net.sf.saxon.expr

Class AxisExpression

java.lang.Object

net.sf.saxon.expr.Expression

net.sf.saxon.expr.AxisExpression

All Implemented Interfaces:

IdentityComparable

public final class AxisExpression
extends Expression

An AxisExpression is always obtained by simplifying a PathExpression. It represents a PathExpression that starts either at the context node, and uses a simple node-test with no filters. For example "*", "title", "./item", "@*", or "ancestor::chapter*".

An AxisExpression delivers nodes in axis order (not in document order). To get nodes in document order, in the case of a reverse axis, the expression should be wrapped in a call on reverse().

Field Summary

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

EFFECTIVE_BOOLEAN_VALUE, EVALUATE_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, MAX_SEQUENCE_LENGTH, MAX_STRING_LENGTH, PROCESS_METHOD, staticProperties, UNBOUNDED_LOWER, UNBOUNDED_UPPER, UPDATE_METHOD, WATCH_METHOD

Constructor Summary

Constructors

Constructor and Description
AxisExpression(byte axis, NodeTest nodeTest) Constructor for an AxisExpression whose origin is the context item

Method Summary

Methods

Modifier and Type	Method and Description
PathMap.PathMapNodeSet	addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet) Add a representation of this expression to a PathMap.
int	computeCardinality() Determine the cardinality of the result of this expression
int	computeSpecialProperties() Get the static properties of this expression (other than its type).
Expression	copy() Copy an expression.
boolean	equals(Object other) Is this expression the same as another expression?
void	export(ExpressionPresenter destination) Diagnostic print of expression structure.
byte	getAxis() Get the axis
ItemType	getContextItemType() Get the static type of the context item for this AxisExpression.
int	getCost() Return the estimated cost of evaluating an expression.
ExpressionCompiler	getExpressionCompiler() Return the compiler of the Axis expression
String	getExpressionName() Get a name identifying the kind of expression, in terms meaningful to a user.
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
int	getIntrinsicDependencies() Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions.
ItemType	getItemType() Determine the data type of the items returned by this expression
NodeTest	getNodeTest() Get the NodeTest.
Set<Expression>	getPreconditions() Find any necessary preconditions for the satisfaction of this expression as a set of boolean expressions to be evaluated on the context node
protected StreamingAdjunct	getStreamingAdjunct() Get a class that supports streamed evaluation of this expression
int	hashCode() get HashCode for comparing two expressions
boolean	isContextPossiblyUndefined() Ask whether there is a possibility that the context item will be undefined
boolean	isSubtreeExpression() Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.
AxisIterator	iterate(NodeInfo origin) Iterate the axis from a given starting node, without regard to context
AxisIterator	iterate(XPathContext context) Evaluate the path-expression in a given context to return a NodeSet
Expression	optimize(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo) Perform optimisation of an expression and its subexpressions.
void	setAxis(byte axis) Set the axis
Expression	simplify() Simplify an expression
Pattern	toPattern(Configuration config, boolean is30) Convert this expression to an equivalent XSLT pattern
String	toShortString() Produce a short string identifying the expression for use in error messages
String	toString() Represent the expression as a string.
Expression	typeCheck(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo) Type-check the expression

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

adoptChildExpression, checkedOperands, checkForUpdatingSubexpressions, checkPermittedContents, clearStreamabilityData, computeDependencies, computeStaticProperties, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluateItem, evaluatePendingUpdates, explain, getCardinality, getConfiguration, getConstructType, getDependencies, getEvaluationMethod, getHostLanguage, getIntegerBounds, getLocation, getNetCost, getObjectName, getPackageData, getParentExpression, getPosture, getPostureAndSweepIfKnown, getProperties, getProperty, getRetainedStaticContext, getSlotsUsed, getSpecialProperties, getStaticBaseURI, getStaticBaseURIString, getStreamability, getSweep, getSystemId, hasVariableBinding, identityHashCode, implementsStaticTypeCheck, isCallOn, isCallOnSystemFunction, isIdentical, isUpdatingExpression, isVacuousExpression, iterateEvents, makeStreamingAdjunct, markTailFunctionCalls, operandList, operands, operandSparseList, optimizeChildren, optimizeForType, process, promote, promoteChildren, resetLocalStaticProperties, restoreParentPointers, setEvaluationMethod, setFiltered, setFlattened, setLocation, setParentExpression, setPostureAndSweep, setRetainedStaticContext, setRetainedStaticContextLocally, setStaticProperty, simplifyChildren, staticTypeCheck, suppressValidation, toStreamingPattern, typeCheckChildren, typeError, unordered

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

AxisExpression

public AxisExpression(byte axis,
              NodeTest nodeTest)

Constructor for an AxisExpression whose origin is the context item

Parameters:

axis - The axis to be used in this AxisExpression: relevant constants are defined in class AxisInfo.

nodeTest - The conditions to be satisfied by selected nodes. May be null, indicating that any node on the axis is acceptable

See Also:

AxisInfo

Method Detail

setAxis

public void setAxis(byte axis)

Set the axis

Parameters:

axis - the new axis

getExpressionName

public String getExpressionName()

Get a name identifying the kind of expression, in terms meaningful to a user.

