StAX

From Wikipedia, the free encyclopedia

Streaming API for XML (StAX) is an application programming interface (API) to read and write XML documents in the Java programming language.

Traditionally, XML APIs are either:

  • tree based - the entire document is read into memory as a tree structure for random access by the calling application
  • event based - the application registers to receive events as entities are encountered within the source document.

Both have advantages; the former (for example, DOM) allows for random access to the document, the latter (e.g. SAX) requires a small memory footprint and is typically much faster.

These two access metaphors can be thought of as polar opposites. A tree based API allows unlimited, random, access and manipulation, while an event based API is a 'one shot' pass through the source document.

StAX was designed as a median between these two opposites. In the StAX metaphor, the programmatic entry point is a cursor that represents a point within the document. The application moves the cursor forward - 'pulling' the information from the parser as it needs. This is different from an event based API - such as SAX - which 'pushes' data to the application - requiring the application to maintain state between events as necessary to keep track of location within the document.

Contents

[edit] Origins

StAX has its roots in a number of incompatible pull APIs for XML, most notably XMLPULL, the authors of which (Stefan Haustein and Aleksandr Slominski) collaborated with, amongst others BEA Systems, Oracle, Sun, Breeze Factor and James Clark.

[edit] Examples

From JSR-173 Specification• Final, V1.0 (used under fair use).

Quote:

The following Java API shows the main methods for reading XML in the cursor approach.
// Java 
public interface XMLStreamReader { 
  public int next() throws XMLStreamException; 
  public boolean hasNext() throws XMLStreamException; 
  public String getText(); 
  public String getLocalName(); 
  public String getNamespaceURI(); 
  // ...othermethodsnotshown 
}
The writing side of the API has methods that correspond to the reading side for “StartElement” and “EndElement” event types.
// Java 
public interface XMLStreamWriter { 
  public void writeStartElement(String localName) throws XMLStreamException;
  public void writeEndElement() throws XMLStreamException; 
  public void writeCharacters(String text) throws XMLStreamException; 
  // ...othermethodsnotshown 
}
5.3.1 XMLStreamReader
This example illustrates how to instantiate an input factory, create a reader and iterate over the elements of an XML document.
XMLInputFactory f = XMLInputFactory.newInstance(); 
XMLStreamReader r = f.createXMLStreamReader(... ); 
while (r.hasNext()) { 
    r.next(); 
}

[edit] Implementations

[edit] See also

Competing and complementary ways to process XML in Java (ordered loosely based on initial date of introduction):

  • DOM Document Object Model is the first standardized, language/platform-independent tree-based xml processing model. (note: there are also alternate Java tree models like JDOM, Dom4j, XOM).
  • SAX The standard "XML push" API
  • JAXB Java XML Binding API: works on top of another parser (usually streaming parser), binds contained data to/from Java objects.
  • Javolution provides a real-time StAX-like implementation which does not force object creation (e.g. String) and has smaller effect on memory footprint/garbage collection (Note: to reduce object creation, most StAX implementations maintain lookup tables to retrieve and reuse frequently used String objects).

[edit] External links

Download JSR specification document as a pdf here: download now