String sql2="select count(*) from table where column=?";
PreparedStatement pst2=conn.prepareStatement(sql2);
pst2.setString(1,str);
if(次数!=0){
session.setAttribute("errmsg2", "用户名已存在,请重新输入");
}else{
}
if(次数!=0)这里的次数应该怎么定义的