3,random.jsp,生成验证码图片的页面
<%@ page autoflush="false"  import="java.util.*,java.awt.*,java.awt.image.*,com.sun.image.codec.jpeg.*,java.util.*" %>

<%
//set chinese char
//cody by jarryli@gmail.com;
//homepage:jiarry.126.com
request.setcharacterencoding("gbk");
response.setcharacterencoding("gbk");
response.setcontenttype("text/html; charset=gbk");
%>
<%
string chose="0123456789abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";

char display[]={'0',' ','0',' ','0',' ','0'},ran[]={'0','0','0','0'},temp;

random rand=new random();

for(int i=0;i<4;i++)
{

 temp=chose.charat(rand.nextint(chose.length()));

 display[i*2]=temp;

 ran[i]=temp;
 }
 
string random=string.valueof(display);

session.setattribute("random",string.valueof(ran));
 %>
<%

        out.clear();
        response.setcontenttype("image/jpeg");
        response.addheader("pragma","no-cache");
        response.addheader("cache-control","no-cache");
        response.adddateheader("expries",0);
        int width=80, height=30;
        bufferedimage image = new bufferedimage(width, height, bufferedimage.type_int_rgb);
        graphics g = image.getgraphics();
        //以下填充背景颜色
        g.setcolor(color.green);
        g.fillrect(0, 0, width, height);
       //设置字体颜色
        g.setcolor(color.red);
        font font=new font("arial",font.plain,20);
        g.setfont(font);
        //g.drawstring(random,5,14);
        g.drawstring(random,5,20);
        g.dispose();
        servletoutputstream outstream = response.getoutputstream();
        jpegimageencoder encoder =jpegcodec.createjpegencoder(outstream);
        encoder.encode(image);
        outstream.close();
   %>

4,net.js,封装好的xmlhttp对象，可以很方便的调用

/* namespacing object */
var net=new object();
 
net.ready_state_uninitialized=0;
net.ready_state_loading=1;
net.ready_state_loaded=2;
net.ready_state_interactive=3;
net.ready_state_complete=4;
/*--- content loader object for cross-browser requests ---*/
net.contentloader=function(url,on_load,on_error,method,params,contenttype){
  this.req=null;
  this.on_load=on_load;
  this.on_error=(on_error) ? on_error : this.defaulterror;
  this.loadxmldoc(url,method,params,contenttype);
}
net.contentloader.prototype.loadxmldoc=function(url,method,params,contenttype){
  if (!method)
  {
 method="get";
  }
  if (!contenttype && method=="post")
  {
 contenttype='application/x-www-form-urlencoded';
  }
  if (window.xmlhttprequest)
  {
 this.req=new xmlhttprequest();
  }
  else if (window.activexobject)
  {
   //add try catch;
   try {
      this.req = new activexobject("msxml2.xmlhttp");
   }catch (e1){
     try {
      this.req = new activexobject("microsoft.xmlhttp");    
      } catch (e2){
    }
   }
     //
 //this.req=new activexobject("microsoft.xmlhttp");
  }
  if (this.req)
  {
 try
 {
   var loader=this;
   this.req.onreadystatechange=function()
   {
  net.contentloader.onreadystate.call(loader);
   }
   this.req.open(method,url,true);
   if (contenttype)
   {
    this.req.setrequestheader('content-type', contenttype);
   }
   this.req.send(params);
 }
 catch (err)
 {
   this.on_error.call(this);
 }
  }
}
net.contentloader.onreadystate=function(){
  var req=this.req;
  var ready=req.readystate;
  if (ready==net.ready_state_complete){
 var httpstatus=req.status;
 if (httpstatus==200 || httpstatus==0){
   this.on_load.call(this);
 }else{
   this.on_error.call(this);
 }
  }
}
net.contentloader.prototype.defaulterror=function(){
  alert("error fetching data!"
 +"\n\nreadystate:"+this.req.readystate
 +"\nstatus: "+this.req.status
 +"\nheaders: "+this.req.getallresponseheaders());
}