Trail: Java API for XML Processing (JAXP)
Lesson: Streaming API for XML
Why StAX?
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Why StAX?

The StAX project was spearheaded by BEA with support from Sun Microsystems, and the JSR 173 specification passed the Java Community Process final approval ballot in March, 2004 ( http://jcp.org/en/jsr/detail?id=173). The primary goal of the StAX API is to give “parsing control to the programmer by exposing a simple iterator based API. This allows the programmer to ask for the next event (pull the event) and allows state to be stored in procedural fashion.” StAX was created to address limitations in the two most prevalent parsing APIs, SAX and DOM.

Streaming versus DOM

Generally speaking, there are two programming models for working with XML infosets: streaming and the document object model (DOM).

The DOM model involves creating in-memory objects representing an entire document tree and the complete infoset state for an XML document. Once in memory, DOM trees can be navigated freely and parsed arbitrarily, and as such provide maximum flexibility for developers. However, the cost of this flexibility is a potentially large memory footprint and significant processor requirements, because the entire representation of the document must be held in memory as objects for the duration of the document processing. This may not be an issue when working with small documents, but memory and processor requirements can escalate quickly with document size.

Streaming refers to a programming model in which XML infosets are transmitted and parsed serially at application runtime, often in real time, and often from dynamic sources whose contents are not precisely known beforehand. Moreover, stream-based parsers can start generating output immediately, and infoset elements can be discarded and garbage collected immediately after they are used. While providing a smaller memory footprint, reduced processor requirements, and higher performance in certain situations, the primary trade-off with stream processing is that you can only see the infoset state at one location at a time in the document. You are essentially limited to the “cardboard tube” view of a document, the implication being that you need to know what processing you want to do before reading the XML document.

Streaming models for XML processing are particularly useful when your application has strict memory limitations, as with a cellphone running the Java Platform, Micro Edition (Java ME platform), or when your application needs to process several requests simultaneously, as with an application server. In fact, it can be argued that the majority of XML business logic can benefit from stream processing, and does not require the in-memory maintenance of entire DOM trees.

Pull Parsing versus Push Parsing

Streaming pull parsing refers to a programming model in which a client application calls methods on an XML parsing library when it needs to interact with an XML infoset—that is, the client only gets (pulls) XML data when it explicitly asks for it.

Streaming push parsing refers to a programming model in which an XML parser sends (pushes) XML data to the client as the parser encounters elements in an XML infoset—that is, the parser sends the data whether or not the client is ready to use it at that time.

Pull parsing provides several advantages over push parsing when working with XML streams:

StAX Use Cases

The StAX specification defines a number of use cases for the API:

A complete discussion of all these use cases is beyond the scope of this lesson. Please refer to the StAX specification for further information.

Comparing StAX to Other JAXP APIs

As an API in the JAXP family, StAX can be compared, among other APIs, to SAX, TrAX, and JDOM. Of the latter two, StAX is not as powerful or flexible as TrAX or JDOM, but neither does it require as much memory or processor load to be useful, and StAX can, in many cases, outperform the DOM-based APIs. The same arguments outlined above, weighing the cost/benefits of the DOM model versus the streaming model, apply here.

With this in mind, the closest comparisons can be made between StAX and SAX, and it is here that StAX offers features that are beneficial in many cases; some of these include:

The following table  summarizes the comparative features of StAX, SAX, DOM, and TrAX (table adapted from “Does StAX Belong in Your XML Toolbox?” at http://www.developer.com/xml/article.php/3397691 by Jeff Ryan).

Table: XML Parser API Feature Summary

Feature

StAX

SAX

DOM

TrAX

API Type

Pull, streaming

Push, streaming

In memory tree

XSLT Rule

Ease of Use

High

Medium

High

Medium

XPath Capability

No

No

Yes

Yes

CPU and Memory Efficiency

Good

Good

Varies

Varies

Forward Only

Yes

Yes

No

No

Read XML

Yes

Yes

Yes

Yes

Write XML

Yes

No

Yes

Yes

Create, Read, Update, Delete

No

No

Yes

No


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