<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<!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 language=javascript>
function checkform(){
if (form1.userName.value==""){ //通过form的name获取form对象，再通过userName获取form下的对象。如果有多个name相同，则获取的是对象数组
alert("请输入数字");
document.getElementById("yanzi").focus();
return(false);
}
}
</script>
</head>
<body>
<form name="form1" method="POST" action="/test/Servlet" onsubmit="return checkform();"> //必须要有return，没有return会提交表单
<input type="text" name="userName" id="yanzi" size="10" onKeypress="if (event.keyCode < 45 || event.keyCode > 57) event.returnValue = false;"> //非字母可以输入，控制键盘的输入
<input type="submit" value="确定并开始" name="submit">
</form>
</body>
</html>