Unable to handle javascript prompt pop ups

Sir,

have a url, as soon as i navigate to that url, "authentication required" javascript prompt pop ups..This dialog pop up has textboxes
<username> ,<password>, buttons ok & cancel...

i need to switch to this alert & enter username, pwd & click ok....

[b:2e50mjss]Issue:[/b:2e50mjss][b:2e50mjss]:Switch to alert never takes place....[/b:2e50mjss]

[b:2e50mjss]Reference: code as below:[/b:2e50mjss]

package Ruff_structured_SCripts_CP;

import java.util.concurrent.TimeUnit;

import org.apache.http.auth.UsernamePasswordCredentials;
import org.openqa.selenium.Alert;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.security.Credentials;

public class Doc_upload {

public static void main(String[] args) {
// TODO Auto-generated method stub
String userName="user1";
WebDriver w=new FirefoxDriver();
w.get("sonso");
w.manage().timeouts().implicitlyWait(5, TimeUnit.SECONDS);
System.out.println("wait ended");
try{
Alert alert=w.switchTo().alert();
//w.switchTo().alert().authenticateUsing((Credentials) new UsernamePasswordCredentials(userName, "Password1090"));
//alert.accept();
alert.dismiss();
alert=w.switchTo().alert();
alert.dismiss();
//System.out.println(alert.getText());
System.out.println("alert handling ended");
}
catch(Exception e)
{
System.out.println("unble to switch to alert");
System.out.println(e.getMessage());
System.out.println(e.getStackTrace());
}


}

}

[b:2e50mjss]Issue:[/b:2e50mjss][b:2e50mjss]:Switch to alert never takes place....[/b:2e50mjss] [b:2e50mjss]& NO Exception occurs [/b:2e50mjss]
[b:2e50mjss]Request for your inputs sir[/b:2e50mjss]