Crawl Using Java

For Crawling in Java, you'll require Jsoup lirary. It provides a very convenient API for extracting and manipulating data, using the best of DOM, CSS, and jquery-like methods.
A simple example of Jsoup:

Document doc = Jsoup.connect("http://en.wikipedia.org/").get();
Elements newsHeadlines = doc.select("#mp-itn b a");

Add it to your project from pom:

<dependency>
    <groupId>org.jsoup</groupId>
    <artifactId>jsoup</artifactId>
    <version>1.10.2</version>
</dependency>

public getmoreURL {
 Document doc = null;
 String currentURL;
 for (HashMap tmpMap: urlInfos) {
  currentURL = tmpMap.get("url");
  doc = Jsoup.connect(currentURL).get();
  Elements articleLead = doc.select("article.lead-story");
  if (currentURL.contains("xyz")) {
   for (Element elem: articleLead) {
    int size = elem.select("a").size();
    Element anchor = elem.select("a").get(1);
    String title = anchor.text();
    String URL = anchor.attr("href");
    if (title.length() == 0) title = elem.select("a").get(2).text();
    if (!dbUtils.find("url", URL)) urlQueue.add(URL);
    if (!alreadyInList(URL)) {
     HashMap info = new HashMap();
     info.put("url", URL);
     urlInfos.add(info);
     retrieveRelatedStory(URL);
    } else System.out.println("Title - " + title + " already in the list.");
   }
   Elements articleElement = doc.select("article.teaser");
   for (Element elem: articleElement) {
    int size = elem.select("a").size();
    Element anchor = elem.select("a").get(1);
    String title = anchor.text();
    String URL = anchor.attr("href");
    if (title.length() == 0) title = elem.select("a").get(2).text();
    if (!dbUtils.find("url", URL)) {
     urlQueue.add(URL);
     crawURLsFromHomePage(title, URL);
     retrieveRelatedStory(URL);
    } else System.out.println("Title - " + title + " already in the list.");
   }
  } else {
   if (!alreadyInList(currentURL)) {
    HashMap tempRelated = new HashMap();
    tempRelated.put("url", currentURL);
    urlInfos.add(tempRelated);
    retrieveRelatedStory(currentURL);
   }
   retrieveRelatedStory(currentURL);
  }
 }
}

Refer following URL :
https://ksah.in/introduction-to-web-scraping-with-java/
http://jaunt-api.com/