Overrides:

getExpressionName in class Expression

Returns:

a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in explain() output displaying the expression.

simplify

public Expression simplify()
                    throws XPathException

Simplify an expression

Overrides:

simplify in class Expression

Returns:

the simplified expression (or the original if unchanged, or if modified in-situ)

Throws:

XPathException - if an error is discovered during expression rewriting

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                   ContextItemStaticInfo contextInfo)
                     throws XPathException

Type-check the expression

Overrides:

typeCheck in class Expression

Parameters:

visitor - an expression visitor

contextInfo - Information available statically about the context item: whether it is (possibly) absent; its static type; its streaming posture.

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)

getContextItemType

public ItemType getContextItemType()

Get the static type of the context item for this AxisExpression. May be null if not known.

Returns:

the statically-inferred type, or null if not known

optimize

public Expression optimize(ExpressionVisitor visitor,
                  ContextItemStaticInfo contextInfo)

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

contextInfo - 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

getCost

public int getCost()

Return the estimated cost of evaluating an expression. This is a very crude measure based on the syntactic form of the expression (we have no knowledge of data values). We take the cost of evaluating a simple scalar comparison or arithmetic expression as 1 (one), and we assume that a sequence has length 5. The resulting estimates may be used, for example, to reorder the predicates in a filter expression so cheaper predicates are evaluated first.

Overrides:

getCost in class Expression

Returns:

an estimate of the gross cost of evaluating the expression, including the cost of evaluating its operands.

equals

public boolean equals(Object other)

Is this expression the same as another expression?

Overrides:

equals in class Object

hashCode

public int hashCode()

get HashCode for comparing two expressions

Overrides:

hashCode in class Object

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

computeSpecialProperties

public int computeSpecialProperties()

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.

Overrides:

computeSpecialProperties in class Expression

Returns:

the special properties, as a bit-significant integer

getItemType

public final ItemType getItemType()

Determine the data type of the items returned by this expression

Specified by:

getItemType in class Expression

Returns:

Type.NODE or a subtype, based on the NodeTest in the axis step, plus information about the content type if this is known from schema analysis

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".

Overrides:

getIntrinsicDependencies in class Expression

Returns:

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

computeCardinality

public final int computeCardinality()

Determine the cardinality of the result 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

isSubtreeExpression

public boolean isSubtreeExpression()

Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.

Overrides:

isSubtreeExpression in class Expression

Returns:

true if the expression has no dependencies on the context node, or if the only dependencies on the context node are downward selections using the self, child, descendant, attribute, and namespace axes.

getAxis

public byte getAxis()

Get the axis

Returns:

the axis number, for example AxisInfo.CHILD

getNodeTest

public NodeTest getNodeTest()

Get the NodeTest. Returns null if the AxisExpression can return any node.

Returns:

the node test, or null if all nodes are returned

addToPathMap

public PathMap.PathMapNodeSet addToPathMap(PathMap pathMap,
                                  PathMap.PathMapNodeSet pathMapNodeSet)

Add a representation of this expression to a PathMap. The PathMap captures a map of the nodes visited by an expression in a source tree.

Overrides:

addToPathMap in class Expression

Parameters:

pathMap - the PathMap to which the expression should be added

pathMapNodeSet - the PathMapNodeSet to which the paths embodied in this expression should be added

Returns:

the pathMapNode representing the focus established by this expression, in the case where this expression is the first operand of a path expression or filter expression

isContextPossiblyUndefined

public boolean isContextPossiblyUndefined()

Ask whether there is a possibility that the context item will be undefined

Returns:

true if this is a possibility

toPattern

public Pattern toPattern(Configuration config,
                boolean is30)
                  throws XPathException

Convert this expression to an equivalent XSLT pattern

Overrides:

toPattern in class Expression

Parameters:

config - the Saxon configuration

is30 - true if this is XSLT 3.0

Returns:

the equivalent pattern

Throws:

XPathException - if conversion is not possible

getImplementationMethod

public int getImplementationMethod()

Description copied from class: Expression

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.

Specified by:

getImplementationMethod in class Expression

Returns:

the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

iterate

public AxisIterator iterate(XPathContext context)
                     throws XPathException

Evaluate the path-expression in a given context to return a NodeSet

Overrides:

iterate in class Expression

Parameters:

context - the evaluation context

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

iterate

public AxisIterator iterate(NodeInfo origin)

Iterate the axis from a given starting node, without regard to context

Parameters:

origin - the starting node

Returns:

the iterator over the axis

getExpressionCompiler

public ExpressionCompiler getExpressionCompiler()

Return the compiler of the Axis expression

Overrides:

getExpressionCompiler in class Expression

Returns:

the relevant ExpressionCompiler

export

public void export(ExpressionPresenter destination)

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

Specified by:

export in class Expression

Parameters:

destination - the expression presenter used to display the structure

toString

public String toString()

Represent the expression as a string. The resulting string will be a valid XPath 3.0 expression with no dependencies on namespace bindings other than the binding of the prefix "xs" to the XML Schema namespace.

Overrides:

toString in class Expression

Returns:

the expression as a string in XPath 3.0 syntax

toShortString

public String toShortString()

Description copied from class: Expression

Produce a short string identifying the expression for use in error messages

Overrides:

toShortString in class Expression

Returns:

a short string, sufficient to identify the expression

getStreamingAdjunct

protected StreamingAdjunct getStreamingAdjunct()

Get a class that supports streamed evaluation of this expression

Overrides:

getStreamingAdjunct in class Expression

Returns:

the relevant StreamingAdjunct, or null if none is available

getPreconditions

public Set<Expression> getPreconditions()

Find any necessary preconditions for the satisfaction of this expression as a set of boolean expressions to be evaluated on the context node

Returns:

A set of conditions, or null if none have been computed