Writing a web crawler in the java programming language

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.

Writing a web crawler in the java programming language

More or less in this case means that you have to be able to make minor adjustments to the Java source code yourself and compile it.

A developer's thoughts on estimating software development

This web page discusses the Java classes that I originally wrote to implement a multithreaded webcrawler in Java. To understand this text, it is therefore necessary to download the Java source code for the multithreaded webcrawler This code is in the public domain.

You can do with the code whatever you like, and there is no warranty of any kind. However, I appreciate feedback. So feel free to send me an email if you have suggestions or questions, or if you have made modifications or extensions to the source code.

True, there are a lot of programs out there. A free and powerful crawler available is e. There are also tutorials on how to write a webcrawler in Java, even directly from Sun.

Although wget is powerful, for my purposes originally: The task would probably have been feasible with wget, but it was just easier to write my own stuff with Java. Besides, the whole multithreading stuff I wrote originally for the webcrawler could be reused in another context.

Sun's tutorial webcrawler on the other hand lacks some important features. For instance, it is not really multithreaded although the actual crawling is spawned off in a separate thread.

And for learning purposes, I think Sun's example distracts the user a bit from the actual task because they use an applet instead of an application. Especially if one is crawling sites from multiple servers, the total crawling time can be significantly reduced, if many downloads are done in parallel.

The skeleton of a crawler

Processing items in a queue Webcrawling can be regarded as processing items in a queue. When the crawler visits a web page, it extracts links to other web pages. So the crawler puts these URLs at the end of a queue, and continues crawling to a URL that it removes from the front of the queue.

It is obvious, that every algorithm that just works by processing items that are independent of each other can easily be parallelized.

writing a web crawler in the java programming language

Therefore, it is desirable to write a few classes that handle multithreading that can be reused. In fact, the classes that I wrote for web crawling originally were reused exactly as they are for a machine learning program.

Java provides easy-to-use classes for both multithreading and handling of lists. A queue can be regarded as a special form of a linked list.

For multithreaded webcrawling, we just need to enhance the functionality of Javas classes a little. In the webcrawling setting, it is desirable that one and the same webpage is not crawled multiple times. We therefore do not only use a queue, but also a set that contains all URLs that have so far been gathered.

Only if a new URL is not in this set, it is added to the queue. Implementation of the queue We may also want to limit either the number of webpages we visit or the link depth.

The methods in the queue interface resemble the desired functionality. Note that if we limit the depth level we need more than one queue.

If we only had one queue, the crawler could not easily determine the link depth of the URL it is just visiting. But regardless of the link depth we allow, two queues are sufficient.

When all URLs in queue 1 are processed, we switch the queues. Because of the generality of the problem, we can allow general Java Objects to be stored in the actual implementation of the queue. Implementation of a thread controller As mentioned above, Java has a good interface for handling threads.JavaCC facilitates designing and implementing your own programming language in Java.

You can build handy little languages for problems at hand or build complex compilers for languages such as Java or C++. Or you can write tools that parse Java source code and perform automatic analysis or transformation tasks.

Oct 30,  · Re: Web Crawler in the Java gimbal2 Mar 7, AM (in response to ) This demo was written using JDK A Web Crawler is a program that navigates the Web and finds new or updated pages for indexing.

The Crawler starts with seed websites or a wide range of popular URLs (also known as the frontier) and searches in depth and width for hyperlinks to extract.

A Web Crawler must be kind and robust. Kindness. Inside the timberdesignmag.com class we instantiate a spiderLeg object which does all the work of crawling the site.

But where do we instantiate a spider object? We can write a simple test class (timberdesignmag.com) and method to do this. package timberdesignmag.comr; public class SpiderTest { /** * This is our test. It's a web-oriented language, meaning lots of library functions to do all the odd little things you'll need to do in a project like this.

It has a good lib for this built in (CURL), and it's a dead simple language. Learn how to write automated tests for a Web API using the popular Java Apache HttpClient library to achieve faster and more reliable delivery of Quality Assurance within the SDLC.

In today's world, software development companies are pressured to deliver a product faster than ever before.

How to write a multi-threaded webcrawler in Java