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StAX API

The StAX API exposes methods for iterative, event-based processing of XML documents. XML documents are treated as a filtered series of events, and infoset states can be stored in a procedural fashion. Moreover, unlike SAX, the StAX API is bidirectional, enabling both reading and writing of XML documents.

The StAX API is really two distinct API sets: a cursor API and an iterator API. These two API sets explained in greater detail later in this chapter, but their main features are briefly described below.

Cursor API

As the name implies, the StAX cursor API represents a cursor with which you can walk an XML document from beginning to end. This cursor can point to one thing at a time, and always moves forward, never backward, usually one infoset element at a time.

The two main cursor interfaces are XMLStreamReader and XMLStreamWriter. XMLStreamReader includes accessor methods for all possible information retrievable from the XML Information model, including document encoding, element names, attributes, namespaces, text nodes, start tags, comments, processing instructions, document boundaries, and so forth; for example:

public interface XMLStreamReader {
  public int next() throws XMLStreamException;
  public boolean hasNext() throws XMLStreamException;
  public String getText();
  public String getLocalName();
  public String getNamespaceURI();
  // ... other methods not shown
} 

You can call methods on XMLStreamReader, such as getText and getName, to get data at the current cursor location. XMLStreamWriter provides methods that correspond to StartElement and EndElement event types; for example:

public interface XMLStreamWriter {
  public void writeStartElement(String localName) \
    throws XMLStreamException;
  public void writeEndElement() \
    throws XMLStreamException;
  public void writeCharacters(String text) \
    throws   XMLStreamException;
// ... other methods not shown
} 

The cursor API mirrors SAX in many ways. For example, methods are available for directly accessing string and character information, and integer indexes can be used to access attribute and namespace information. As with SAX, the cursor API methods return XML information as strings, which minimizes object allocation requirements.

Iterator API

The StAX iterator API represents an XML document stream as a set of discrete event objects. These events are pulled by the application and provided by the parser in the order in which they are read in the source XML document.

The base iterator interface is called XMLEvent, and there are subinterfaces for each event type listed in Table 17-2, below. The primary parser interface for reading iterator events is XMLEventReader, and the primary interface for writing iterator events is XMLEventWriter. The XMLEventReader interface contains five methods, the most important of which is nextEvent(), which returns the next event in an XML stream. XMLEventReader implements java.util.Iterator, which means that returns from XMLEventReader can be cached or passed into routines that can work with the standard Java Iterator; for example:

public interface XMLEventReader extends Iterator {
  public XMLEvent nextEvent() throws XMLStreamException;
  public boolean hasNext();
  public XMLEvent peek() throws XMLStreamException;
  ...
} 

Similarly, on the output side of the iterator API, you have:

public interface XMLEventWriter {
  public void flush() throws XMLStreamException;
  public void close() throws XMLStreamException;
  public void add(XMLEvent e) throws XMLStreamException;
  public void add(Attribute attribute) \
    throws XMLStreamException;
  ...
} 

Iterator Event Types

Table 17-2 lists the thirteen XMLEvent types defined in the event iterator API.

Table 17-2 XMLEvent Types 

Event Type	Description
StartDocument	Reports the beginning of a set of XML events, including encoding, XML version, and standalone properties.
StartElement	Reports the start of an element, including any attributes and namespace declarations; also provides access to the prefix, namespace URI, and local name of the start tag.
EndElement	Reports the end tag of an element. Namespaces that have gone out of scope can be recalled here if they have been explicitly set on their corresponding StartElement.
Characters	Corresponds to XML CData sections and CharacterData entities. Note that ignorable whitespace and significant whitespace are also reported as Character events.
EntityReference	Character entities can be reported as discrete events, which an application developer can then choose to resolve or pass through unresolved. By default, entities are resolved. Alternatively, if you do not want to report the entity as an event, replacement text can be substituted and reported as Characters.
ProcessingInstruction	Reports the target and data for an underlying processing instruction.
Comment	Returns the text of a comment
EndDocument	Reports the end of a set of XML events.
DTD	Reports as java.lang.String information about the DTD, if any, associated with the stream, and provides a method for returning custom objects found in the DTD.
Attribute	Attributes are generally reported as part of a StartElement event. However, there are times when it is desirable to return an attribute as a standalone Attribute event; for example, when a namespace is returned as the result of an XQuery or XPath expression.
Namespace	As with attributes, namespaces are usually reported as part of a StartElement, but there are times when it is desirable to report a namespace as a discrete Namespace event.

Note that the DTD, EntityDeclaration, EntityReference, NotationDeclaration, and ProcessingInstruction events are only created if the document being processed contains a DTD.

