Enterprise J2ME Developing Mobile Java Applications [Electronic resources] نسخه متنی

15.1 What Is XML?

XML is the acronym of the eXtensible Markup Language. Like any other markup language, it uses nested text tags to enclose content and represent data structure. XML itself does not define a specific set of tags and structures to use. You can define any tags to use in your XML document as long as your applications and other communication parties understand them. Hence, XML is extensible. For example, you can use the following XML document to describe a small computer parts inventory.

Listing 15.1. A sample XML document

<product category="CPU">
<description>Athlon 1.5GHz</description>
<maker>AMD</maker>
<UnitPrice>100.0</UnitPrice>
<Availability>10000</Availability>
</product>
<product category="printer">
<description>Inkjet color printer</description>
<maker>HP</maker>
<UnitPrice>120.0</UnitPrice>
<Availability>1000</Availability>
</product>

Markup data languages have been around since the 1970s. In fact, the HTML itself is a markup language. What makes XML so special? Well, there are several reasons.

XML has rich expression power. Nested tag elements allow us to easily express hierarchical data structures. More importantly, together with technologies such as XML Schema, XML supports strong data typing. Strong typed data are fundamental to object-oriented systems (i.e., Java applications). Please see Chapter 16 for more on XML Schema and XML data types.

XML is both machine and human friendly. Unlike HTML, XML has strict syntax requirements for easy and fast parsing. For example, every XML start tag must have a matching end tag, and they must be properly nested. Since an XML document contains descriptive tag information, it is easy for humans to read and understand.

XML promotes open standard. XML's extensibility and flexibility allows it to be adopted across many industries. To become a ubiquitous data exchange format, XML must be standardized. XML namespaces and schema are already standardized by W3C. Many industry-specific and application-specific XML formats are also being standardized. Today, XML is the most interoperable data format across many platforms.

XML is quickly becoming the technology of choice for enterprise data exchange and integration. To develop mobile enterprise applications, our mobile clients must talk to XML-powered backend systems.