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1 ?xml version=1.0 encoding=UTF-8 standalone=yes? 2 book
3 bk:cover
xmlns:bk=http:any.website.netbook
4 bk:titleA Book About Namespacesbk:title 5 bk:authorAnonymousbk:title
6 bk:isbn number=1881378241 7 bk:cover
8 bk2:chapter
xmlns:bk2=http:any.website.netbook
9 ch_name=Introduction 10 bk2:paragraph
11 In this chapter we start from the beginning. 12 ...
13 bk2:paragraph 14 . . .
15 bk2:chapter
As can be seen, the namespace identifier must be declared only in the outermost element. In our case, there are two top-level elements: bk:cover and bk:chapter, and their embedded
elements just inherit the namespace attributes. All the elements of the namespace are prefixed with the appropriate prefix, in our case “bk.” The actual prefix’s name is not important, so in the
above example I define “bk” and “bk2” as prefixes for the same namespace in different scopes. Notice also that an element can have an arbitrary number of namespace attributes, each
defining a different prefix and referring to a different namespace.
In the second form, the prefix is omitted, so the elements of this namespace are not qualified. The namespace attribute is bound to the default namespace. For the above example Listing 6-3, the
second form can be declared as:
Listing 6-4: Example of using a default namespace. 1 ?xml version=1.0 encoding=UTF-8 standalone=yes?
2 book 3 cover
xmlns=http:any.website.netbook
4 titleA Book About Namespacestitle 5 authorAnonymoustitle
6 isbn number=1881378241 7 cover
8 . . .
Notice that there can be at most one default namespace declared within a given scope. In Listing 6-4, we can define another default namespace in the same document, but its scope must not
overlap with that of the first one.
6.1.4 XML Parsers
The parsers define standard APIs to access and manipulate the parsed XML data. The two most popular parser APIs are DOM Document Object Model based and SAX Simple API for XML.
See Appendix E for a brief review of DOM.
SAX and DOM offer complementary paradigms to access the data contained in XML documents. DOM allows random access to any part of a parsed XML document. To use DOM APIs, the
parsed objects must be stored in the working memory. Conversely, SAX provides no storage and presents the data as a linear stream. With SAX, if you want to refer back to anything seen earlier
you have to implement the underlying mechanism yourself. For example, with DOM an Scope of b
k
Scope of b k2
Scope of default n.s.
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application program can import an XML document, modify it in arbitrary order, and write back any time. With SAX, you cannot perform the editing arbitrarily since there is no stored document
to edit. You would have to edit it by filtering the stream, as it flows, and write back immediately.
Event-Oriented Paradigm: SAX
SAX Simple API for XML is a simple, event-based API for XML parsers. The benefit of an event-based API is that it does not require the creation and maintenance of an internal
representation of the parsed XML document. This makes possible parsing XML documents that are much larger than the available system memory would allow, which is particularly important
for small terminals, such as PDAs and mobile phones. Because it does not require storage behind its API, SAX is complementary to DOM.
SAX provides events for the following structural information for XML documents: • The start and end of the document
• Document type declaration DTD • The start and end of elements
• Attributes of each element • Character data
• Unparsed entity declarations • Notation declarations
• Processing instructions
Initiating the parser
. . .
Parser parser =
ParserFactory. makeParser
com.sun.xml.parser.Parser;
parser.setDocumentHandlernew
DocumentHandlerImpl;
parser.parse input;
. . .
DocumentHandler Interface
public void startDocumentthrows
SAXException{} public void
endDocumentthrows
SAXException{} public void
startElementString name,
AttributeList attrs throws SAXException{}
public void endElementString
namethrows SAXException{} public void
characterschar buf [],
int offset, int lenthrows SAXException{}
?xml ... element attr1=“val1”
This is a test. element
element attr1=“val2” end of the document
startDocument startElement
characters endElement
endDocument
Document Handler
Event triggering in SAX parser:
Figure 6-3: SAX parser Java example.
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Object-Model Oriented Paradigm: DOM
DOM Document Object Model
Practical Issues
Additional features relevant for both event-oriented and object-model oriented parsers include: • Validation against a DTD
• Validation against an XML Schema • Namespace awareness, i.e., the ability to determine the namespace URI of an element or
attribute These features affect the performance and memory footprint of a parser, so some parsers do not
support all the features. You should check the documentation for the particular parser as to the list of supported features.
6.2 XML Schemas
Although there is no universal definition of schema, generally scholars agree that schemas are abstractions or generalizations of our perceptions of the world around us, which is molded by our
experience. Functionally, schemas are knowledge structures that serve as heuristics which help us evaluate new information. An integral part of schema is our expectations of people, place, and
things. Schemas provide a mechanism for describing the logical structure of information, in the sense of what elements can or should be present and how they can be arranged. Deviant news
results in violation of these expectations, resulting in schema incongruence.
In XML, schemas are used to make a class of documents adhere to a particular interface and thus allow the XML documents to be created in a uniform way. Stated another way, schemas allow a
document to communicate meta-information to the parser about its content, or its grammar. Meta- information includes the allowed sequence and arrangementnesting of tags, attribute values and
their types and defaults, the names of external files that may be referenced and whether or not they contain XML, the formats of some external non-XML data that may be referenced, and the
entities that may be encountered. Therefore, schema defines the document production rules. XML documents conforming to a particular schema are said to be valid documents. Notice that
having a schema associated with a given XML document is optional. If there is a schema for a given document, it must appear before the first element in the document.
Here is a simple example to motivate the need for schemas. In Section 6.1.1 above I introduced an XML representation of a correspondence letter and used the tags letter, sender,
name , address, street, city, etc., to mark up the elements of a letter. What if
somebody used the same vocabulary in a somewhat different manner, such as the following?
Listing 6-5: Variation on the XML example document from Listing 6-1.
