Package net.sf.saxon.expr.instruct

## Class TraceExpression

- java.lang.Object
- - net.sf.saxon.expr.Expression
  - - net.sf.saxon.expr.instruct.Instruction
    - - net.sf.saxon.expr.instruct.TraceExpression

- All Implemented Interfaces:
    `ExportAgent`, `TailCallReturner`, `Locatable`, `IdentityComparable`, `Traceable`

      public class TraceExpression
      extends Instruction
      implements Traceable

  A wrapper expression used to trace expressions in XSLT and XQuery.

- - ### Field Summary

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

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

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `TraceExpression​(Expression child)` | Create a trace expression that traces execution of a given child expression

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `checkForUpdatingSubexpressions()` | Check to ensure that this expression does not contain any inappropriate updating subexpressions.
    `Expression` | `copy​(RebindingMap rebindings)` | Copy an expression.
    `Item` | `evaluateItem​(XPathContext context)` | Evaluate an expression as a single item.
    `void` | `evaluatePendingUpdates​(XPathContext context, PendingUpdateList pul)` | Evaluate an updating expression, adding the results to a Pending Update List.
    `void` | `export​(ExpressionPresenter out)` | Export the expression structure.
    `Expression` | `getBody()` |
    `int` | `getCardinality()` | Determine the static cardinality of the expression.
    `Expression` | `getChild()` |
    `int` | `getDependencies()` | Determine which aspects of the context the expression depends on.
    `java.lang.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` | `getInstructionNameCode()` | Get the namecode of the instruction for use in diagnostics
    `ItemType` | `getItemType()` | Get the item type of the items returned by evaluating this instruction
    `int` | `getNetCost()` | Return the estimated cost of evaluating an expression.
    `java.util.Iterator<java.lang.String>` | `getProperties()` | Get an iterator over all the properties available.
    `java.lang.Object` | `getProperty​(java.lang.String name)` | Get a named property of the instruction/expression
    `java.lang.String` | `getStreamerName()` | Get the (partial) name of a class that supports streaming of this kind of expression
    `boolean` | `isUpdatingExpression()` | Determine whether this is an updating expression as defined in the XQuery update specification
    `boolean` | `isVacuousExpression()` | Determine whether this is a vacuous expression as defined in the XQuery update specification
    `SequenceIterator` | `iterate​(XPathContext context)` | Return an Iterator to iterate over the values of a sequence.
    `boolean` | `mayCreateNewNodes()` | Determine whether this instruction potentially creates new nodes.
    `java.lang.Iterable<Operand>` | `operands()` | Get the immediate sub-expressions of this expression, with information about the relationship of each expression to its parent expression.
    `TailCall` | `processLeavingTail​(Outputter output, XPathContext context)` | Execute this instruction, with the possibility of returning tail calls if there are any.
    `void` | `setProperty​(java.lang.String name, java.lang.Object value)` | Set a named property of the instruction/expression
    `java.lang.String` | `toShortString()` | Produce a short string identifying the expression for use in error messages

    - ### Methods inherited from class net.sf.saxon.expr.instruct.Instruction

      `alwaysCreatesNewNodes, assembleParams, assembleTunnelParams, computeCardinality, computeSpecialProperties, dynamicError, evaluateAsString, getSourceLocator, isInstruction, isXSLT, process, someOperandCreatesNewNodes`

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

      `addToPathMap, adoptChildExpression, allowExtractingCommonSubexpressions, checkedOperands, checkPermittedContents, computeDependencies, computeHashCode, computeStaticProperties, dynamicError, effectiveBooleanValue, equals, explain, getConfiguration, getCost, getEvaluationMethod, getExtraProperty, getIntegerBounds, getInterpretedExpression, getIntrinsicDependencies, getLocalRetainedStaticContext, getLocation, getObjectName, getPackageData, getParentExpression, getRetainedStaticContext, getScopingExpression, getSlotsUsed, getSpecialProperties, getStaticBaseURI, getStaticBaseURIString, getStaticType, getStaticUType, getTracingTag, hasCompatibleStaticContext, hashCode, hasSpecialProperty, hasVariableBinding, identityHashCode, implementsStaticTypeCheck, isCallOn, isEqual, isIdentical, isLiftable, isMultiThreaded, isStaticPropertiesKnown, isSubtreeExpression, markTailFunctionCalls, operandList, operandSparseList, optimize, optimizeChildren, prepareForStreaming, resetLocalStaticProperties, restoreParentPointers, setEvaluationMethod, setExtraProperty, setFiltered, setFlattened, setLocation, setParentExpression, setRetainedStaticContext, setRetainedStaticContextLocally, setRetainedStaticContextThoroughly, setStaticProperty, simplify, simplifyChildren, staticTypeCheck, suppressValidation, toPattern, toString, typeCheck, typeCheckChildren, typeError, unordered, verifyParentPointers`

