net.sf.saxon.expr

Class ComputedExpression

java.lang.Object

net.sf.saxon.expr.ComputedExpression

All Implemented Interfaces:

Serializable, SourceLocator, Container, Expression, InstructionInfoProvider

Direct Known Subclasses:

Assignation, AxisExpression, BinaryExpression, CastAsQName, ContextItemExpression, ErrorExpression, FilterExpression, FunctionCall, IfExpression, Instruction, IntegerRangeTest, IsLastExpression, NumberInstruction, PathExpression, PositionRange, SimpleContentConstructor, SimpleExpression, SimpleMappingExpression, SingleNodeExpression, SliceExpression, SortExpression, SuppliedParameterReference, TailExpression, TupleExpression, TupleSorter, UnaryExpression, VariableReference

public abstract class ComputedExpression
extends Object
implements Serializable, Expression, InstructionInfoProvider, Container

This class is an abstract superclass for different kinds of expression. This includes XSLT instructions, which are treated just like XPath expressions. Every expression is either a constant Value, or a ComputedExpression.

There are three principal methods for evaluating an expression: iterate(), which an iterator over the result of the expression as a sequence; evaluateItem(), which returns an object that is an instance of net.sf.saxon.om.Item; and process(), which pushes the results of the expression to a Receiver. All three methods take an XPathContext object to supply the evaluation context; for an expression that is a Value, this argument is ignored and may be null. This root class provides an implementation of iterate() in terms of evaluateItem() that works only for singleton expressions, and an implementation of evaluateItem() in terms of iterate() that works only for non-singleton expressions. Subclasses of expression must therefore provide either iterate() or evaluateItem() or process(): they do not have to provide all three.

Note that the methods that take an XPathContext argument are run-time methods. The methods without such an argument are compile-time methods. Run-time methods must not modify the state of the Expression object.

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
protected int	locationId
protected int	staticProperties

Fields inherited from interface net.sf.saxon.expr.Expression

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

Constructor Summary

Constructors

Constructor and Description
ComputedExpression()

Method Summary

Modifier and Type	Method and Description
void	adoptChildExpression(Expression child)
void	checkPermittedContents(SchemaType parentType, StaticContext env, boolean whole) Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type.
protected abstract int	computeCardinality()
int	computeDependencies() Compute the dependencies of an expression, as the union of the dependencies of its subexpressions.
int	computeSpecialProperties()
void	computeStaticProperties() Compute the static properties.
Expression	doPromotion(Expression subexpression, PromotionOffer offer) Promote a subexpression if possible, and if the expression was changed, carry out housekeeping to reset the static properties and correct the parent pointers in the tree
protected void	dynamicError(String message, String code, XPathContext context) Method used in subclasses to signal a dynamic error
protected void	dynamicError(String message, XPathContext context) Method used in subclasses to signal a dynamic error
boolean	effectiveBooleanValue(XPathContext context) Get the effective boolean value of the expression.
String	evaluateAsString(XPathContext context) Evaluate an expression as a String.
Item	evaluateItem(XPathContext context) Evaluate an expression as a single item.
int	getCardinality() Determine the static cardinality of the expression.
int	getColumnNumber() Get the column number of the expression
protected int	getConstructType() Get the type of this expression for use in tracing and diagnostics
int	getDependencies() Determine which aspects of the context the expression depends on.
Executable	getExecutable() Get the executable containing this expression
int	getHostLanguage() Get the host language (XSLT, XQuery, XPath) used to implement the code in this container
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
InstructionInfo	getInstructionInfo() Get InstructionInfo for this expression
int	getIntrinsicDependencies() Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions.
int	getLineNumber() Get the line number of the expression
int	getLocationId() Get the location ID of the expression
LocationProvider	getLocationProvider() Get the LocationProvider allowing location identifiers to be resolved.
Container	getParentExpression() Get the expression that immediately contains this expression.
String	getPublicId() Get the publicId of the module containing the expression (to satisfy the SourceLocator interface)
int[]	getSlotsUsed() Get the local variables (identified by their slot numbers) on which this expression depends.
int	getSpecialProperties() Get the static properties of this expression (other than its type).
String	getSystemId() Get the systemId of the module containing the expression
boolean	hasBadParentPointer() Diagnostic method: search the tree for an expression whose parent expression is incorrectly set
SequenceIterator	iterate(XPathContext context) Return an Iterator to iterate over the values of a sequence.
Iterator	iterateSubExpressions() Get the immediate sub-expressions of this expression.
boolean	markTailFunctionCalls() Mark tail-recursive calls on stylesheet functions.
void	process(XPathContext context) Process the instruction, without returning any tail calls
Expression	promote(PromotionOffer offer) Offer promotion for this subexpression.
void	resetStaticProperties() Reset the static properties.
void	setLocationId(int id) Set the location ID on an expression.
void	setParentExpression(Container parent)
Expression	simplify(StaticContext env) Simplify an expression.
void	suppressValidation(int validationMode) Suppress validation on contained element constructors, on the grounds that the parent element is already performing validation.
protected void	typeError(String message, String errorCode, XPathContext context) Method used in subclasses to signal a runtime type error
protected void	typeError(String message, XPathContext context) Method used in subclasses to signal a runtime type error

