net.sf.saxon.pattern
Class PatternSponsor

java.lang.Object
  extended by net.sf.saxon.expr.Expression
      extended by net.sf.saxon.pattern.PatternSponsor
All Implemented Interfaces:
Serializable, SourceLocator, LocationProvider, SaxonLocator, InstructionInfo, Locator

public class PatternSponsor
extends Expression

The PatternSponsor class allows a Pattern to be treated like an expression. Although patterns are not evaluated at run-time in the same way as expressions, they need to be manipulated in much the same way as expressions at compile time: for example variables need to be bound, dependencies need to be analyzed, and so on. This is especially true of patterns appearing in the xsl:number and xsl:for-each-group instructions (less so for the more common match patterns in xsl:template).

This class implements the Expression interface, so that an Expression can have a PatternSponsor as a subexpression; it wraps a Pattern.

A Pattern is treated as a boolean expression that returns true if the pattern matches the context node. It is evaluated in that way only when obtained through the s9api XPathCompiler.compilePattern(String) method.

See Also:
Serialized Form

Field Summary
 
Fields inherited from class net.sf.saxon.expr.Expression
EFFECTIVE_BOOLEAN_VALUE, EVALUATE_METHOD, EVENT_FEED_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, locationId, MAX_SEQUENCE_LENGTH, MAX_STRING_LENGTH, PROCESS_METHOD, PUSH_SELECTION, staticProperties, UNBOUNDED_LOWER, UNBOUNDED_UPPER, WATCH_METHOD
 
Constructor Summary
PatternSponsor(Pattern pattern)
          Create a sponsor expression for a pattern
 
Method Summary
 void checkPermittedContents(SchemaType parentType, StaticContext env, boolean whole)
          Check statically that the results of the expression are capable of constructing the content of a given schema type.
protected  int computeCardinality()
          Compute the static cardinality of this expression
 Expression copy()
          Copy an expression.
 boolean effectiveBooleanValue(XPathContext context)
          Get the effective boolean value of the expression.
 CharSequence evaluateAsString(XPathContext context)
          Evaluate an expression as a String.
 Item evaluateItem(XPathContext context)
          Evaluate an expression as a single item.
 void explain(ExpressionPresenter out)
          Diagnostic print of expression structure.
 int getCardinality()
          Determine the static cardinality of the expression.
 Container getContainer()
          Get the container that immediately contains this expression.
 int getDependencies()
          Determine which aspects of the context the expression depends on.
 int getImplementationMethod()
          An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
 ItemType getItemType(TypeHierarchy th)
          Determine the data type of the expression, if possible.
 Pattern getPattern()
          Get the wrapped pattern
 boolean hasLoopingSubexpression(Expression child)
          Treat all subexpressions of a pattern as "looping" subexpresions, that is, assume they are evaluated repeatedly
 SequenceIterator iterate(XPathContext context)
          Return an Iterator to iterate over the values of a sequence.
 Iterator<Expression> iterateSubExpressions()
          Get the immediate sub-expressions of this expression.
 Expression optimize(ExpressionVisitor visitor, ExpressionVisitor.ContextItemType contextItemType)
          Perform optimisation of an expression and its subexpressions.
 void process(XPathContext context)
          Process the instruction, without returning any tail calls
 Expression promote(PromotionOffer offer, Expression parent)
          Offer promotion for this subexpression.
 boolean replaceSubExpression(Expression original, Expression replacement)
          Replace one subexpression by a replacement subexpression
 Expression simplify(ExpressionVisitor visitor)
          Simplify an expression.
 Expression typeCheck(ExpressionVisitor visitor, ExpressionVisitor.ContextItemType contextItemType)
          Perform type checking of an expression and its subexpressions.
 
Methods inherited from class net.sf.saxon.expr.Expression
addToPathMap, adoptChildExpression, checkForUpdatingSubexpressions, computeDependencies, computeSpecialProperties, computeStaticProperties, doPromotion, dynamicError, evaluatePendingUpdates, explain, getColumnNumber, getColumnNumber, getConstructType, getEvaluationMethod, getExecutable, getExpressionName, getHostLanguage, getIntegerBounds, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getObjectName, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasVariableBinding, implementsStaticTypeCheck, isSubtreeExpression, isUpdatingExpression, isVacuousExpression, iterateEvents, iterateSameFocusSubExpressions, markTailFunctionCalls, resetLocalStaticProperties, setContainer, setEvaluationMethod, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation, toString, typeError
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

PatternSponsor

public PatternSponsor(Pattern pattern)
Create a sponsor expression for a pattern

Parameters:
pattern - the pattern being wrapped
Method Detail

getPattern

public Pattern getPattern()
Get the wrapped pattern

Returns:
the wrapped pattern

getImplementationMethod

public int getImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.