    - ### Methods inherited from class java.lang.Object

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

    - ### Methods inherited from interface net.sf.saxon.expr.Locatable

      `getLocation`

    - ### Methods inherited from interface net.sf.saxon.trace.Traceable

      `gatherProperties, getObjectName`

- - ### Constructor Detail

    - #### TraceExpression

          public TraceExpression​(Expression child)

      Create a trace expression that traces execution of a given child expression
      Parameters:
        `child` - the expression to be traced. This will be available to the TraceListener as the value of the "expression" property of the InstructionInfo.

  - ### Method Detail

    - #### getChild

          public Expression getChild()

    - #### getBody

          public Expression getBody()

    - #### operands

          public java.lang.Iterable<Operand> operands()

      Description copied from class: `Expression`
      Get the immediate sub-expressions of this expression, with information about the relationship of each expression to its parent expression. Default implementation works off the results of iterateSubExpressions()

      If the expression is a Callable, then it is required that the order of the operands returned by this function is the same as the order of arguments supplied to the corresponding call() method.

      Specified by:
        `operands` in class `Instruction`
      Returns:
        an iterator containing the sub-expressions of this expression

    - #### setProperty

          public void setProperty​(java.lang.String name,
                                  java.lang.Object value)

      Set a named property of the instruction/expression
      Parameters:
        `name` - the name of the property
        `value` - the value of the property

    - #### getProperty

          public java.lang.Object getProperty​(java.lang.String name)

      Get a named property of the instruction/expression
      Overrides:
        `getProperty` in class `Expression`
      Parameters:
        `name` - the name of the property
      Returns:
        the value of the property

    - #### getProperties

          public java.util.Iterator<java.lang.String> getProperties()

      Get an iterator over all the properties available. The values returned by the iterator will be of type String, and each string can be supplied as input to the getProperty() method to retrieve the value of the property.
      Overrides:
        `getProperties` in class `Expression`
      Returns:
        an iterator over the properties

    - #### getExpressionName

          public java.lang.String getExpressionName()

      Get a name identifying the kind of expression, in terms meaningful to a user.
      Overrides:
        `getExpressionName` in class `Instruction`
      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.

    - #### getStreamerName

          public java.lang.String getStreamerName()

      Get the (partial) name of a class that supports streaming of this kind of expression
      Overrides:
        `getStreamerName` in class `Expression`
      Returns:
        the partial name of a class that can be instantiated to provide streaming support in Saxon-EE, or null if there is no such class

    - #### copy

          public Expression copy​(RebindingMap rebindings)

      Description copied from class: `Expression`
      Copy an expression. This makes a deep copy.
      Specified by:
        `copy` in class `Expression`
      Parameters:
        `rebindings` - a mutable list of (old binding, new binding) pairs that is used to update the bindings held in any local variable references that are copied.
      Returns:
        the copy of the original expression

    - #### isUpdatingExpression

          public boolean isUpdatingExpression()

      Determine whether this is an updating expression as defined in the XQuery update specification
      Overrides:
        `isUpdatingExpression` in class `Expression`
      Returns:
        true if this is an updating expression

    - #### isVacuousExpression

          public boolean isVacuousExpression()

      Determine whether this is a vacuous expression as defined in the XQuery update specification
      Overrides:
        `isVacuousExpression` in class `Expression`
      Returns:
        true if this expression is vacuous

