postgres user authentication

Hi experts,

I am trying to authenticate a user with postgres DB. I have created a postgres user and when I use the method below, it seems to work fine for a database installation on windows - i.e it returns a null Connection object when the authentication fails, however for a postgres installation on unix, it returns a non null Connection object even when the authentication fails. I am relying on this method to authenticate a user based on if the connection object is null or not.
I am using the jdbc driver from  postgresql.jdbc.jar with Java 5.0

Am I missing some thing trivial here. I cant understand why its behaving differently in the above cases. Any suggestions regarding this or any alternate ways of authenticating a postgres user from java code like a third party library would be appreciated.

public static Connection getConnection(String JNDIName, String username, String password) throws FrameworkException {

    Connection conn = null;
    try {
      DataSource ds = ServiceLocator.getInstance().getDatasource(JNDIName, true);

      if (username == null || (username != null && "".equals(username))) {
        //Retrieve the connection with the DS configured user/passowrd
        conn = ds.getConnection();
      }
      else {
        conn = ds.getConnection(username, password);
      }
    }
    catch (SQLException se) {
      throw new FrameworkException("Error retrieving connection for: " + JNDIName, se);
    }
    catch (ServiceLocatorException sle) {
      throw new FrameworkException("Could not retrieve a datasource for " + JNDIName, sle);
    }
    return conn;
  }