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Package overview: net.sf.saxon.instruct
This package provides classes for the compiled representation of the various elements
and other instructions found in an XSLT stylesheet. The same constructs are also used for
evaluating similar constructs in XQuery, in particular, expressions that construct new nodes.
Instances of these classes are constructed when the stylesheet or query is compiled. In the
case of XSLT, the objects
representing the compile-time stylesheet (in package net.sf.saxon.style) can then be
discarded and garbage-collected.
The most important class is Instruction, which represents an XSLT Instruction. In most cases
these instructions have a one-to-one relationship with instructions in the original source XSLT
stylesheet, and the names of the subclasses (for example ApplyImports, ApplyTemplates, Choose)
reflect this.
In XSLT 1.0, XSLT instructions and XPath expressions were quite distinct, and were evaluated in different
ways: XSLT instructions in "push" mode (they were described as "writing to the result tree"), and XPath expressions
in "pull" mode (reading from the source tree). This distinction is no longer present in the XSLT 2.0 processing
model, and the boundary between the Instruction and Expression classes is therefore
a rather fuzzy one. Both instructions and expressions can now be evaluated in either push or pull mode.
Flow-of-control constructs such as conditional expressions and FOR expressions are evaluated in either mode
depending on their parent expression.
Michael H. Kay
Saxonica Limited
9 February 2005