# Web technologies/2014-2015/Laboratory 5
## Parsing XML documents

An important issue when dealing with XML is parsing the documents. There are several parser types including:
* DOM parsers:
** allow the navigation of the XML document as it were a tree.
** the main drawback is that the document needs to be completely loaded into memory before actually parsing it.
** DOM documents can be either created by parsing an XML file, or by users which want to create an XML file programmatic.
* SAX parsers:
** event-driven API in which the XML document is read sequentially by using callbacks that are triggered when different element types are meet.
** overcomes the DOM’s memory problem, and is fast and efficient at reading files sequentially.
** its problem comes from the fact that it is quite difficult to read random information from inside an XML file.
* FlexML parsers:
** follow the SAX approach and rely on events during the parsing process.
** it does not constitute a parsing library by itself, but instead it converts the DTD file into a parser specification usable with the classical Flex parser generator.
* Pull parsers:
** use an iterator design pattern in order to sequentially read various XML items such as elements, attributes or data.
** this method allows the programmer to write recursive-descent parsers:
*** applications in which the structure of the code that handles the parsing looks like the XML they process.
*** examples of parsers from this category include: StAX13, and the .NET System.Xml.XmlReader.
* Non-extractive parsers:
** a new technology in which the object oriented modeling of the XML is replaced with 64-bit Virtual Token Descriptors.
** one of the most expressive parser belonging to this category is VTD-XML.
Note: We will use the Apache Xerxes library for these exercises. You can find more information here.

### SAX

SAX (Simple API for XML) is a serial access XML parser. A SAX parser can be found in the Xerces library found here.
The following fragment of code shows how we could use SAX to parse an XML document:

#### Using Python3

Links:
SAX Tutorial
Python3 SAX documentation

### DOM

DOM (Document Object Model) is a convention for representing XML documents. A DOM parser can be found in the Xerces library found here.
DOM handles XML files as being made of the following types of nodes:
* Document node
* Element nodes
* Attribute nodes
* Leaf nodes:
** Text nodes
** Comment nodes
** Processing instruction nodes
** CDATA nodes
** Entity reference nodes
** Document type nodes
* Non-tree nodes;

#### Using Xerces and Java:

The following fragment of code shows how we could use DOM to traverse an XML tree:
DOM also allows users to create a new XML document or change the structure of an already existing one:

#### Using Python3:

To programmatically create an XML document using the xml.dom module:
Links:
* DOM tutorial
* W3C DOM tutorial
* Java XML Parsing Tutorials
* Python3 DOM documentation

## Exercises

* Parse the XML created in your assignment from Laboratory 3 using both SAX and DOM. Print out the parsing time of each method (hint: use System.currentTimeMillis to get the start and end time).
* Create the XML from your assignment in Laboratory 3 using DOM. Print the result to an XML file.
** In order to save resulted xml to file you can use:
Or you can use the writexml method from the minimal DOM implementation, minidom if you are using Python.
Gabriel Iuhasz, 2019-10-30, iuhasz.gabriel@e-uvt.ro