Recurrency not working well in Web Crawler project

For excerise I want to make my own Web Crawler but I have a problem with recurrent invocation of my crawl method. It should start for every link in my links array and goes so on until I decide to abort whole program but it only goes for first element in that array so it simply goes back and forth without any progress. How can I fix this?

Crawler.java

package regularmikey.mikecrawler;

import java.io.IOException;

import org.jsoup.HttpStatusException;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

public class Crawler implements Runnable {

    private Elements links;
    private Document doc;
    private String start_url;


    public Crawler(){};
    public Crawler(String url){start_url = url;};


    public void crawl(String url)  {

        try {
            System.out.println(url);
            doc = Jsoup.connect(url).get();

            String title = doc.title();
            System.out.println("title : " + title);

            links = doc.select("a[href]");
            for (Element link : links) {
                if(AdressValidator.validAddress(link.attr("href"))) {
                crawl(link.attr("href"));
                }
            }
        } catch (org.jsoup.UnsupportedMimeTypeException e) {

            e.printStackTrace();
        } catch (IOException e) {

            e.printStackTrace();
        }

    }


    public void run() {

            crawl(start_url);

    }
}

App.java

package regularmikey.mikecrawler;

public class App 
{
    public static void main( String[] args )
    {
       Thread thread = new Thread(new Crawler("http://facebook.com"));
       thread.run();

    }
}