServletActionContext.getResponse();
      response.setContentType("text/html;charset=utf-8");
      PrintWriter out;
          if(validate.equals(valnumber)){
     session.invalidate();//清空session
     message="validate success";
     try {
    out = ServletActionContext.getResponse().getWriter();
    out.println(message);
    out.flush();
          out.close();
      } catch (IOException e) {
    e.printStackTrace();
      }
    }else
     session.removeAttribute("validatenumber");
     message="validate is error";
     try {
    out = ServletActionContext.getResponse().getWriter();
    out.println(message);
    out.println("<table>");
    out.println("<tr>");
    out.println("<td>userName:<input type='text' id='name' value='"+name+"'/></td>");
    out.println("</tr>");
    out.println("<tr>");
    out.println("<td>userPassword:<input type='password' id='password' value='"+password+"'/></td>");
    out.println("</tr>");
    out.println("<tr>");
    out.println("<td>validate:<input type='text' id='validate'/><img     src=\"http://localhost:8088/uploadfile/images.jsp?a="+Math.random()+"\"/><a href='' onclick='changeCode()'>看不清,换一张?</a></td>");
    out.println("</tr>");
    out.println("</table>");
    out.flush();
          out.close();
     } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
      }

   }
      private String validate;
      private String message;
      private String name;
      private String password;
    public String getMessage() {
        return message;
    }
    public void setMessage(String message) {
        this.message = message;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    //    public Image getImage() {
//        return image;
//    }
//    public void setImage(Image image) {
//        this.image = image;
//    }
    public String getValidate() {
        return validate;
    }
    public void setValidate(String validate) {
        this.validate = validate;
    }
}

登录页面：
login.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="/webwork" prefix="ww" %>
<%
    String path = request.getContextPath();
%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
        <script src="<%=path%>/js/prototype.js"></script>
    <script>
      function getXML(){
      var name = document.getElementById("name").value;
      var password = document.getElementById("password").value;
      var validate = document.getElementById("validate").value;
      var url="<%=path %>/upload/creatimage.action?password="+password+"&validate="+validate;
      var myAjax = new Ajax.Request(
      url,
      {
          method:'post',
          onComplete:showResponse,
          parameters:"name="+name,
          asynchronous:true
      }
      );
      }
      function showResponse(xmlrequest){
      var text = xmlrequest.responseText;
      document.getElementById("uploadanotherfile").innerHTML=text;
         }
     function changeCode()
      {
       document.getElementById("cc").src="<%=path%>/images.jsp?a="+Math.random();

      }
</script>
</head>
<body>
<form action="<%=path %>/upload/creatimage.action" method="post">
<div id="uploadanotherfile">
<table>
<tr>
<td>userName:<input type="text" id="name" value="<ww:property value="name"/>"/></td>
</tr>
<tr>
<td>userPassword:<input type="password" id="password" value="<ww:property value="password"/>"/></td>
</tr>
<tr>
<td>validate：<input type="text" id="validate"><img   alt="" id="cc" src="<%=path %>/images.jsp"/><a href="#" onclick="changeCode();">看不清,换一张?</a></td>
</tr>
</table>
</div>
<input type="button" name="button" value="submit" onclick="getXML();"/>
</form>
</body>
</html>

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getOutputStream() has already been calle ... | ————

2011-09-03 00:58
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