Class ComputedExpression

java.lang.Object
net.sf.saxon.expr.ComputedExpression
All Implemented Interfaces:
Serializable, SourceLocator, Container, Expression, InstructionInfoProvider
Direct Known Subclasses:
Assignation, AxisExpression, BinaryExpression, CompareToIntegerConstant, 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:
  • Field Details

    • staticProperties

      protected int staticProperties
    • locationId

      protected int locationId
  • Constructor Details

    • ComputedExpression

      public ComputedExpression()
  • Method Details

    • 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)
    • 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
    • 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.
      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
      Parameters:
      env - the static context
      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
    • 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.
    • 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(int nameCode, int arity)
      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, String code, XPathContext context) throws DynamicError
      Method used in subclasses to signal a dynamic 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