C#编程中XML详解：构造、验证、处理与应用

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CHAPTER 1 WHAT IS XML?

NOTE Although this chapter presents some short examples of XML, you aren’t

expected to understand all that’s going on just yet. The idea is simply to intro-

duce the important concepts behind the language so that throughout the book

you can see not only how to use XML, but also why it works the way it does.

STEPS LEADING UP TO XML: DATA REPRESENTATION

AND MARKUPS

There are two main uses for XML: One is a way to represent low-level data, for example conﬁ gura-

tion ﬁ les. The second is a way to add metadata to documents; for example, you may want to stress a

particular sentence in a report by putting it in italics or bold.

The ﬁ rst usage for XML is meant as a replacement for the more traditional ways this has been done

before, usually by means of lists of name/value pairs as is seen in Windows’ INI or Java’s Property

ﬁ les. The second application of XML is similar to how HTML ﬁ les work. The document text is con-

tained in an overall container, the

<body> element, with individual phrases surrounded by <i> or <b>

tags. For both of these scenarios there has been a multiplicity of techniques devised over the years.

The problem with these disparate approaches has been more apparent than ever, since the increased

use of the Internet and extensive existence of distributed applications, particularly those that rely on

components designed and managed by different parties. That problem is one of intercommunication.

It’s certainly possible to design a distributed system that has two components, one outputting data

using a Windows INI ﬁ le and the other which turns it into a Java Properties format. Unfortunately, it

means a lot of development on both sides, which shouldn’t really be necessary and detracts resources

from the main objective, developing new functionality that delivers business value.

XML was conceived as a solution to this kind of problem; it is meant to make passing data between

different components much easier and relieve the need to continually worry about different formats

of input and output, freeing up developers to concentrate on the more important aspects of coding

such as the business logic. XML is also seen as a solution to the question of whether ﬁ les should be

easily readable by software or by humans; XML’s aim is to be both. You’ll be examining the distinc-

tion between data-oriented and document-centric XML later in the book, but for now let’s look a

bit more deeply into what the choices were before XML when there was need to store or communi-

cate data in an electronic format.

This section takes a mid-level look at data representation, without taking too much time to explain

low-level details such as memory addresses and the like. For the purposes here you can store data in

ﬁ les two ways: as binary or as text.

Binary Files

A binary ﬁ le, at its simplest, is just a stream of bits (1s and 0s). It’s up to the application that created

the binary ﬁ le to understand what all of the bits mean. That’s why binary ﬁ les can only be read and

produced by certain computer programs, which have been speciﬁ cally written to understand them.

For example, when saving a document in Microsoft Word, using a version before 2003, the ﬁ le cre-

ated (which has a doc extension) is in a binary format. If you open the ﬁ le in a text editor such as

Notepad, you won’t be able to see a picture of the original Word document; the best you’ll be able to

see is the occasional line of text surrounded by gibberish rather than the prose, which could be in a

number of formats such as bold or italic. The characters in the document other than the actual text

are metadata, literally information about information. Mixing data and metadata is both common

and straightforward in a binary ﬁ le. Metadata can specify things such as which words should be

shown in bold, what text is to be displayed in a table, and so on. To interpret this ﬁ le you the need

the help of the application that created it. Without the help of a converter that has in-depth knowl-

edge of the underlying binary format, you won’t be able to open a document created in Word with

another similar application such as WordPerfect. The main advantage of binary formats is that they

are concise and can be expressed in a relatively small space. This means that more ﬁ les can be stored

(on a hard drive, for example) but, more importantly nowadays, less bandwidth is used when trans-

porting these ﬁ les across networks.

Text Files

The main difference between text and binary ﬁ les is that text ﬁ les are human and machine readable.

Instead of a proprietary format that needs a speciﬁ c application to decipher it, the data is such that

each group of bits represents a character from a known set. This means that many different applica-

tions can read text ﬁ les. On a standard Windows machine you have a choice of Notepad, WordPad,

and others, including being able to use command-line–based utilities such as Edit. Non-Windows

machines have a similar wide range insert of programs available, such as Emacs and Vim.

NOTE The way that characters are represented by the underlying data stream

is referred to as a ﬁ le’s encoding. The speciﬁ c encoding used is often present

as the ﬁ rst few bytes in the ﬁ le; an application checks these bytes upon opening

the ﬁ le and then knows how to display and manipulate the data. There is also a

default encoding if these ﬁ rst few bytes are not present. XML also has other

ways of specifying how a ﬁ le was encoded, and you’ll see these later on.

