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Extensible Markup Language Parsing Techniques

Abstract XML is the language used to develop web applications. XML is a set of rules for designing structured data in a text format as opposed to binary format, which is useful for man, and machine both. A parser is used for syntactical and lexical analysis. XML parser extract the information from the XML document which is very much needed in all Web applications Simple object access protocol is a protocol that lets the program to send XML over HTTP to invoke methods on remote objects. An XML parser can serve as an engine for implementing this or a comparable protocol. XML parser can also be used to send data messages formatted as XML over HTTP. By adding XML and HTTP capabilities to application, software developers can begin to offer alternatives to traditional browsers that have significant value to their customers. This paper presents an XML parser that implements a subset of the XML specification. This is useful to all developers and users for checking the welformedness and validation of an XML documents.

1. Introduction The World Wide Web Consortium has created an SGML working group to build a set of specifications that are easy and straightforward to use. The subset called XML has the advantages of SGML that is extensibility, structure, and validation in a language and is very easy to learn, use and implement than full SGML. XML is fully internationalized for both European and Asian languages, with all conforming processors required to support the Unicode character set in both its UTF-8 and UTF-16 encoding. The language is designed for the quickest possible client-side processing consistent with its primary purpose as an electronic publishing and data interchange format. Most application needs to save some configuration data, and often need to transmit or receive data to or from other applications. This is especially true for software that interacts with the Internet. If you need a format for interchanging such data, one solution is to design your own binary format. Besides having some advantages of storing complex structures, list, arrays etc., it has got some drawbacks, such as binary format will not be easy to understand and modification will have compatibility problems. As an alternative we can use a text-based format, which is easy to use, but not powerful. XML provides a more general solution. It is text-based, hierarchical format that has an advantage of both binary and text based worlds. It is easy to use but is also powerful. Even it was primarily designed for the Web, it can be used for any application that needs to store data or communicate with other applications. This paper presents an XML Parser that implements a subset of the XML specification. The goal of an XML Parser is to extract information from the XML documents. It gets

an input an XML file and then it starts parsing it. There are two different ways of doing it. One way is to use an event-driven approach. The SAX parser is the model for

this approach. The second approach builds a tree to represent 11111010101100000011010001011100111100000011000010011100111101101001111000111010010100101101111010111110100100000111000001011100001101100011111011011010101100101001111001110110011100000001100010111100110101001001110010011000010100100111111000010110101111101111101010110000001101000101110011110000001100001001110011110110100111100011101001010010110111101011111010010000011100000101110000110110001111101101101010110010100111100111011001110000000110001011110011010100100111001001100001010010011111100001011010111110111110101011000000110100010111001111000000110000100111001111011010011110001110100101001011011110101111101001000001110000010111000011011000111110110110101011001010011110011101100111000000011000101111001101010010011100100110000101001001111110000101101011111011111010101100000011010001011100111100000011000010011100111101101001111000111010010100101101111010111110100100000111000001011100001101100011111011011010101100101001111001110110011100000001100010111100110101001001110010011000010100100111111000010110101111101111101010110000001101000101110011110000001100001001110011110110100111100011101001010010110111101011111010010000011100000101110000110110001111101101101010110010100111100111011001110000000110001011110011010100100111001001100001010010011111100001011010111110111110101011000000110100010111001111000000110000100111001111011010011110001110100101001011011110101111101001000001110000010111000011011000111110110110101011001010011110011101100111000000011000101111001101010010011100100110000101001001111110000101101011111011111010101100000011010001011100111100000011000010011100111101101001111000111010010100101101111010111110100100000111000001011100001101100011111011011010101100101001111001110110011100000001100010111100110101001001110010011000010100100111111000010110101111101111101010110000001101000101110011110000001100001001110011110110100111100011101001010010110111101011111010010000011100000101110000110110001111101101101010110010100111100111011001110000000110001011110011010100100111001001100001010010011111100001011010111110111110101011000000110100010111001111000000110000100111001111011010011110001110100101001011011110101111101001000001110000010111000011011000111110110110101011001010011110011101100111000000011000101111001101010010011100100110000101001001111110000101101011111011111010101100000011010001011100111100000011000010011100111101101001111000111010010100101101111010111110100100000111000001011100001101100011111011011010101100101001111001110110011100000001100010111100110101001001110010011000010100100111111000010110101111101111101010110000001101000101110011110000001100001001110011110110100111100011101001010010110111101011111010010000011100000101110000110110001111101101101010110010100111100111011001110000000110001011110011010100100111001001100001010010011111100001011010111110111110101011000000110100010111001111000000110000100111001111011010011110001110100101001011011110101111101001000001110000010111000011011000111110110110101011001010011110011101100111000000011000101111001101010010011100100110000101001001111110000101101011111011111010101100000011010001011100111100000011000010011100111101101001111000111010010100101101111010111110100100000111000001011100001001010000110101100001101001011010110100100100110010110010101100000111010011011011110110000001001001010100110100000111101010110100001110100111100011110010111101100001000010011010110111000101111011010001000100000111011011110000011110000011110010111100011100100101000011010110000110100101101011010010010011001011001010110000011101001101101111011000000100100101010011010000011110101011010000111010011110001111001011110110000100001001101011011100010111101101000100010000011101101111000001111000001111001011110001110010010100001101011000011010010110101101001001001100101100101011000001110100110110111101100000010010010101001101000001111010101101000011101001111000111100101111011000010000100110101101110001011110110100010001000001110110111100000111100000111100101111000111001001010000110101100001101001011010110100100100110010110010101100000111010011011011110110000001001001010100110100000111101010110100001110100111100011110010111101100001000010011010110111000101111011010001000100000111011011110000011110000011110010111100011100100101000011010110000110100101101011010010010011001011001010110000011101001101101111011000000100100101010011010000011110101011010000111010011110001111001011110110000100001001

