Oracle Application Server 10g Essentials [Electronic resources] نسخه متنی

10.2 XML Standards

The World
Wide
Web Consortium, a group of individuals that participate in the
establishment of web-related standards, has created many XML-related
standards, including XML 1.0, XML's first formal
specification, and more recently, XML Schema. Oracle Application
Server provides support for these established XML standards both in
the database via the
mod_plsql module and in the
application server for web-based applications written in any of the
following:

C or C++
that runs in mod_fastcgi

Java that runs in OC4J

Web Services

10.2.1 XML

XML Version 1.0 is XML's
first formal specification (recommendation) from the W3C. It defines
the syntax used to create a well-formed XML document. In addition, it
defines the structure of a DTD, a schema that, in turn, can validate
one of its XML document instances.

Oracle Application Server is XML 1.0-compliant, which means that the
server's internal use of XML, along with the tools
and APIs it provides, all comply with the XML Version 1.0
recommendation from the W3C.

10.2.2 XML Namespaces

XML Namespaces exist to eliminate
potentially ambiguous XML element (tag) names when XML applications
are combined to create a new XML application. An XML Namespace
associates a URI with a namespace prefix. The prefix is added to an
element name in an XML document to maintain the element
name's uniqueness within the document. This
qualification becomes necessary when element names from two different
XML applications used in another XML application use the same element
name. Without the prefix, the elements with the same name collide;
this creates ambiguity that, in turn, causes confusion about the
element's meaning. The associated namespace URI
creates a context for an otherwise ambiguous element.

The XML Namespace Version 1.0 recommendation isn't
part of the XML 1.0 recommendation. It post-dates XML 1.0 and
isn't a dependent specification. XML Namespaces may
be used if necessary, but they are by no means required for an XML
document to be compliant with the 1.0 recommendation.

Oracle Application Server supports the XML
Namespace's 1.0 recommendation from the W3C.

10.2.3 XML Path Language

XML Path Language is a non-XML based
language that identifies a particular part, or parts, of an XML
document. XPath treats an XML document and its content as a
tree structure, in much the same way operating
systems treat file systems. By doing so, it allows you to write an
expression that identifies a specific element, attribute, text,
comment, processing instruction, or namespace within an XML document.
XPath expressions can also specify Boolean values, numbers, or
strings.

Oracle Application Server supports XPath Version 1.0.

10.2.4 Extensible Stylesheet Language Transformations

EXtensible
Stylesheet Language for Transformations (XSLT) is an
XML application that
allows you to specify a set of rules that transform an XML document
into another document format. Although typically used for XML-to-XML
document transformation, XSLT can be used to
create a text document of any kind.

An XSLT stylesheet is an XML document that contains rules for
transformation. The rules in an XSL stylesheet use XPath notation to
identify the targets of rules. Using an XSL stylesheet, an XSLT
processor transforms an input XML document to another format using
the rules specified in a stylesheet.

Oracle Application Server supports XSLT Version 1.0.

10.2.5 XML Schemas

An
XML Schema is an XML document that
describes the composition of a valid XML document in much greater
detail than is possible using a DTD. An XML Schema adds the concepts
of datatypes (both simple and complex), datatype inheritance, and
element and attribute constraints to the structural capabilities of a
DTD. However, XML Schemas improve on DTDs at the expense of
additional complexity.

Oracle Application Server provides support for parts 0-2 of the
W3C's XML Schema recommendation from May 2, 2001.

10.2.6 XML Parsers

XML parsers can read an XML document from an
operating system file or stream and present the XML
document's information as a tree structure in memory
after the entire file is read, or in an event-based fashion as a
document is read. Each approach has its strengths and weaknesses:

Tree-structured approach, or document model

This approach makes it feasible to query any part of an XML document
at any time. It also allows you to modify an XML document, but it can
require a great deal of memory.

Event-based model

This approach conserves memory consumption, but it can process a
document only as it is read.

Because of the opposing constraints, two types of XML parsers exist:
Document Object Model (DOM) parsers and Simple API for XML (SAX)
parsers.

10.2.6.1 Document Object Model

The Document Object Model (DOM) is a
specification for reading and modifying (and saving) an XML document
as a tree structure. The elements that make up a particular XML
document create a hierarchical structure that can be represented as a
tree with the first element of an XML document as the root. In many
ways, this tree structure, shown in Figure 10-1, is
similar to the one presented by most modern operating
systems' file systems.

Figure 10-1. A tree of DOM nodes for Example 10-1

At its core, DOM is a set of APIs that represent an XML document as a
set of object nodes, as in Figure 10-1. A node type
exists for each kind of XML structural and content syntax.
DOM's strength is that it allows random access to
any part of an XML document.

Oracle Application Server supports DOM Version 2.0. It provides DOM
APIs for C, C++, Java, and PL/SQL.

10.2.6.2 Simple API for XML

The Simple API for XML (SAX) is a
specification for reading an XML document that reports each XML
artifactfor example, an element or an attributeas an
event as a document is read. The events are reported by calling the
callback functions of a programmatically registered document handler.
SAX's strength is its low memory consumption.

Example 10-4 lists the callback events generated when
parsing the XML document from Example 10-1. The
events in this list are named using their corresponding function
names in the Java interface
org.sax.xml.ContentHandler. Any values passed to
the callback functions are shown after the function names.

Example 10-4. A list of SAX events for Example 10-1

setDocumentLocator
startDocument
startElement: localName=person, qualifiedName=person
ignorableWhitespace: 
startElement: localName=name, qualifiedName=name
ignorableWhitespace: 
startElement: localName=last_name, qualifiedName=last_name
characters: Doe
ignorableWhitespace: 
startElement: localName=first_name, qualifiedName=first_name
characters: Jane
ignorableWhitespace: 
ignorableWhitespace: 
startElement: localName=birth_date, qualifiedName=birth_date
characters: 1980-01-01
ignorableWhitespace: 
startElement: localName=gender, qualifiedName=gender
characters: Female
ignorableWhitespace: 
endDocument

Oracle Application Server supports SAX Version 2.0. It provides SAX
APIs for languages that support callback functions: C, C++, and Java.