The ability to be read and understood by both humans and machines is not the only advantage

of text ﬁ les; they are also comparatively easier to parse than binary ﬁ les. The main disadvantage

however, is their size. In order for text ﬁ les to contain metadata (for example, a stretch of text to be

marked as important), the relevant words are usually surrounded by characters denoting this extra

information, which are somehow differentiated from the actual text itself. The most common exam-

ples of this can be found in HTML, where angle brackets are special symbols used to convey the

meaning that anything within them refers to how the text should be treated rather than the actual

data. For example, if I want mark a phrase as important I can wrap it like so:

<strong>returns must include the item order number</strong>

Steps Leading up to XML: Data Representation and Markups

❘

CHAPTER 1 WHAT IS XML?

Another disadvantage of text ﬁ les is their lack of support for metadata. If you open a Word docu-

ment that contains text in an array of fonts with different styles and save it as a text ﬁ le, you’ll just

get a plain rendition; all of the metadata has been lost. What people were looking for was some way

to have the best of both worlds — a human-readable ﬁ le that could also be read by a wide range

of applications, and could carry metadata along with its content. This brings us to the subject of

markup.

A Brief History of Markup

The advantages of text ﬁ les made it the preferred choice over binary ﬁ les, yet the disadvantages were

still cumbersome enough that people wanted to also standardize how metadata could be added.

Most agreed that markup, the act of surrounding text that conveyed information about the text,

was the way forward, but even with this agreed there was still much to be decided. The main two

questions were:

➤ How can metadata be differentiated from the basic text?

➤ What metadata is allowed?

For example, some documents needed the ability to mark text as bold or italic whereas others

were more concerned with who the original document author was, when was it created, and

who had subsequently modiﬁ ed it. To cope with this problem a deﬁ nition called Standard

Generalized Markup Language was released, commonly shortened to SGML. SGML is a step

removed from deﬁ ning an actual markup language, such as the Hyper Text Markup Language,

or HTML. Instead it relays how markup languages are to be deﬁ ned. SGML allows you to

create your own markup language and then deﬁ ne it using a standard syntax such that any

SGML-aware application can consume documents written in that language and handle them

accordingly. As previously noted, the most ubiquitous example of this is HTML. HTML uses

angular brackets (< and >) to separate metadata from basic text and also deﬁ nes a list of

what can go into these brackets, such as

em for emphasizing text, tr for table, and td for

representing tabular data.

THE BIRTH OF XML

SGML, although well thought-out and capable of deﬁ ning many different types of markup, suffered

from one major failing: it was very complicated. All the ﬂ exibility came at a cost, and there were

still relatively few applications that could read the SGML deﬁ nition of a markup language and use

it to correctly process documents. The concept was correct, but it needed to be simpler. With this

goal in mind, a small working group and a larger number of interested parties began working in the

mid-1990s on a subset of SGML known as Extensible Markup Language (XML). The ﬁ rst working

draft was published in 1996 and two years later the W3C published a revised version as a recom-

mendation on February 10, 1998.

NOTE The World Wide Web Consortium (W3C) is the main international stan-

dards organization for the World Wide Web. It has a number of working groups

targeting di erent aspects of the Web that discuss standardization and docu-

mentation of the di erent technologies used on the Internet. The standards doc-

uments go through various stages such as Working Draft and Candidate

Recommendation before ﬁ nally becoming a Recommendation. This process can

take many years. The reason that the ﬁ nal agreement is called a recommenda-

tion rather than a standard is that you are still free to ignore what it says and use

your own. All web developers know the problems in developing applications that

work across all browsers, and many of these problems arise because the

browser vendors did not follow a W3C recommendation or they did not imple-

ment features before the recommendation was ﬁ nalized. Most of the XML tech-

nologies discussed in this book have a W3C recommendation associated with

them, although some don’t have a full recommendation because they are still in

draft form. Additionally, some XML-related standards originate from outside the

W3C, such as SAX which is discussed in Chapter 11, “Event Driven Programming.”

and therefore they also don’t have o cial W3C recommendations.

XML therefore derived as a subset of SGML, whereas HTML is an application of SGML. XML

doesn’t dictate the overall format of a ﬁ le or what metadata can be added, it just speciﬁ es a few

rules. That means it retains a lot of the ﬂ exibility of SGML without most of the complexity. For

example, suppose you have a standard text ﬁ le containing a list of application users:

Joe Fawcett

Danny Ayers

Catherine Middleton

This ﬁ le has no metadata; the only reason you know it’s a list of people is your own knowledge and

experience of how names are typically represented in the western world. Now look at these names

as they might appear in an XML document:

</applicationUsers>

Immediately it’s more apparent what the individual pieces of data are, although an application still

wouldn’t know just from that ﬁ le how to treat a

user or what firstName means. Using the XML

format rather than the plain text version, it’s much easier to map these data items within the appli-

cation itself so they can be handled correctly.

The two common features of virtually all XML ﬁ le are called elements and attributes. In the pre-

ceding example, the elements are

applicationUsers and user, and the attributes are firstName

and

lastName.

The Birth of XML

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