the XML document, limiting the size of XML documents that can be parsed by this approach. The DOM parser is the model for this approach.

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2.Extensible Markup Language The eXtensible Markup Language came out of the world of the Standard Generalized Markup Language (SGML). Initially XML was developed to overcome the shortcomings of HTML, a markup language containing stylistic information. The aim of the XML’s developers was to create a language that was easy to use over Internet, supported by a wide variety of applications, compatible with SGML and legible to humans. XML separates content from style as its ancestor, SGML. A typical XML is hierarchical. It is made up of elements defined by tags. A document type definition (DTD), or XML schema, is used to define the structure of a document. An XML document is referred to as well formed if it conforms to the XML Standards, and correct (or valid) if it complies with a DTD or Schema. At the core of an XML application is an XML Parser. All XML parsers will check that the documents they receive are well formed, and most also check to see if these documents are valid.

2.1 Valid XML documents have following validating criteria’s (i) Meets validity constraints (ii) Validity constraints referred to as VC or VCs (iii) Parser checks and determines if validity constraints are fulfilled (iv) No parser errors (v) Has to contain a DTD or reference one (vi) Xml documents without a DTD must always be well formed

2.2 Welformed XML documents have following well-formness criteria’s (i) Contains one or more elements (+ means one or more) (ii) There is exactly one root element also called a document element (iii) Elements are properly nested (a) Children are nested inside their parent (b) Elements without child elements exist by themselves (iv) Every child element has one parent element (v) A child element is said to be in the content of the parent (vi) A child cannot be in the content of any other element that is in the content of the parent (vii) The parent can have 0, 1, or more children (* means zero or more) (viii) Well Formed Constraints referred to as WFC or WFCs (ix) Violation of a WFC is a Fatal Error and the parser is supposed to stop sending document data in the normal manner (i.e., flags an exception in java or triggers an error message handling routine...)

3. XML Parsing The XML Parser must be compliant with a complete set of W3C interfaces to ensure interoperability with applications and web-based technologies. The functionality of an XML parser includes: (i) Checking well formedness of a document

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(ii) Searching with faster algorithms (iii) Validation of an XML document

4. Types of Parsers There are two classes of XML Parsers: Validating parsers and non-validating parsers. All true XML parsers must report violations of the XML specification constraints for being well formed. Validating

