net.sf.saxon.expr
Class Expression

java.lang.Object
  net.sf.saxon.expr.Expression

All Implemented Interfaces:

Serializable, SourceLocator, Container, EvaluableItem, SequenceIterable, InstructionInfoProvider

Direct Known Subclasses:

Assignation, AxisExpression, BinaryExpression, CompareToIntegerConstant, ContextItemExpression, ErrorExpression, FilterExpression, FunctionCall, IfExpression, IndexedFilterExpression, IndexedLookupExpression, Instruction, IntegerRangeTest, IsLastExpression, Literal, NumberInstruction, PathExpression, PatternMatchExpression, PatternSponsor, PositionRange, SimpleContentConstructor, SimpleExpression, SimpleMappingExpression, SingleNodeExpression, SliceExpression, SortExpression, StreamingCopy, SuppliedParameterReference, TailExpression, TupleExpression, TupleSorter, UnaryExpression, VariableReference

public abstract class Expression
extends Object
implements SequenceIterable, EvaluableItem, Serializable, InstructionInfoProvider, Container

Interface supported by an XPath expression. This includes both compile-time and run-time methods.

See Also:

Serialized Form

Field Summary

static int	EVALUATE_METHOD
static int	ITERATE_METHOD
static int	PROCESS_METHOD

Constructor Summary

Expression()

Method Summary

void	adoptChildExpression(Expression child) Set up a parent-child relationship between this expression and a given child expression.
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.
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.
abstract void	display(int level, PrintStream out, Configuration config) Diagnostic print of expression structure.
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
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
Procedure	getContainingProcedure() Get the Procedure containing this expression.
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.
abstract ItemType	getItemType(TypeHierarchy th) Determine the data type of the expression, if possible.
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.
int	markTailFunctionCalls(int nameCode, int arity) Mark tail-recursive calls on stylesheet functions.
Expression	optimize(Optimizer opt, StaticContext env, ItemType 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) Offer promotion for this subexpression.
boolean	replaceSubExpression(Expression original, Expression replacement) Replace one subexpression by a replacement subexpression
void	resetStaticProperties() Reset the static properties.
void	setLocationId(int id) Set the location ID on an expression.
void	setParentExpression(Container parent)
static void	setParentExpression(Expression child, 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.
Expression	typeCheck(StaticContext env, ItemType contextItemType) Perform type checking of an expression and its subexpressions.

Methods inherited from class java.lang.Object

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

Field Detail

EVALUATE_METHOD

public static final int EVALUATE_METHOD

See Also:

Constant Field Values

ITERATE_METHOD

public static final int ITERATE_METHOD

See Also:

Constant Field Values

PROCESS_METHOD

public static final int PROCESS_METHOD

See Also:

Constant Field Values

Constructor Detail

Expression

public Expression()

Method Detail

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.

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.

Returns:

the simplified expression

Throws:

XPathException - if an error is discovered during expression rewriting

typeCheck

public Expression typeCheck(StaticContext env,
                            ItemType contextItemType)
                     throws XPathException

Perform type checking of an expression and its subexpressions. This is the second phase of static optimization.

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.

If the implementation returns a value other than "this", then it is required to ensure that the parent pointer and location information in the returned expression have been set up correctly. It should not rely on the caller to do this, although for historical reasons many callers do so.

Parameters:

env - the static context of the expression

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:

StaticError - if an error is discovered during this phase (typically a type error)

XPathException

optimize

public Expression optimize(Optimizer opt,
                           StaticContext env,
                           ItemType contextItemType)
                    throws XPathException

Perform optimisation of an expression and its subexpressions. This is the third and final phase of static optimization.

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.

Parameters:

opt - the optimizer in use. This provides access to supporting functions; it also allows different optimization strategies to be used in different circumstances.

env - the static context of the expression

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:

StaticError - if an error is discovered during this phase (typically a type error)

XPathException

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.

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

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

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).

getItemType

public abstract 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.

Parameters:

th -

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 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

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

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.

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 EvaluableItem

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.

Specified by:

iterate in interface SequenceIterable

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.

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)
                        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.

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

Parameters:

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

Throws:

XPathException

display

public abstract void display(int level,
                             PrintStream out,
                             Configuration config)

Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Parameters:

level - indentation level for this expression

out - Output destination

config -

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.

Throws:

XPathException

setParentExpression

public void setParentExpression(Container parent)

setParentExpression

public static void setParentExpression(Expression child,
                                       Container parent)

adoptChildExpression

public void adoptChildExpression(Expression child)

Set up a parent-child relationship between this expression and a given child expression.

Note: many calls on this method are now redundant, but are kept in place for "belt-and-braces" reasons. The rule is that an implementation of simplify(), typeCheck(), or optimize() that returns a value other than "this" is required to set the location information and parent pointer in the new child expression. However, in the past this was often left to the caller, which did it by calling this method, either unconditionally on return from one of these methods, or after testing that the returned object was not the same as the original.

Parameters:

child - the child 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

getContainingProcedure

public Procedure getContainingProcedure()

Get the Procedure containing this expression. Returns null if the expression is not contained in a Procedure.

getLocationProvider

public LocationProvider getLocationProvider()

Get the LocationProvider allowing location identifiers to be resolved.

Specified by:

getLocationProvider in interface Container

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

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.

computeSpecialProperties

public int computeSpecialProperties()

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

replaceSubExpression

public boolean replaceSubExpression(Expression original,
                                    Expression replacement)

Replace one subexpression by a replacement subexpression

Specified by:

replaceSubExpression in interface Container

Parameters:

original - the original subexpression

replacement - the replacement subexpression

Returns:

true if the original subexpression is found

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.

markTailFunctionCalls

public int markTailFunctionCalls(int nameCode,
                                 int arity)

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

Returns:

0 if no tail call was found; 1 if a tail call on a different function was found; 2 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.

getInstructionInfo

public InstructionInfo getInstructionInfo()

Get InstructionInfo for this expression

Specified by:

getInstructionInfo in interface InstructionInfoProvider

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