How to validate username from MySql with JSP - mysql

Question

hello guys i am try to validate username from the database with the username that the user entered in the html from, assume
un//be the variable where username entered now from html form is stored
now how to retrieve all the columns of the uname from user table
uname //column name in mysql for usernames
user //table name in mysql
and check weather the username i.e,un entered now is present or not in the database
i am using
Connection con = DriverManager.getConnection("jdbc:mysql://localhost/mebps","root","admin");
Statement stmt = (Statement) con.createStatement();
ResultSet rs = stmt.executeQuery("select un from userinfo");
while(rs.next())
{
if(rs.getString("uname") == un)
{
out.println("user is present");
}
}

Answer 1

There are at least two major mistakes:
You're comparing string instances by == instead of comparing their values by equals() method. The proper line would be if (rs.getString("uname").equals(un)).
You're not letting the DB do the job of returning the right row, instead you're copying the entire DB table into Java's memory and doing the comparison in Java. This is very inefficient. Make use of SQL powers the smart way so that it always returns exactly the information you need. There's for example a WHERE clause.
On an unrelated note, you seem not to be closing DB resources properly after use. This will result in resource leaking which is also a bad idea as it may cause your application to crash on long term. Further, the column name uname and un are not the same. But I'll assume it to be careless oversimplifying of the example.
Here's a minor rewrite:
public boolean exist(String username) throws SQLException {
Connection connection = null;
PreparedStatement statement = null;
ResultSet resultSet = null;
boolean exist = false;
try {
connection = database.getConnection();
statement = connection.prepareStatement("SELECT uname FROM userinfo WHERE uname=?");
statement.setString(1, username);
resultSet = statement.executeQuery();
exist = resultSet.next();
} finally {
if (resultSet != null) try { resultSet.close(); } catch (SQLException ignore) {}
if (statement != null) try { statement.close(); } catch (SQLException ignore) {}
if (connection != null) try { connection.close(); } catch (SQLException ignore) {}
}
return exist;
}
You see, if there's a match, then it returns true (at least one record), otherwise false (no one record). No need to copy the entire table into Java's memory and crawl through it in Java.
Last but not least, this code doesn't belong in a JSP file, but in a normal Java class, starting with a servlet. See also our servlets wiki page to learn more about it.