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface net.sf.saxon.expr.Expression

display, getItemType, optimize, typeCheck

Field Detail

staticProperties

protected int staticProperties

locationId

protected int locationId

Constructor Detail

ComputedExpression

public ComputedExpression()

Method Detail

getParentExpression

public Container getParentExpression()

Get the expression 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.

Specified by:

getParentExpression in interface 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.

setParentExpression

public void setParentExpression(Container parent)

adoptChildExpression

public void adoptChildExpression(Expression child)

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 prefered.

Specified by:

getImplementationMethod in interface Expression

setLocationId

public void setLocationId(int id)

Set the location ID on an expression.

getLocationId

public final int getLocationId()

Get the location ID of the expression

getLineNumber

public int getLineNumber()

Get the line number of the expression

Specified by:

getLineNumber in interface SourceLocator

getColumnNumber

public int getColumnNumber()

Get the column number of the expression

Specified by:

getColumnNumber in interface SourceLocator

getSystemId

public String getSystemId()

Get the systemId of the module containing the expression

Specified by:

getSystemId in interface SourceLocator

getPublicId

public final String getPublicId()

Get the publicId of the module containing the expression (to satisfy the SourceLocator interface)

Specified by:

getPublicId in interface SourceLocator

getExecutable

public Executable getExecutable()

Get the executable containing this expression

Specified by:

getExecutable in interface Container

getLocationProvider

public LocationProvider getLocationProvider()

Get the LocationProvider allowing location identifiers to be resolved.

Specified by:

getLocationProvider in interface Container

simplify

public Expression simplify(StaticContext env)
                    throws XPathException

Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression). The default implementation does nothing.

Specified by:

simplify in interface Expression

Returns:

the simplified expression

Throws:

XPathException - if an error is discovered during expression rewriting

promote

public Expression promote(PromotionOffer offer)
                   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.

Specified by:

promote in interface 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

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

doPromotion

public final Expression doPromotion(Expression subexpression,
                                    PromotionOffer offer)
                             throws XPathException

Promote a subexpression if possible, and if the expression was changed, carry out housekeeping to reset the static properties and correct the parent pointers in the tree

Throws:

XPathException

getSpecialProperties

public final int getSpecialProperties()

Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.

Specified by:

getSpecialProperties in interface Expression

Returns:

a set of flags indicating static properties of this expression

computeStaticProperties

public final void computeStaticProperties()

Compute the static properties. This should only be done once for each expression.

resetStaticProperties

public final void resetStaticProperties()

Reset the static properties. This should be done whenever the expression is changed in a way that might affect the properties. It causes the properties to be recomputed next time they are needed.

computeCardinality

protected abstract int computeCardinality()

computeSpecialProperties

public int computeSpecialProperties()

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.

Specified by:

getCardinality in interface Expression

Returns:

one of the values Cardinality.ONE_OR_MORE, Cardinality.ZERO_OR_MORE, Cardinality.EXACTLY_ONE, Cardinality.ZERO_OR_ONE, Cardinality.EMPTY. This default implementation returns ZERO_OR_MORE (which effectively gives no information).

