register.jsp:
注意：1、contextName就是你的项目名称。2、form中method设置为get.
<%@ page language="java" import="java.util.*" pageEncoding="GBK"%>
<html>
<head>
<script>
function UserExist(){
var name= document.form1.username.value
document.form1.action="/contextName/userName.check?username="+name
document.form1.submit()
}
</script>
</head>
<body>
<form name="form1" action="SaveServlet" method="get" onsubmit="return check()">
<table>
<tr>
<td>用户名：</td><td><input type="text" name="username" >
<input type="button" name="bn1" value="检测用户名是否存在" onclick="UserExist()">
</td></tr>
<tr><td colspan="2"><input type="submit" name="subm" value="确定"></td></tr>
</table>
</form>
</body>
</html>
web.xml添加下面定义
<servlet>
<servlet-name>CheckUserNameServlet</servlet-name>
<servlet-class>com.company.project.servlet.CheckUserNameServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>CheckUserNameServlet</servlet-name>
<url-pattern>*.check</url-pattern>
</servlet-mapping>
CheckUserNameServlet中doGet()方法最后一句话可以修改成这样：
out.println("<a href='javascript:history.go(-1)'>返回</a>")
$.ajax({type: "POST",
url: "some.php",
data: "name=John&location=Boston",
success: function(msg){
alert( "Data Saved: " + msg )
}
})
其中 success: function(msg) 的 msg 就是servlet 传回的值