Overrides:
getImplementationMethod in class Expression
Returns:
the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

simplify

public Expression simplify(ExpressionVisitor visitor)
                    throws XPathException
Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression). The default implementation does nothing.

Overrides:
simplify in class Expression
Parameters:
visitor - an expression visitor
Returns:
the simplified expression
Throws:
XPathException - if an error is discovered during expression rewriting

optimize

public Expression optimize(ExpressionVisitor visitor,
                           ExpressionVisitor.ContextItemType contextItemType)
                    throws XPathException
Perform optimisation of an expression and its subexpressions.

This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

Overrides:
optimize in class Expression
Parameters:
visitor - an expression visitor
contextItemType - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
Returns:
the original expression, rewritten if appropriate to optimize execution
Throws:
XPathException - if an error is discovered during this phase (typically a type error)

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ExpressionVisitor.ContextItemType contextItemType)
                     throws XPathException
Perform type checking of an expression and its subexpressions.

This checks statically that the operands of the expression have the correct type; if necessary it generates code to do run-time type checking or type conversion. A static type error is reported only if execution cannot possibly succeed, that is, if a run-time type error is inevitable. The call may return a modified form of the expression.

This method is called after all references to functions and variables have been resolved to the declaration of the function or variable. However, the types of such functions and variables may not be accurately known if they have not been explicitly declared.

Overrides:
typeCheck in class Expression
Parameters:
visitor - an expression visitor
contextItemType - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
Returns:
the original expression, rewritten to perform necessary run-time type checks, and to perform other type-related optimizations
Throws:
XPathException - if an error is discovered during this phase (typically a type error)

computeCardinality

protected int computeCardinality()
Description copied from class: Expression
Compute the static cardinality of this expression

Specified by:
computeCardinality in class Expression
Returns:
the computed cardinality, as one of the values StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.EXACTLY_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_MORE

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Specified by:
copy in class Expression
Returns:
the copy of the original expression

hasLoopingSubexpression

public boolean hasLoopingSubexpression(Expression child)
Treat all subexpressions of a pattern as "looping" subexpresions, that is, assume they are evaluated repeatedly

Overrides:
hasLoopingSubexpression in class Expression
Parameters:
child - the child expression
Returns:
true, always

promote

public Expression promote(PromotionOffer offer,
                          Expression parent)
                   throws XPathException
Offer promotion for this subexpression. The offer will be accepted if the subexpression is not dependent on the factors (e.g. the context item) identified in the PromotionOffer. By default the offer is not accepted - this is appropriate in the case of simple expressions such as constant values and variable references where promotion would give no performance advantage. This method is always called at compile time.

Overrides:
promote in class Expression
Parameters:
offer - details of the offer, for example the offer to move expressions that don't depend on the context to an outer level in the containing expression
parent -
Returns:
if the offer is not accepted, return this expression unchanged. Otherwise return the result of rewriting the expression to promote this subexpression
Throws:
XPathException - if any error is detected

getCardinality

public int getCardinality()

Determine the static cardinality of the expression. This establishes how many items there will be in the result of the expression, at compile time (i.e., without actually evaluating the result.

This method should always return a result, though it may be the best approximation that is available at the time.

Overrides:
getCardinality in class Expression
Returns:
one of the values StaticProperty.ALLOWS_ONE, StaticProperty.ALLOWS_ZERO_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.EMPTY.

getItemType

public ItemType getItemType(TypeHierarchy th)
Determine the data type of the expression, if possible. All expression return sequences, in general; this method determines the type of the items within the sequence, assuming that (a) this is known in advance, and (b) it is the same for all items in the sequence.

This method should always return a result, though it may be the best approximation that is available at the time.