    - #### checkForUpdatingSubexpressions

          public void checkForUpdatingSubexpressions()
                                              throws XPathException

      Check to ensure that this expression does not contain any inappropriate updating subexpressions. This check is overridden for those expressions that permit updating subexpressions.
      Overrides:
        `checkForUpdatingSubexpressions` in class `Expression`
      Throws:
        `XPathException` - if the expression has a non-permitted updating subexpression

    - #### getImplementationMethod

          public int getImplementationMethod()

      Description copied from class: `Instruction`
      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. For instructions this is the process() method.
      Overrides:
        `getImplementationMethod` in class `Instruction`
      Returns:
        the implementation method, for example `Expression.ITERATE_METHOD` or `Expression.EVALUATE_METHOD` or `Expression.PROCESS_METHOD`

    - #### processLeavingTail

          public TailCall processLeavingTail​(Outputter output,
                                             XPathContext context)
                                      throws XPathException

      Execute this instruction, with the possibility of returning tail calls if there are any. This outputs the trace information via the registered TraceListener, and invokes the instruction being traced.
      Specified by:
        `processLeavingTail` in interface `TailCallReturner`
      Specified by:
        `processLeavingTail` in class `Instruction`
      Parameters:
        `output` - the destination for the result
        `context` - the dynamic execution context
      Returns:
        either null, or a tail call that the caller must invoke on return
      Throws:
        `XPathException` - if execution of the target expression fails

    - #### getItemType

          public ItemType getItemType()

      Get the item type of the items returned by evaluating this instruction
      Overrides:
        `getItemType` in class `Instruction`
      Returns:
        the static item type of the instruction

    - #### 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.
      Overrides:
        `getCardinality` in class `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 `StaticProperty.DEPENDS_ON_CONTEXT_ITEM` and `StaticProperty.DEPENDS_ON_CURRENT_ITEM`. 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.
      Overrides:
        `getDependencies` in class `Expression`
      Returns:
        a set of bit-significant flags identifying the dependencies of the expression

    - #### mayCreateNewNodes

          public final boolean mayCreateNewNodes()

      Determine whether this instruction potentially creates new nodes.
      Overrides:
        `mayCreateNewNodes` in class `Instruction`
      Returns:
        true if the instruction creates new nodes under some input conditions (or if it can't be proved that it doesn't)

    - #### getNetCost

          public int getNetCost()

      Return the estimated cost of evaluating an expression. For a TraceExpression we return zero, because ideally we don't want trace expressions to affect optimization decisions.
      Overrides:
        `getNetCost` in class `Instruction`
      Returns:
        zero

    - #### 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.
      Overrides:
        `evaluateItem` in class `Instruction`
      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.
      Overrides:
        `iterate` in class `Instruction`
      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

    - #### getInstructionNameCode

          public int getInstructionNameCode()

      Description copied from class: `Instruction`
      Get the namecode of the instruction for use in diagnostics
      Overrides:
        `getInstructionNameCode` in class `Instruction`
      Returns:
        a code identifying the instruction: typically but not always the fingerprint of a name in the XSLT namespace

    - #### export

          public void export​(ExpressionPresenter out)
                      throws XPathException

      Export the expression structure. The abstract expression tree is written to the supplied output destination. Note: trace expressions are omitted from the generated SEF file.
      Specified by:
        `export` in interface `ExportAgent`
      Specified by:
        `export` in class `Expression`
      Parameters:
        `out` - the expression presenter used to display the structure
      Throws:
        `XPathException` - if the export fails, for example if an expression is found that won't work in the target environment.

    - #### evaluatePendingUpdates

          public void evaluatePendingUpdates​(XPathContext context,
                                             PendingUpdateList pul)
                                      throws XPathException

      Evaluate an updating expression, adding the results to a Pending Update List. The default implementation of this method, which is used for non-updating expressions, throws an UnsupportedOperationException
      Overrides:
        `evaluatePendingUpdates` in class `Expression`
      Parameters:
        `context` - the XPath dynamic evaluation context
        `pul` - the pending update list to which the results should be written
      Throws:
        `XPathException` - if evaluation fails

    - #### toShortString

          public java.lang.String toShortString()

      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