Chapter 4 XML Processing

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DOM may have to load the entire document into memory so that the document

can be edited or data retrieved, whereas SAX allows the document to be processed

as it is parsed. However, despite its initial slowness, it is better to use the DOM

model when the source document must be edited or processed multiple times.

You should also try to use JAXB whenever the document content has a direct

representation, as domain specific objects, in Java. If you don't use JAXB, then

you must manually map document content to domain specific objects, and this

process often (when SAX is too cumbersome to apply see page 166) requires an

intermediate DOM representation of the document. Not only is this intermediate

DOM representation transient, it consumes memory resources and must be tra 

versed when mapping to the domain specific objects. With JAXB, you can auto 

matically generate the same code, thus saving development time, and, depending

on the JAXB implementation, it may not create an intermediate DOM representa 

tion of the source document. In any case, JAXB uses less memory resources as a

JAXB content tree is by nature smaller than an equivalent DOM tree.

When using higher level technologies such as XSLT, keep in mind that they

may rely on lower level technologies like SAX and DOM, which may affect per 

formance, possibly adversely. 

When building complex XML transformation pipelines, use the JAXP class

SAXTransformerFactory

 to process the results of one style sheet transformation

with another style sheet. You can optimize performance by avoiding the creation

of in memory data structures such as DOM trees by working with SAX events

until at the last stage in the pipeline.

As an alternative, you may consider using APIs other than the four discussed

previously. JDOM and dom4j are particularly appropriate for applications that

implement a document centric processing model and that must manipulate a

DOM representation of the documents. 

JDOM, for example, achieves the same results as DOM but, because it is

more generic, it can address any document model. Not only is it optimized for

Java, but developers find JDOM easy to use because it relies on the Java

Collection

 API. JDOM documents can be built directly from, and converted to,

SAX events and DOM trees, allowing JDOM to be seamlessly integrated in XML

processing pipelines and in particular as the source or result of XSLT transforma 

tions.

Another alternative API is dom4j, which is similar to JDOM. In addition to

supporting tree style processing, the dom4j API has built in support for Xpath.

For example, the 

org.dom4j.Node

 interface defines methods to select nodes

according to an Xpath expression. dom4j also implements an event based pro