Specified by:
getItemType in class Expression
Parameters:
th - the type hierarchy cache
Returns:
a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known at compile time)

getDependencies

public int getDependencies()
Determine which aspects of the context the expression depends on. The result is a bitwise-or'ed value composed from constants such as StaticProperty.DEPENDS_ON_CONTEXT_ITEM and StaticProperty.DEPENDS_ON_CURRENT_ITEM. The default implementation combines the intrinsic dependencies of this expression with the dependencies of the subexpressions, computed recursively. This is overridden for expressions such as FilterExpression where a subexpression's dependencies are not necessarily inherited by the parent expression.

Overrides:
getDependencies in class Expression
Returns:
a set of bit-significant flags identifying the dependencies of the expression

iterateSubExpressions

public Iterator<Expression> iterateSubExpressions()
Get the immediate sub-expressions of this expression. Default implementation returns a zero-length array, appropriate for an expression that has no sub-expressions.

Overrides:
iterateSubExpressions in class Expression
Returns:
an iterator containing the sub-expressions of this expression

replaceSubExpression

public boolean replaceSubExpression(Expression original,
                                    Expression replacement)
Replace one subexpression by a replacement subexpression

Overrides:
replaceSubExpression in class Expression
Parameters:
original - the original subexpression
replacement - the replacement subexpression
Returns:
true if the original subexpression is found

getContainer

public Container getContainer()
Get the container that immediately contains this expression. This method returns null for an outermost expression; it also return null in the case of literal values. For an XPath expression occurring within an XSLT stylesheet, this method returns the XSLT instruction containing the XPath expression.

Overrides:
getContainer in class Expression
Returns:
the expression that contains this expression, if known; return null if there is no containing expression or if the containing expression is unknown.

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException
Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.

Overrides:
evaluateItem in class Expression
Parameters:
context - The context in which the expression is to be evaluated
Returns:
the node or atomic value that results from evaluating the expression; or null to indicate that the result is an empty sequence
Throws:
XPathException - if any dynamic error occurs evaluating the expression

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException
Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.

Overrides:
iterate in class Expression
Parameters:
context - supplies the context for evaluation
Returns:
a SequenceIterator that can be used to iterate over the result of the expression
Throws:
XPathException - if any dynamic error occurs evaluating the expression

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)
                              throws XPathException
Get the effective boolean value of the expression. Returns true if the underlying pattern matches the context node, otherwise false.

Overrides:
effectiveBooleanValue in class Expression
Parameters:
context - The context in which the expression is to be evaluated
Returns:
the effective boolean value
Throws:
XPathException - if any dynamic error occurs evaluating the expression

evaluateAsString

public CharSequence evaluateAsString(XPathContext context)
                              throws XPathException
Evaluate an expression as a String. This function must only be called in contexts where it is known that the expression will return a single string (or where an empty sequence is to be treated as a zero-length string). Implementations should not attempt to convert the result to a string, other than converting () to "". This method is used mainly to evaluate expressions produced by compiling an attribute value template.

Overrides:
evaluateAsString in class Expression
Parameters:
context - The context in which the expression is to be evaluated
Returns:
the value of the expression, evaluated in the current context. The expression must return a string or (); if the value of the expression is (), this method returns "".
Throws:
XPathException - if any dynamic error occurs evaluating the expression
ClassCastException - if the result type of the expression is not xs:string?

process

public void process(XPathContext context)
             throws XPathException
Process the instruction, without returning any tail calls

Overrides:
process in class Expression
Parameters:
context - The dynamic context, giving access to the current node, the current variables, etc.
Throws:
XPathException - if a dynamic error occurs

explain

public void explain(ExpressionPresenter out)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Specified by:
explain in class Expression
Parameters:
out - the expression presenter used to display the structure

checkPermittedContents

public void checkPermittedContents(SchemaType parentType,
                                   StaticContext env,
                                   boolean whole)
                            throws XPathException
Check statically that the results of the expression are capable of constructing the content of a given schema type.

Overrides:
checkPermittedContents in class Expression
Parameters:
parentType - The schema type
env - the static context
whole - true if this expression is expected to make the whole content of the type, false if it is expected to make up only a part
Throws:
XPathException - if the expression doesn't match the required content type


Copyright (c) 2004-2011 Saxonica Limited. All rights reserved.