com.saxonica.ee.stream.adjunct

Class StreamingAdjunct

java.lang.Object

com.saxonica.ee.stream.adjunct.StreamingAdjunct

Direct Known Subclasses:

ApplyImportsAdjunct, ApplyTemplatesAdjunct, AtomicSequenceConverterAdjunct, AxisExpressionAdjunct, CallTemplateAdjunct, CodepointsToStringAdjunct, ComposingAdjunct, ContextItemExprAdjunct, CopyAdjunct, DeepEqualAdjunct, DistinctValuesAdjunct, EmptyTextNodeRemoverAdjunct, FoldAdjunct, ForExpressionAdjunct, ForkAdjunct, GeneralComparisonAdjunct, IndexOfAdjunct, InspectionAdjunct, IsLastExprAdjunct, LastAdjunct, LetExpressionAdjunct, LiteralAdjunct, MapEntryAdjunct, MergeInstrAdjunct, MinimaxAdjunct, NewMapAdjunct, NextIterationAdjunct, NextMatchAdjunct, ParentConstructorAdjunct, ReverseAdjunct, RootAdjunct, RootExpressionAdjunct, SimpleNodeConstructorAdjunct, SingletonAtomizerAdjunct, SnapshotFnAdjunct, SortExpressionAdjunct, StreamInstrAdjunct, SubscriptExpressionAdjunct, SumAdjunct, TransmissionAdjunct, UseAttributeSetsAdjunct, VariableReferenceAdjunct, VennExpressionAdjunct

public class StreamingAdjunct
extends Object

This abstract class supplements an Expression with methods used at compile-time to support streaming, by generating a Watch that allows the expression to be evaluated in push mode.

The default implementation is adequate only for expressions that take single items as their input. For other kinds of expression, either a WatchMaker or FeedMaker, or both, need to be supplied. If the input sequence is atomized (as with the sum() function), then only a FeedMaker is required; a TypedValueWatch will automatically be added to the Streaming Route to perform atomization of the supplied sequence. For expressions that do not perform atomization, both a WatchMaker (for use when the input is striding or consuming) and a FeedMaker (for use when it is climbing or grounded) need to be provided.

Neither method should be called in cases where streamability analysis has decided that the expression is not streamable. In such cases the methods may throw an exception.

There is a fallback implementation that allows any expression to be evaluated as a Feed, but it involves buffering the input in memory, and performing an implicit copy-of() to create a grounded copy, so it is only really suitable for the case where the input is a single atomic value.

Constructor Summary

Constructors

Constructor and Description
StreamingAdjunct()

Method Summary

Methods

Modifier and Type	Method and Description
PostureAndSweep	computeStreamability(boolean allowExtensions, ContextItemStaticInfo contextInfo, List<String> reasons) Compute the posture and sweep of this expression as defined in the W3C streamability specifications.
Configuration	getConfiguration() Get the Saxon Configuration
protected FeedMaker	getDefaultFeedMaker(int arg) Get a FeedMaker which will generate a feed to handle streamed input of operand "arg".
Expression	getExpression() Get the expression being processed by this StreamingAdjunct
FeedMaker	getFeedMaker(int arg) Get a FeedMaker, a factory class that creates a Feed to evaluate this expression in streaming mode.
WatchMaker	getWatchMaker(boolean forGrouping) Get a WatchMaker, a factory class that creates a Watch to evaluate this expression in streaming mode.
void	setConfiguration(Configuration config) Set the Saxon Configuration
void	setExpression(Expression expression) Set the expression being processed by this StreamingAdjunct
Pattern	toStreamingPattern(Configuration config) Convert this expression to a streaming pattern (a pattern used internally to match nodes during push processing of an event stream)

Methods inherited from class java.lang.Object

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

Constructor Detail

StreamingAdjunct

public StreamingAdjunct()

Method Detail

setConfiguration

public void setConfiguration(Configuration config)

Set the Saxon Configuration

Parameters:

config - the Configuration

getConfiguration

public Configuration getConfiguration()

Get the Saxon Configuration

Returns:

the configuration

setExpression

public void setExpression(Expression expression)

Set the expression being processed by this StreamingAdjunct

Parameters:

expression - the expression being processed

getExpression

public Expression getExpression()

Get the expression being processed by this StreamingAdjunct

Returns:

the expression being processed

computeStreamability

public PostureAndSweep computeStreamability(boolean allowExtensions,
                                   ContextItemStaticInfo contextInfo,
                                   List<String> reasons)

Compute the posture and sweep of this expression as defined in the W3C streamability specifications. This must be implemented in all subclasses that don't use the general streamability rules. The implementation should have the side-effect of computing the streamability of all subexpressions, recursively.

Parameters:

allowExtensions - if false, the definition of "guaranteed streamability" in the W3C specification is used. If true, Saxon extensions are permitted, which make some

contextInfo - Information about the context item type and posture

reasons - the caller may supply a list, in which case the implementation may add to this; the contents are human-readable messages explaining why the expression is not streamable.

Returns:

the posture and sweep of the expression

toStreamingPattern

public Pattern toStreamingPattern(Configuration config)

Convert this expression to a streaming pattern (a pattern used internally to match nodes during push processing of an event stream)

Parameters:

config - the Saxon configuration

Returns:

the equivalent pattern if conversion succeeds; otherwise null

getWatchMaker

public WatchMaker getWatchMaker(boolean forGrouping)
                         throws XPathException

Get a WatchMaker, a factory class that creates a Watch to evaluate this expression in streaming mode. This method will only be called if the getImplementationMethod() method of the expression includes the value Expression.WATCH_METHOD

Parameters:

forGrouping - true if the selected items are being grouped

Returns:

a Watchmaker to evaluate the expression, or null if it is not streamable with respect to the specificed argument.

Throws:

XPathException - in the event of a failure

getFeedMaker

public FeedMaker getFeedMaker(int arg)
                       throws XPathException

Get a FeedMaker, a factory class that creates a Feed to evaluate this expression in streaming mode. In many cases the StreamingAdjunct is itself a FeedMaker, in which case it returns itself. But where an expression has several operands, it will typically create different feeds depending on which argument is the streamed input, and in such cases the individual StreamingAdjunct makes the decision which feed to use, by returning an appropriate FeedMaker.

Parameters:

arg - identifies the argument with respect to which the expression is to be inverted: usually 0 to indicate the first argument is the value that is streamed. Argument numbering is the same as the sequence of arguments/operands returned by the method Expression.operands()

Returns:

a FeedMaker to evaluate the expression, or null if it is not streamable with respect to the specificed argument.

Throws:

XPathException - in the event of a failure

getDefaultFeedMaker

protected FeedMaker getDefaultFeedMaker(int arg)
                                 throws XPathException

Get a FeedMaker which will generate a feed to handle streamed input of operand "arg".

Parameters:

arg - identifies which operand of the expression is the streamed (consuming) input

Returns:

a FeedMaker

Throws:

XPathException - if a dynamic error occurs