In-memory crawler

Crawler.java

import org.jsoup.Jsoup;
import org.jsoup.nodes.*;
import org.jsoup.select.Elements;

import java.io.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Crawler
{
    public int urlID;
    public ConcurrentHashMap<String, Set<Integer>> ii;
    public ConcurrentHashMap<Integer, String> urls;
    public ConcurrentHashMap<String, Integer> urlToIndex;
    public Set<Integer> crawled;
    public ConcurrentLinkedQueue<Integer> queue;

    public Crawler() {
        urlID = 0;
        ii = new ConcurrentHashMap<>();
        urls = new ConcurrentHashMap<>();
        urlToIndex = new ConcurrentHashMap<>();
        queue = new ConcurrentLinkedQueue<>();
        crawled = Collections.newSetFromMap(new ConcurrentHashMap<>());
    }

    public boolean urlInDB(String url)  {
        return crawled.contains(url);
    }

    public synchronized void addURL(String url) {
        if (crawled.contains(url) || !url.contains("purdue.edu") || !url.contains("http"))
            return;
        int currID = urlID;
        urlID += 1;
        urls.put(currID, url);
        urlToIndex.put(url, currID);
        queue.offer(currID);
    }

    public void crawl() {
        while (!queue.isEmpty()) {
            int currID = queue.poll();
            String url = urls.get(currID);
            if (urlInDB(url))
                continue;
            try {
                crawl(url);
                crawled.add(urlID);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public void crawl(String urlScanned) throws IOException {
        System.out.println(Thread.currentThread() + " " + crawled.size() + " - Crawling " + urlScanned);
        Document doc = Jsoup.connect(urlScanned).get();
        Elements links = doc.select("a[href]");
        links.forEach((Element a) -> addURL(a.attr("abs:href")));

        String text = doc.body().text();
        Pattern p = Pattern.compile("[A-Za-z]*");
        for (String str : text.split(" ")) {
            Matcher m = p.matcher(str);
            if (m.matches()) {
                String word = str.toLowerCase().replace("\"", "\'");
                Set<Integer> set = ii.getOrDefault(word, new HashSet<>());
                set.add(urlToIndex.get(urlScanned));
                ii.put(word, set);
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Crawler crawler = new Crawler();
        String root = "http://cs.purdue.edu";
        try {
            crawler.crawl(root);
        } catch (Exception e) {}

        Thread.sleep(5000);
        for (int i = 0; i < 8; i++) {
            Thread thread = new Thread(() -> crawler.crawl());
            thread.start();
        }
    }
}