enira.net

We are all living in a technological wasteland.

RSS
people
Aug 14 2013

Webscraping – Part 2 – Basic XPath HTML queries in JAVA

By | Filled under: Java, Webscraping

Introduction

So, this tutorial continues to build on the first part. If you haven’t read this, I recommend to do so. As it contains vital information that I won’t repeat in this post.

Note: please download and open the eclipse project. The run methods can be found in the Scraping02 class.

Additional queries

Sometimes it is possible that the HTML structure is so fucked up that you need to take matters in your own hand. In run1 I’ll show you how to loop trough all elements of the images table one by one.

So ‘node.getFirstChild()’ is the <td> tag, the ‘node.getFirstChild().getNextSibling()’ is the first <img> tag. To loop trough you will need to get ‘next.getNextSibling().getNextSibling()’ as the first nextSibling is the current element.

Example:

private void run1(Document document) {
	System.out.println("run1:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		Node node = (Node) xpath.evaluate("//*[@class='images']//td",
				document, XPathConstants.NODE);
 
		Node next = node.getFirstChild().getNextSibling();
		do {
			String image = next.getAttributes().getNamedItem("src")
					.toString();
 
			// result is: src="image1.jpg", so split
			System.out.println(image.split("\"")[1]);
 
			next = next.getNextSibling().getNextSibling();
		} while (next != null);
 
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run1:
image1.jpg
image2.jpg
image3.jpg
image4.jpg
image5.jpg

This hasn’t been covered in the past but it is quite useful for web scraping. XPath queries can select the attribute values too. This can be done using the ‘@’ character. This comes quite handy when selecting links from <a href=””> tags. In run2 the query selects the links (href attribute) from all <a>tags in the tag defined by a class named ‘links’.

private void run2(Document document) {
	System.out.println("run2:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate(
				"//*[@class='links']//a/@href", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run2:
http://enira.net/links/link1.htm
http://enira.net/links/link2.htm
http://enira.net/links/link3.htm
http://enira.net/links/link4.htm

In run3 I’ve combined run2 together with run1. Here you can see that HtmlCleaner corrects thags that aren’t correct. (In this case the <img> tag). The XPath query reads: select all elements with a class ‘images, from this class select the ‘img’ element with a td parent at any level/depth and get the src attribute.

private void run3(Document document) {
	System.out.println("run3:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate(
				"//*[@class='images']//td/img/@src", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run3:
image1.jpg
image2.jpg
image3.jpg
image4.jpg
image5.jpg

So that’s about it, now you should be able to scrape the web quite easily. I think I’ve covered enough for you to get started scraping pages using XPath.

Downloads:

Example project (Eclipse): Scraping02.zip (132 KB)

No Comments |
Aug 14 2013

Webscraping – Part 1 – Basic XPATH HTML queries in JAVA

By | Filled under: Java, Webscraping

Introduction

So occasionally I want to scrape a web page of links or images. Being a programmer I hate such a tedious tags and I always end up writing a script. In the past I always used a combination of substring(), indexOf or some other string formatting functions. However websites are also/still XML files! This enables a much easier method to search content: XPATH.

This little tutorial will handle XPATH queries on a plain HTML page in Java. I am using an adapted tutorial that can be found at: http://manual.calibre-ebook.com/xpath.html.

Setup

The Java library that I will use in this tutorial is HtmlCleaner. HtmlCleaner is open-source HTML parser written in Java and it cleans up any ill written HTML code. You can download it at: http://htmlcleaner.sourceforge.net/download.php.

 
Right click and go to ‘Properties’. Go to the tab ‘Libraries’ and press ‘Add JARs’.

I also provided a test file. In the project you can find it in the ‘src/resources’ folder. The function readDocument() will read this file and create a usable Document object.

private Document readDocument() {
	String content = null;
	try {
		content = FileUtils
				.readLargeTextFileUTF8("src/resources/index.html");
	} catch (IOException e) {
 
	}
 
	TagNode tagNode = new HtmlCleaner().clean(content);
	Document doc = null;
	try {
		doc = new DomSerializer(new CleanerProperties()).createDOM(tagNode);
		return doc;
	} catch (ParserConfigurationException e) {
		e.printStackTrace();
	} catch (Exception e) {
		e.printStackTrace();
	}
 
	return null;
}

This function will be used by all the run examples.

Selecting by tagname

So this first example will select all h2 tags from the top level. Please note: ‘//’ defines an element directly one level from the root. HtmlCleaner considers the body tag as the root of the document.

private void run1(Document document) {
	System.out.println("run1:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate("//h2", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run1:
Chapter One
Chapter Two

This next example will search all ‘div’ elements and show the p tags inside the div. Note: This example takes only the div tags on the top level!

private void run2(Document document) {
	System.out.println("run2:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate("//div/p", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run2:
A very short ebook to demonstrate the use of XPath.

This next example will get also the same result as the second run, however in this case the div element can be any nested div child, and doesn’t needs to be a sub child from the root node.

private void run3(Document document) {
	System.out.println("run3:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate("//*/div/p", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run3:
A very short ebook to demonstrate the use of XPath.

This next example will select based on name of the element. In this case XPATH will select all <h1> and <h2> tags.

private void run4(Document document) {
	System.out.println("run4:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate(
				"//*[name()='h1' or name()='h2']", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run4:
A very short ebook
Chapter One
Chapter Two

 

Selecting by attributes

In XPATH it is also possible to select based on attributes. Next example selects the content of all tags containing an attribute style=”.

private void run5(Document document) {
	System.out.println("run5:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate("//*[@style]", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run5:
Written by Kovid Goyal

 

Next example will select all chapter classes. This is done by adding a class selector like in run5, but specify it to match only ‘chapter’ attribute values.

private void run6(Document document) {
	System.out.println("run6:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate("//*[@class='chapter']",
				document, XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run6:
Chapter One
Chapter Two

Now let’s get a little bit advanced: next example will select all <h1> tags which have a class named ‘bookTitle’. The xpath query reads: From the top level select all h1 elements with an attribute class that matches the value ‘bookTitle’.

private void run7(Document document) {
	System.out.println("run7:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	String str;
	try {
		str = (String) xpath.evaluate("//h1[@class='bookTitle']", document,
				XPathConstants.STRING);
 
		System.out.println(str);
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run7:
A very short ebook

 

Selecting by tag content

XPATH is also able to select content based on text containing a certain value. In run8 will show you how to select all <h2> tags containing the text string ‘One’.  Note: XPath is case sensitive!

private void run8(Document document) {
	System.out.println("run8:");
 
	XPath xpath = XPathFactory.newInstance().newXPath();
	try {
		NodeList nodes = (NodeList) xpath.evaluate(
				"//h2[contains(., 'One')]", document,
				XPathConstants.NODESET);
 
		for (int i = 0; i < nodes.getLength(); i++) {
			System.out.println(nodes.item(i).getTextContent());
		}
	} catch (XPathExpressionException e) {
		e.printStackTrace();
	}
 
	System.out.println("");
}

Output:

run8:
Chapter One

Downloads:

Example project (Eclipse): Scraping01.zip (132 KB)

No Comments |