I list these, because they are actually the most used ones in most of the tutorials available.
It turns out I was able to do it in about lines of code spread over two classes. How does it work? You give it a URL to a web page and word to search for.
The spider will go to that web page and collect all of the words on the page as well as all of the URLs on the page. There are only two classes, so even a text editor and a command line will work.
But what if Page B contains a bunch more links to other pages, and one of those pages links back to Page A? Remember that a set, by definition, contains unique entries.
In other words, no duplicates. All the pages we visit will be unique or at least their URL will be unique. We can enforce this idea by choosing the right data structure, in this case a set.
Why is pagesToVisit a List? This is just storing a bunch of URLs we have to visit next. When the crawler visits a page it collects all the URLs on that page and we just append them to this list.
Recall that Lists have special methods that Sets ordinarily do not, such as adding an entry to the end of a list or adding an entry to the beginning of a list.
Every time our crawler visits a webpage, we want to collect all the URLs on that page and add them to the end of our big list of pages to visit.
Assuming we have values in these two data structures, can you think of a way to determine the next site to visit? Okay, so we can determine the next URL to visit, but then what? We still have to do all the work of HTTP requests, parsing the document, and collecting words and links.
This is an idea of separating out functionality.
What are our inputs? A word to look for and a starting URL. We use all of our three fields in the Spider class as well as our private method to get the next URL. We assume the other class, SpiderLeg, is going to do the work of making HTTP requests and handling responses, as well as parsing the document.
This separation of concerns is a big deal for many reasons, but the gist of it is that it makes code more readable, maintainable, testable, and flexible. Earlier we decided on three public methods that the SpiderLeg class was going to perform.
But because this is all neatly bundled up in this package for us, we just have to write a few lines of code ourselves. But how do we start using jsoup?
You import the jsoup jar into your project! Nothing too fancy going on here. Great, and if we remember the other thing we wanted this second class SpiderLeg. This turns out to be surprisingly easy: Okay, so this second class SpiderLeg. Remember that we store the links in a private field in the first method?
This method should only be used after a successful crawl. This is because some web servers get confused when robots visit their page. Ready to try out the crawler? Remember that we wrote the Spider. But where do we instantiate a spider object? We can write a simple test class SpiderTest. It creates a spider which creates spider legs and crawls the web.Things have been a bit slow around here recently, so I figured to keep things alive I may as well start a series of posts.
As most of my freelancing work recently has been building web scraping scripts and/or scraping data from particularly tricky sites for clients, it would appear that scraping data from websites is extremely popular at the moment.
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