Sample Event Mapping

As an example of how the event iterator API maps an XML stream, consider the following XML document:

<?xml version="1.0"?>
<BookCatalogue xmlns="http://www.publishing.org">
  <Book>
    <Title>Yogasana Vijnana: the Science of Yoga</Title>
    <ISBN>81-40-34319-4</ISBN>
    <Cost currency="INR">11.50</Cost>
  </Book>
</BookCatalogue> 

This document would be parsed into eighteen primary and secondary events, as shown below. Note that secondary events, shown in curly braces ({}), are typically accessed from a primary event rather than directly.

Table 17-3 Sample Iterator API Event Mapping 

#	Element/Attribute	Event
1	version="1.0"	StartDocument
2	isCData = false data = "\n" IsWhiteSpace = true	Characters
3	qname = BookCatalogue:http://www.publishing.org attributes = null namespaces = {BookCatalogue" -> http://www.publishing.org"}	StartElement
4	qname = Book attributes = null namespaces = null	StartElement
5	qname = Title attributes = null namespaces = null	StartElement
6	isCData = false data = "Yogasana Vijnana: the Science of Yoga\n\t" IsWhiteSpace = false	Characters
7	qname = Title namespaces = null	EndElement
8	qname = ISBN attributes = null namespaces = null	StartElement
9	isCData = false data = "81-40-34319-4\n\t" IsWhiteSpace = false	Characters
10	qname = ISBN namespaces = null	EndElement
11	qname = Cost attributes = {"currency" -> INR} namespaces = null	StartElement
12	isCData = false data = "11.50\n\t" IsWhiteSpace = false	Characters
13	qname = Cost namespaces = null	EndElement
14	isCData = false data = "\n" IsWhiteSpace = true	Characters
15	qname = Book namespaces = null	EndElement
16	isCData = false data = "\n" IsWhiteSpace = true	Characters
17	qname = BookCatalogue:http://www.publishing.org namespaces = {BookCatalogue" -> http://www.publishing.org"}	EndElement
18		EndDocument

There are several important things to note in the above example:

The events are created in the order in which the corresponding XML elements are encountered in the document, including nesting of elements, opening and closing of elements, attribute order, document start and document end, and so forth.
As with proper XML syntax, all container elements have corresponding start and end events; for example, every StartElement has a corresponding EndElement, even for empty elements.
Attribute events are treated as secondary events, and are accessed from their corresponding StartElement event.
Similar to Attribute events, Namespace events are treated as secondary, but appear twice and are accessible twice in the event stream, first from their corresponding StartElement and then from their corresponding EndElement.
Character events are specified for all elements, even if those elements have no character data. Similarly, Character events can be split across events.
The StAX parser maintains a namespace stack, which holds information about all XML namespaces defined for the current element and its ancestors. The namespace stack is exposed through the javax.xml.namespace.NamespaceContext interface, and can be accessed by namespace prefix or URI.

Choosing Between Cursor and Iterator APIs

It is reasonable to ask at this point, "What API should I choose? Should I create instances of XMLStreamReader or XMLEventReader? Why are there two kinds of APIs anyway?"

Development Goals

The authors of the StAX specification targeted three types of developers:

Library and infrastructure developers - Create application servers, JAXM, JAXB, JAX-RPC and similar implementations; need highly efficient, low-level APIs with minimal extensibility requirements.
J2ME developers - Need small, simple, pull-parsing libraries, and have minimal extensibility needs.
Java EE and Java SE developers - Need clean, efficient pull-parsing libraries, plus need the flexibility to both read and write XML streams, create new event types, and extend XML document elements and attributes.

Given these wide-ranging development categories, the StAX authors felt it was more useful to define two small, efficient APIs rather than overloading one larger and necessarily more complex API.

Comparing Cursor and Iterator APIs

Before choosing between the cursor and iterator APIs, you should note a few things that you can do with the iterator API that you cannot do with cursor API:

Objects created from the XMLEvent subclasses are immutable, and can be used in arrays, lists, and maps, and can be passed through your applications even after the parser has moved on to subsequent events.
You can create subtypes of XMLEvent that are either completely new information items or extensions of existing items but with additional methods.
You can add and remove events from an XML event stream in much simpler ways than with the cursor API.

Similarly, keep some general recommendations in mind when making your choice:

If you are programming for a particularly memory-constrained environment, like J2ME, you can make smaller, more efficient code with the cursor API.
If performance is your highest priority--for example, when creating low-level libraries or infrastructure--the cursor API is more efficient.
If you want to create XML processing pipelines, use the iterator API.
If you want to modify the event stream, use the iterator API.
If you want to your application to be able to handle pluggable processing of the event stream, use the iterator API.
In general, if you do not have a strong preference one way or the other, using the iterator API is recommended because it is more flexible and extensible, thereby "future-proofing" your applications.

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