net.sf.saxon.expr

Class ComputedExpression

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.

Field Summary
protected intlocationId
protected intstaticProperties
Method Summary
voidadoptChildExpression(Expression child)
voidcheckPermittedContents(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 intcomputeCardinality()
intcomputeDependencies()
Compute the dependencies of an expression, as the union of the dependencies of its subexpressions.
intcomputeSpecialProperties()
voidcomputeStaticProperties()
Compute the static properties.
ExpressiondoPromotion(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 voiddynamicError(String message, XPathContext context)
Method used in subclasses to signal a dynamic error
protected voiddynamicError(String message, String code, XPathContext context)
Method used in subclasses to signal a dynamic error
booleaneffectiveBooleanValue(XPathContext context)
Get the effective boolean value of the expression.
StringevaluateAsString(XPathContext context)
Evaluate an expression as a String.
ItemevaluateItem(XPathContext context)
Evaluate an expression as a single item.
intgetCardinality()
Determine the static cardinality of the expression.
intgetColumnNumber()
Get the column number of the expression
protected intgetConstructType()
Get the type of this expression for use in tracing and diagnostics
intgetDependencies()
Determine which aspects of the context the expression depends on.
ExecutablegetExecutable()
Get the executable containing this expression
intgetHostLanguage()
Get the host language (XSLT, XQuery, XPath) used to implement the code in this container
intgetImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
InstructionInfogetInstructionInfo()
Get InstructionInfo for this expression
intgetIntrinsicDependencies()
Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions.
intgetLineNumber()
Get the line number of the expression
intgetLocationId()
Get the location ID of the expression
LocationProvidergetLocationProvider()
Get the LocationProvider allowing location identifiers to be resolved.
ContainergetParentExpression()
Get the expression that immediately contains this expression.
StringgetPublicId()
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.
intgetSpecialProperties()
Get the static properties of this expression (other than its type).
StringgetSystemId()
Get the systemId of the module containing the expression
booleanhasBadParentPointer()
Diagnostic method: search the tree for an expression whose parent expression is incorrectly set
SequenceIteratoriterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.
IteratoriterateSubExpressions()
Get the immediate sub-expressions of this expression.
booleanmarkTailFunctionCalls()
Mark tail-recursive calls on stylesheet functions.
voidprocess(XPathContext context)
Process the instruction, without returning any tail calls
Expressionpromote(PromotionOffer offer)
Offer promotion for this subexpression.
voidresetStaticProperties()
Reset the static properties.
voidsetLocationId(int id)
Set the location ID on an expression.
voidsetParentExpression(Container parent)
Expressionsimplify(StaticContext env)
Simplify an expression.
voidsuppressValidation(int validationMode)
Suppress validation on contained element constructors, on the grounds that the parent element is already performing validation.
protected voidtypeError(String message, XPathContext context)
Method used in subclasses to signal a runtime type error
protected voidtypeError(String message, String errorCode, XPathContext context)
Method used in subclasses to signal a runtime type error

Field Detail

locationId

protected int locationId

staticProperties

protected int staticProperties

Method Detail

adoptChildExpression

public void adoptChildExpression(Expression child)

checkPermittedContents

public 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. 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.

computeCardinality

protected abstract int computeCardinality()

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

computeSpecialProperties

public int computeSpecialProperties()

computeStaticProperties

public final void computeStaticProperties()
Compute the static properties. This should only be done once for each expression.

doPromotion

public final 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

dynamicError

protected void dynamicError(String message, XPathContext context)
Method used in subclasses to signal a dynamic error

dynamicError

protected void dynamicError(String message, String code, XPathContext context)
Method used in subclasses to signal a dynamic error

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)
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.

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 String evaluateAsString(XPathContext context)
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.

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?

evaluateItem

public Item evaluateItem(XPathContext context)
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.

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

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.

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).

getColumnNumber

public int getColumnNumber()
Get the column number of the expression

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

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.

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

getExecutable

public Executable getExecutable()
Get the executable containing this expression

getHostLanguage

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

Returns: typically XSLT or XQUERY

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.

getInstructionInfo

public InstructionInfo getInstructionInfo()
Get InstructionInfo for this expression

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

getLineNumber

public int getLineNumber()
Get the line number of the expression

getLocationId

public final int getLocationId()
Get the location ID of the expression

getLocationProvider

public LocationProvider getLocationProvider()
Get the LocationProvider allowing location identifiers to be resolved.

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.

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

getPublicId

public final String getPublicId()
Get the publicId of the module containing the expression (to satisfy the SourceLocator interface)

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.

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.

Returns: a set of flags indicating static properties of this expression

getSystemId

public String getSystemId()
Get the systemId of the module containing the expression

hasBadParentPointer

public boolean hasBadParentPointer()
Diagnostic method: search the tree for an expression whose parent expression is incorrectly set

iterate

public SequenceIterator iterate(XPathContext context)
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.

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

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.

Returns: an iterator containing the sub-expressions of this expression

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.

process

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

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

promote

public Expression promote(PromotionOffer offer)
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.

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: net.sf.saxon.trans.XPathException if any error is detected

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.

setLocationId

public void setLocationId(int id)
Set the location ID on an expression.

setParentExpression

public void setParentExpression(Container parent)

simplify

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

Returns: the simplified expression

Throws: XPathException if an error is discovered during expression rewriting

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.

typeError

protected void typeError(String message, XPathContext context)
Method used in subclasses to signal a runtime type error

typeError

protected void typeError(String message, String errorCode, XPathContext context)
Method used in subclasses to signal a runtime type error