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 XPathContext.VARIABLES and XPathContext.CURRENT_NODE. 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.

Specified by:

getDependencies in interface Expression

Returns:

a set of bit-significant flags identifying the dependencies of the expression

computeDependencies

public int computeDependencies()

Compute the dependencies of an expression, as the union of the dependencies of its subexpressions. (This is overridden for path expressions and filter expressions, where the dependencies of a subexpression are not all propogated). This method should be called only once, to compute the dependencies; after that, getDependencies should be used.

Returns:

the depencies, as a bit-mask

getIntrinsicDependencies

public int getIntrinsicDependencies()

Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions. For example, position() has an intrinsic dependency on the context position, while (position()+1) does not. The default implementation of the method returns 0, indicating "no dependencies".

Returns:

a set of bit-significant flags identifying the "intrinsic" dependencies. The flags are documented in class net.sf.saxon.value.StaticProperty

iterateSubExpressions

public Iterator 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.

Specified by:

iterateSubExpressions in interface Expression

Returns:

an iterator containing the sub-expressions of this expression

suppressValidation

public void suppressValidation(int validationMode)

Suppress validation on contained element constructors, on the grounds that the parent element is already performing validation. The default implementation does nothing.

checkPermittedContents

public void checkPermittedContents(SchemaType parentType,
                                   StaticContext env,
                                   boolean whole)
                            throws XPathException

Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type. It's always OK to say yes, since the check will be repeated at run-time. The process of checking element and attribute constructors against the content model of a complex type also registers the type of content expected of those constructors, so the static validation can continue recursively.

Specified by:

checkPermittedContents in interface 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

markTailFunctionCalls

public boolean markTailFunctionCalls()

Mark tail-recursive calls on stylesheet functions. For most expressions, this does nothing.

Returns:

true if a tail recursive call was found and if this call accounts for the whole of the value.

getSlotsUsed

public int[] getSlotsUsed()

Get the local variables (identified by their slot numbers) on which this expression depends. Should only be called if the caller has established that there is a dependency on local variables.

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.

Specified by:

evaluateItem in interface 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

evaluateAsString

public String 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.

Specified by:

evaluateAsString in interface 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?

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.

Specified by:

iterate in interface 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. This returns false if the value is the empty sequence, a zero-length string, a number equal to zero, or the boolean false. Otherwise it returns true.

Specified by:

effectiveBooleanValue in interface 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

process

public void process(XPathContext context)
             throws XPathException

Process the instruction, without returning any tail calls

Specified by:

process in interface Expression

Parameters:

context - The dynamic context, giving access to the current node, the current variables, etc.

Throws:

XPathException

dynamicError

protected void dynamicError(String message,
                            XPathContext context)
                     throws DynamicError

Method used in subclasses to signal a dynamic error

Throws:

DynamicError

dynamicError

protected void dynamicError(String message,
                            String code,
                            XPathContext context)
                     throws DynamicError

Method used in subclasses to signal a dynamic error

Throws:

DynamicError

typeError

protected void typeError(String message,
                         XPathContext context)
                  throws DynamicError

Method used in subclasses to signal a runtime type error

Throws:

DynamicError

typeError

protected void typeError(String message,
                         String errorCode,
                         XPathContext context)
                  throws DynamicError

Method used in subclasses to signal a runtime type error

Throws:

DynamicError

getInstructionInfo

public InstructionInfo getInstructionInfo()

Get InstructionInfo for this expression

Specified by:

getInstructionInfo in interface InstructionInfoProvider

getConstructType

protected int getConstructType()

Get the type of this expression for use in tracing and diagnostics

Returns:

the type of expression, as enumerated in class Location

hasBadParentPointer

public boolean hasBadParentPointer()

Diagnostic method: search the tree for an expression whose parent expression is incorrectly set

getHostLanguage

public int getHostLanguage()

Get the host language (XSLT, XQuery, XPath) used to implement the code in this container

Specified by:

getHostLanguage in interface Container

Returns:

typically Configuration.XSLT or Configuration.XQUERY