Question

I wanted to make a Java based web crawler for an experiment. I heard that making a Web Crawler in Java was the way to go if this is your first time. However, I have two important questions.

1. How will my program ‘visit’ or ‘connect’ to web pages? Please give a brief explanation. (I understand the basics of the layers of abstraction from the hardware up to the software, here I am interested in the Java abstractions)

2. What libraries should I use? I would assume I need a library for connecting to web pages, a library for HTTP/HTTPS protocol, and a library for HTML parsing.

Answer 1

This is How your program ‘visit’ or ‘connect’ to web pages.

    URL url;
    InputStream is = null;
    DataInputStream dis;
    String line;

    try {
        url = new URL("http://stackoverflow.com/");
        is = url.openStream();  // throws an IOException
        dis = new DataInputStream(new BufferedInputStream(is));

        while ((line = dis.readLine()) != null) {
            System.out.println(line);
        }
    } catch (MalformedURLException mue) {
         mue.printStackTrace();
    } catch (IOException ioe) {
         ioe.printStackTrace();
    } finally {
        try {
            is.close();
        } catch (IOException ioe) {
            // nothing to see here
        }
    }

This will download source of html page.

For HTML parsing see this

Also take a look at jSpider and jsoup