还有一个不知道哪里错的错误！
下面一段是我检查用户名是否可用的代码！
<%@ page contentType= "text/html;charset=GB2312 " %>
<meta http-equiv= "Content-Type " content= "text/html; charset=gb2312 ">
<%@ page import= "java.sql.* " %>
<%!
public String codeToString(String str)
{//处理中文字符串的函数
String s=str;
try
{
byte tempB[]=s.getBytes( "ISO-8859-1 ");
s=new String(tempB);
return s;
}
catch(Exception e)
{
return s;
}
}
%>
<%//接收客户端提交的数据
String username=codeToString(request.getParameter( "username "));
if(username==null)
{
%>
<SCRIPT LANGUAGE= "JavaScript ">
<!--
window.alert( "请输入用户名！ ");
window.close();
//-->
</SCRIPT>
<%
}
String sqlStrings= "select * from userstable where user_name= ' "+username+ " ' " ;
try
{
Connection con;
Statement sql;
ResultSet rs;
Class.forName( "com.mysql.jdbc.Driver ");
con=DriverManager.getConnection ( "jdbc:mysql://localhost:3306/JOB ", "root ", "1 ");
sql=con.createStatement();
rs=sql.executeQuery(sqlStrings);
if(rs.first())
{
%>
<SCRIPT LANGUAGE= "JavaScript ">
<!--
alert( "该用户名已被其他用户注册！ ");
window.close();
//-->
</SCRIPT>
<%
}
else
{
String sqlStringw= "select * from userwtable where user_name= ' "+username+ " ' " ;
try
{
Connection con2;
Statement sql2;
ResultSet rw;
Class.forName( "com.mysql.jdbc.Driver ");
con2=DriverManager.getConnection( "jdbc:mysql://localhost:3306/JOB ", "root ", "1 ");
sql2=con.createStatement();
rw=sql2.executeQuery(sqlStringw);
if(rw.first())
{
%>
<SCRIPT LANGUAGE= "JavaScript ">
<!--
alert( "该用户名已被其他用户注册！ ");
window.close();
//-->
</SCRIPT>
<%
}
else
{
%>
<SCRIPT LANGUAGE= "JavaScript ">
<!--
alert( "你可以使用该用户名！ ");
window.close();
//-->
</SCRIPT>
<%
}
con2.close();
}
catch(SQLException e2)
{
out.print( "SQL异常！ ");
}
}
con.close();
}
catch(SQLException e1)
{
out.print( "SQL异常！ ");
}
%>
<head>
<title> 用户名验证 </title>
</head>
<body>
</body>
</html>
但是当用户名为空时，程序并没有执行
%>
<SCRIPT LANGUAGE= "JavaScript ">
<!--
window.alert( "请输入用户名！ ");
window.close();
//-->
</SCRIPT>
<%这段语句。这是怎么回事啊？
[解决办法]
建使用如下句:
out.println( " <script language= 'javascript '> ....... </script> ")