XML Demystified

CHAPTER 7 XML Parsers and Transformations
of the SAX specification, the Java implementation is probably the most mature and
most widely used.
It’s important to understand that SAX is a standard for an application program
interface (API). It specifies standards for classes that you use to build a SAX parser.
This may sound confusing, especially if you’ve never programmed before.
However, you can probably imagine the many steps that are necessary to read and
transform an XML document. You need to write code for each step in order to build a
parser to transform the XML documents. This is a tedious and time-consuming job.
However, you can minimize the tedium and save time by using the classes of an
API, which other developers have already written. Think of these classes as already
assembled subparts of the parser. You assemble the subparts together to create a
parser.
You aren’t expected to write a parser, but you’ll need a parser in order to transform
your XML document. A SAX parser (a parser that was developed using the SAX
API) is designed to read large XML documents because it starts at the beginning of
the XML document and reads a group of lines, called a block at a time, until it
reaches the end of the document. The entire transformation process occurs in one
reading.
As it reads each block, the SAX parser determines if the block contains an XML
tag or information. If it’s an XML tag, the SAX parser compares the XML tag to the
XSL and then transforms the information based on the XSL instructions. The SAX
parser then reads the next block of the XML document.
A block is discarded once it’s transformed. This frees memory for the next block,
which gives the SAX parser an advantage over a DOM parser. A DOM parser loads
the entire XML document in memory, which you’ll learn about in “The Document
Object Model,” later in this chapter. The SAX parser requires a small amount of
memory to transform a very large XML document.
This advantage is also a disadvantage because a SAX parser cannot reference a
block of an XML document other than the block that’s in memory. This means that
it cannot modify XML information that has already been transformed based on the
block that’s currently being read.
A SAX parser gets one chance at reading each XML tag. Sometimes this is all
you need, though for a more complex transformation, you’ll need to use a DOM
parser that can reference any part of the XML document (see “The Document
Object Model,” later in this chapter).
Components of a SAX Parser
There are four components in a SAX parser: the Content Handler, Error Handler,
DTD Handler, and Entity Resolver.
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