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parsers must also report violations of the constraints expressed by the declarations in the DTD. Non-Validating parsers are required only to check that the DTD is well formed. They are not required to understand and use the DTD for document checking. There are some exceptions, however in particular for attributes. There are two different ways of XML parser to extract information from the XML document. One way is to use an event-driven approach. The parser begins reading the string and sends messages when certain events occur. For example, a message is sent when a start tag is encountered and another event when an end tag is reached. Programs that use these parsers have callback functions to process the events. When a message signals that a desired signal has been found, the program can examine the tag and its accompanying information in detail and act accordingly. The SAX (Simple API for XML) parser is the model for this approach. The second approach builds a tree to represent XML document. Each tag of the document represents a node in the tree. Once built, a program can traverse the tree to process the document or to search for specific tags. Usually these trees reside in memory, limiting the size of XML documents that can be parsed by this approach. In contrast, event-driven parsers do not create a tree and can parse documents of any size. Most XML parsers are either event driven or produce an in-memory DOM (Document Object Model) instance of the document. The one we use depends on the application and memory requirements. Producing a DOM tree requires more memory, but it can provide greater programmatic flexibility. SAX may be suitable for applications that need smaller memory footprints, and it can process the XML document as a stream of events.

5. Developing XML Parsers The following steps are followed for developing an XML parser. (i) Canonical XML XML parsers generally work with canonical XML. This is the XML we are left with after an XML document is preprocessed. I liken this to a C complier that first removes directives and macros, leaving only syntactically correct C code. With XML, preprocessing references to external files (external DTDs) and expansion of entity reference. What is left over is still an XML document, but one that uses a simpler syntax. Is canonical XML useful? Absolutely. Many real-world applications generate XML documents using this simpler syntax. For this reason, most of the XML parser parses documents that conform to canonical XML. (ii) Building Trees DOM (Document Object Model) approach works by building a tree with nodes that are Tag objects. These pointers allow a tag to maintain two lists—one of sibling and one of children. Each tag also contains a list of Attribute objects and a list of Contents objects. (iii) Lexical Analysis It is basically to figure out the pieces (tokens) of the program -- variables, constants, keywords, etc. The following points can summarize its functionality: -

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a) Given a bunch of characters, how do you recognize the key things and figure out the kinds of things they are? b) The code that does lexical analysis is called the scanner. c) First pass: Just chop up everything by white space into words.

6. Parsing Environment The Lexical class performs the role of lexical analyzer and provides two primary member functions. The first is called NextToken, which is called as needed to look for the next token in the string. The second is called GetCharData, which returns strings

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that contain whitespace. These functions are called as needed to build the Tag tree. GetCharData works by collecting every character it encounters into a string until it reaches either the end of the XML document or encounters a start tag.

7. Parsing Process A Parser object requests tokens or character data from the Lexical object. As the tokens and characters data are returned, the Parser builds the tag tree. The Parser uses a recursive descent technique. The member function, Translate, starts the process by getting the root tag of the XML document. The function GetTag follows the syntax by parsing the start tag, the content, and finally the end tag with calls to StartTag (), Content (), and EndTag (), respectively. While parsing the content, Lexical could return a StartTag token, signaling the beginning of another tag. If so, Content () recursively calls GetTag and adds the returned tag to the current tag’s content. Parsing start and end tags proceed similarly. A start tag id defined syntactically as a tag name followed by zero or more attributes between a < character and > character. Match is a function that serves only to move the parser to the next token. From the parser’s perspective it is telling Match, “ I expect to match this token next. If it matches, great. Send it back to me. If it doesn’t match then there is a trouble.” Finally, the use of the parser relies on traversing the resulting Tag tree. The TagIterator class assists with this task. A TagIterator object requires that u identify the tag that is the root of the tree. Once this is done, we can call the member functions Begin () and Next () to move through the tree.

8. Conclusion With the development of Web technology, the developments of markup languages are also having a fast pace to meet the specific requirements of the individual products. This creates a lot of problems for maintaining the common standard among the developed markup languages. Fortunately extensible markup language fulfills these requirements. Giving the facilities to develop individual markup languages keeping the required standard same. This paper ahs presented techniques for the development of XML parser which is very much needed for checking the well formedness and validity of an XML document. These techniques are highly useful for all who wants to develop their own Web documents for Internet applications.

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References [1]. Alfred V.Aho, Ravi Sethi, Jeffery D.Ullman, ” Compliers: Principles, Techniques, And Tools ”, [2]. Simon North, Paul Hermans, ” XML in 21 Days ”, SAMS, 2000. [3]. http://www.xml.com [4]. http://www.webreferences.com/xml/ [5]. http://www.jclark.com/xml/

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