World Wide Web

HTML parsing and Rhino

About a year back I was working on a personal project in IBM. This was a clone of YubNub for the IBM intranet.

For those of you who don’t know YubNub, it is a simple but powerful tool, which allows you to define keywords to reach pages. One of the popular examples is gim which will take you to the Google Image Search results page for the keywords that you entered.

When I built this YubNub clone, I had plans to introduce the feature of defining commands to get data from specific portions of a page. For example, you would be able to fetch the telephone number of a person using a command like: telephone . The way this works is by scraping the content off a page containing the telephone number at a specific section in the person’s profile page.

But wouldn’t it be cool to provide the flexibility to the user to define what to fetch from a page on the Intranet? You can ask the user to define what content to fetch from a page when he creates the command.

Look at the YubNub create command interface. The basic information asked in the page is:

  • Name of the command
  • URL
  • Description

Now imagine having an extra text-field which asks you to enter the XPath to the content that you want to scrape from the resultant page.

In simple words, this means, you are saying, fetch this page, then get this specific portion of the page and only give me that content. You could perhaps pipe that content to some other command or play with that content in umpteen ways. I haven’t followed YubNub of-late, but I am sure there are many commands in YubNub which have similar functionality.

Now in principle, although this is possible there was one major issue I faced. The server had to do the page fetch and then page scraping. Now although there are very good XML parsers out there, there is no good ‘XML’ parser for HTML. And XPath does not work unless the page is XML.

Most pages on the Internet are HTML (or XHTML) and although it is straight-forward to transform them to XML, anyone who has tried it will see that this is not a simple solution. When you try to parse an XHTML page (even popular pages out there) you will run into issues like ‘entity not defined’ or ‘matching element not found’ etc. Although there are tools like Tidy or TagSoup, you are not guaranteed that the output of such tools is a well-formed XML.

On the other hands, browsers are extremely flexible in the way they handle HTML. Traversing through the HTML DOM is really simple and many a times you don’t even realize that your browser has silently corrected 10’s of errors in the page. You can get to any specific portion of the page using HTML DOM functions or using libraries like JQuery.

So what I was looking for, was some tool which had the flexibility of the browser’s HTML handling, but at the same time was able to function on the server.

As if by co-incidence, I ran into this post from John Resig (the person popular for JQuery). John describes one of his projects on bringing the browser environment to Rhino. He also gives an example of how to scrape content from a web-page and send the result to a file.

Wow! This is exactly what I had been looking for. Since Rhino can be embedded in Java, all you would need to do is to make a call to the JS function to scrape content and then pass the content back to Java and continue with your processing.

Although I don’t work on the project anymore, I see requirement of this functionality in many other places. For example, just sometime back, I was looking for a simple tool to fetch Tiddlers from Tiddlywiki and convert them into a simple HTML page. This will help in supporting those browsers which don’t have Javascript enabled. I tried some of the tools out there, but most of them failed. So I planned to write my own. And lo, I came across this same issue. TiddlyWiki content is in HTML and this content is not easy to parse using XML parsers (which is perhaps why many of those tools failed). So how about using Rhino and John’s project to scrape content from the wiki and sending it to a file in a different format?

The project looks very promising. I should follow it closely.