学习理论之后，必定要用实践操作来巩固知识。
下面写一个简单的例子，检测用户名是否唯一（直接撸代码）：
<%@ page language="java" contentType="text/html; charset=GB18030"
pageEncoding="GB18030"%><!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=GB18030"><title>检测用户名是否唯一</title><style type="text/css"><!--#toolTip { position:absolute; left:331px; top:39px; width:98px; height:48px; padding-top:45px; padding-left:25px; padding-right:25px; z-index:1; display:none; color:red; background-image: url(images/tooltip.jpg);}--></style></head><body style="margin: 0px;"><form method="post" action="" name="form1"><table width="509" height="352" border="0" align="center" cellpadding="0" cellspacing="0" background="images/bg.gif">
<tr>
<td height="54"> </td>
</tr>
<tr>
<td height="253" valign="top">
<p style="position:absolute;">
<table width="100%" height="250" border="0" cellpadding="0" cellspacing="0">
<tr>
<td width="18%" height="54" align="right" style="color:#8e6723 "><b>用户名：</b></td>
<td width="49%"><input name="username" type="text" id="username" size="32"></td>
<td width="33%"><img src="images/checkBt.jpg" width="104" height="23" style="cursor:hand;" onClick="checkUser(form1.username);"></td>
</tr>
<tr>
<td height="51" align="right" style="color:#8e6723 "><b>密码：</b></td>
<td><input name="pwd1" type="password" id="pwd1" size="35"></td>
<td rowspan="2"> <p id="toolTip"></p></td>
</tr>
<tr>
<td height="56" align="right" style="color:#8e6723 "><b>确认密码：</b></td>
<td><input name="pwd2" type="password" id="pwd2" size="35"></td>
</tr>
<tr>
<td height="55" align="right" style="color:#8e6723 "><b>E-mail：</b></td>
<td colspan="2"><input name="email" type="text" id="email" size="45"></td>
</tr>
<tr>
<td> </td>
<td colspan="2"><input type="image" name="imageField" src="images/registerBt.jpg"></td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td> </td>
</tr></table></form></body></html><script language="javascript">function createRequest(url) {
http_request = false;
if (window.XMLHttpRequest) { // 非IE浏览器
http_request = new XMLHttpRequest(); //创建XMLHttpRequest对象
} else if (window.ActiveXObject) { // IE浏览器
try {
http_request = new ActiveXObject("Msxml2.XMLHTTP"); //创建XMLHttpRequest对象
} catch (e) {
try {
http_request = new ActiveXObject("Microsoft.XMLHTTP"); //创建XMLHttpRequest对象
} catch (e) {}
}
} if (!http_request) {
alert("不能创建XMLHttpRequest对象实例！");
return false;
}
http_request.onreadystatechange = getResult; //调用返回结果处理函数
http_request.open('GET', url, true); //创建与服务器的连接
http_request.send(null); //向服务器发送请求}function getResult() {
if (http_request.readyState == 4) { // 判断请求状态
if (http_request.status == 200) { // 请求成功，开始处理返回结果
document.getElementById("toolTip").innerHTML=http_request.responseText; //设置提示内容
document.getElementById("toolTip").style.display="block"; //显示提示框
} else { // 请求页面有错误
alert("您所请求的页面有错误！");
}
}
}function checkUser(userName){
if(userName.value==""){
alert("请输入用户名！");userName.focus();return;
}else{
createRequest('checkUser.jsp?user='+userName.value);
}
}</script>此例并没有连接数据库，只是用数组简单表示注册过的用户。
<%@ page language="java" import="java.util.*" pageEncoding="GB18030" %>
<%
String[] userList={"明日科技","mr","mrsoft","wgh"}; //创建一个一维数组
String user=new String(request.getParameter("user").getBytes("ISO-8859-1"),"GB18030"); //获取用户名
Arrays.sort(userList); //对数组排序
int result=Arrays.binarySearch(userList,user); //搜索数组
if(result>-1){
out.println("很抱歉，该用户名已经被注册！"); //输出检测结果
}else{
out.println("恭喜您，该用户名没有被注册！"); //输出检测结果
}
%>