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The core classes from my webcrawler implementation.

Again, not really working code as there are a bunch of dependencies missing. This is really for demonstration purposes.

public class WebCrawler implements Iterable
    private final HashSet visitedPages = new HashSet();
    private final LinkedList workQueue = new LinkedList();
    private final PageProcessor processor;
    // Map of URLs to pages.
    private final Map pages = new HashMap();     
    private final Predicate unvisited = new Predicate() { 
        public boolean satisfies(Object page){
            return !WebCrawler.this.visitedPages.contains(page);}};    

    public WebCrawler(PageProcessor processor, String... urls){
        this.processor = processor;
        for (String url : urls){
    /* Iterator which iterates over all WebPages that haven't yet been visited.
     * It is thoroughly lazy and a web page will never be visited until it turns
     * up in this iterator.*/
    public final Iterator pageIterator = (Iterator)
        new FlatteningIterator(
            new ListeningIterator(
                    new FilterIterator(
                        new PoppingIterator(this.workQueue))){
                @Override public void onNext(Object next){
                    if (next instanceof WebPage){
                            new FilterIterator(unvisited, WebCrawler.this.processor.linkedPages((WebPage)next)));}}});
    public Iterator iterator(){ return IteratorUtils.link(this.visitedPages.iterator(), this.pageIterator); }                        

 * Abstract class representing a mechanism for processing urls into pages. Contains 
 * utility methods and a cacheing strategy.
 * @author david
public abstract class PageProcessor implements Transformer
    private final PageCache cache;
    private final IteratorTransformer iteratorTransformer = new IteratorTransformer(this);
    private Predicate domain;
    public PageProcessor(Predicate domain, PageCache cache){
        this.domain = domain;
        this.cache = cache;}
    public PageProcessor(String domainPrefix, PageCache cache){
        this(StringUtils.startsWith(domainPrefix), cache);}
     * Take the Url and return a WebPage corresponding to it.
    protected abstract T process(String url);
    public T transform(String url){ return this.page(url); }
     * If the page has previously been processed, retrieve it from the internal cache.
     * Else process it and put it in the eternal cache.
    public T page(String url){
        T page = cache.getCachedPage(url);
        if (page == null){
                page = this.process(url);
        return page;}
     * Returns an iterator over all pages linked to by this page.
    public Iterator linkedPages(WebPage page){
        return iteratorTransformer.transform(new FilterIterator(domain, page.getLinkUrls()));}

 * A very simple PageProcessor implementation based on the HTMLParser library
 * which uses a MapBackedPageCache.
 * @author david
public class HtmlParserPageProcessor extends PageProcessor
    private static NodeFilter ALLOWED_TAGS = new NodeFilter(){
        public boolean accept(Node node){ 
            return (node instanceof LinkTag) || (node instanceof TitleTag);}};
    public HtmlParserPageProcessor(Predicate domain){
        super(domain,  new MapBackedPageCache());}
    public HtmlParserPageProcessor(String domain){
        super(domain,  new MapBackedPageCache());}
     * Fetches the resource represented by the URL, parses the HTML and extracts
     * the title element and all the links and uses them to build a WebPage object.
    public WebPage process(String url){
            Parser parser = new Parser(url);
            NodeIterator iterator = parser.parse(ALLOWED_TAGS).elements();
            String title = "";
            List links = new ArrayList();
            while (iterator.hasMoreNodes()){
                Node node = iterator.nextNode();
                if (node instanceof TitleTag) title = ((TitleTag)node).getTitle();
                else if (node instanceof LinkTag) links.add(((LinkTag)node).extractLink());}
            return new WebPage(url, title, links);}               
        catch (Exception e){ throw new RuntimeException(e); }}


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