Since you are using PreparedStatement and not the simple Statement then change this:

pstat = conn.prepareStatement("select username,password from user where username='"+ username + "' and password = '"+password+ "'"); to this:

pstat = conn.prepareStatement(
"select username,password from user where username=? and password = ?"
);

After you get the resultSet and call its methods(rs.next()) and before you break, you must close everything:
rs.close();
pstat.close();
conn.close(); Note: if you want to run a query again, you must reopen the connection, so it would be better if you put the code that opens it in some method so you can call it whenever you want to open a connection

Next time besides the code, post the errors you get and at which line