org.apache.lucene.facet.taxonomy
Class TaxonomyReader

java.lang.Object
  org.apache.lucene.facet.taxonomy.TaxonomyReader

All Implemented Interfaces:

Closeable

Direct Known Subclasses:

DirectoryTaxonomyReader

public abstract class TaxonomyReader
extends Object
implements Closeable

TaxonomyReader is the read-only interface with which the faceted-search library uses the taxonomy during search time.

A TaxonomyReader holds a list of categories. Each category has a serial number which we call an "ordinal", and a hierarchical "path" name:

• The ordinal is an integer that starts at 0 for the first category (which is always the root category), and grows contiguously as more categories are added; Note that once a category is added, it can never be deleted.
• The path is a CategoryPath object specifying the category's position in the hierarchy.

Notes about concurrent access to the taxonomy:

An implementation must allow multiple readers to be active concurrently with a single writer. Readers follow so-called "point in time" semantics, i.e., a TaxonomyReader object will only see taxonomy entries which were available at the time it was created. What the writer writes is only available to (new) readers after the writer's commit() is called.

In faceted search, two separate indices are used: the main Lucene index, and the taxonomy. Because the main index refers to the categories listed in the taxonomy, it is important to open the taxonomy *after* opening the main index, and it is also necessary to reopen() the taxonomy after reopen()ing the main index.

This order is important, otherwise it would be possible for the main index to refer to a category which is not yet visible in the old snapshot of the taxonomy. Note that it is indeed fine for the the taxonomy to be opened after the main index - even a long time after. The reason is that once a category is added to the taxonomy, it can never be changed or deleted, so there is no danger that a "too new" taxonomy not being consistent with an older index.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary

static class

TaxonomyReader.ChildrenIterator An iterator over a category's children.

Field Summary

static int	INVALID_ORDINAL Ordinals are always non-negative, so a negative ordinal can be used to signify an error.
static int	ROOT_ORDINAL The root category (the category with the empty path) always has the ordinal 0, to which we give a name ROOT_ORDINAL.

Constructor Summary

TaxonomyReader()

Method Summary

void	close()
void	decRef() Expert: decreases the refCount of this TaxonomyReader instance.
protected abstract void	doClose() performs the actual task of closing the resources that are used by the taxonomy reader.
protected abstract TaxonomyReader	doOpenIfChanged() Implements the actual opening of a new TaxonomyReader instance if the taxonomy has changed.
protected void	ensureOpen()
TaxonomyReader.ChildrenIterator	getChildren(int ordinal) Returns an iterator over the children of the given ordinal.
abstract Map<String,String>	getCommitUserData() Retrieve user committed data.
abstract int	getOrdinal(CategoryPath categoryPath) Returns the ordinal of the category given as a path.
abstract ParallelTaxonomyArrays	getParallelTaxonomyArrays() Returns a ParallelTaxonomyArrays object which can be used to efficiently traverse the taxonomy tree.
abstract CategoryPath	getPath(int ordinal) Returns the path name of the category with the given ordinal.
int	getRefCount() Returns the current refCount for this taxonomy reader.
abstract int	getSize() Returns the number of categories in the taxonomy.
void	incRef() Expert: increments the refCount of this TaxonomyReader instance.
static <T extends TaxonomyReader> T	openIfChanged(T oldTaxoReader) If the taxonomy has changed since the provided reader was opened, open and return a new TaxonomyReader; else, return null.
boolean	tryIncRef() Expert: increments the refCount of this TaxonomyReader instance only if it has not been closed yet.

Methods inherited from class java.lang.Object

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

Field Detail

ROOT_ORDINAL

public static final int ROOT_ORDINAL

The root category (the category with the empty path) always has the ordinal 0, to which we give a name ROOT_ORDINAL. getOrdinal(CategoryPath) of an empty path will always return ROOT_ORDINAL, and getPath(int) with ROOT_ORDINAL will return the empty path.

See Also:

Constant Field Values

INVALID_ORDINAL

public static final int INVALID_ORDINAL

Ordinals are always non-negative, so a negative ordinal can be used to signify an error. Methods here return INVALID_ORDINAL (-1) in this case.

See Also:

Constant Field Values

Constructor Detail

TaxonomyReader

public TaxonomyReader()

Method Detail

openIfChanged

public static <T extends TaxonomyReader> T openIfChanged(T oldTaxoReader)
                                              throws IOException

If the taxonomy has changed since the provided reader was opened, open and return a new TaxonomyReader; else, return null. The new reader, if not null, will be the same type of reader as the one given to this method.

This method is typically far less costly than opening a fully new TaxonomyReader as it shares resources with the provided TaxonomyReader, when possible.

Throws:

IOException

doClose

protected abstract void doClose()
                         throws IOException

performs the actual task of closing the resources that are used by the taxonomy reader.

Throws:

IOException

doOpenIfChanged

protected abstract TaxonomyReader doOpenIfChanged()
                                           throws IOException

Implements the actual opening of a new TaxonomyReader instance if the taxonomy has changed.

Throws:

IOException

See Also:

openIfChanged(TaxonomyReader)

ensureOpen

protected final void ensureOpen()
                         throws AlreadyClosedException

Throws:

AlreadyClosedException - if this IndexReader is closed

close

public final void close()
                 throws IOException

Specified by:

close in interface Closeable

Throws:

IOException

decRef

public final void decRef()
                  throws IOException

Expert: decreases the refCount of this TaxonomyReader instance. If the refCount drops to 0 this taxonomy reader is closed.

Throws:

IOException

getParallelTaxonomyArrays

public abstract ParallelTaxonomyArrays getParallelTaxonomyArrays()
                                                          throws IOException

Returns a ParallelTaxonomyArrays object which can be used to efficiently traverse the taxonomy tree.

Throws:

IOException

getChildren

public TaxonomyReader.ChildrenIterator getChildren(int ordinal)
                                            throws IOException

Returns an iterator over the children of the given ordinal.

Throws:

IOException

getCommitUserData

public abstract Map<String,String> getCommitUserData()
                                              throws IOException

Retrieve user committed data.

Throws:

IOException

See Also:

TaxonomyWriter.setCommitData(Map)

getOrdinal

public abstract int getOrdinal(CategoryPath categoryPath)
                        throws IOException

Returns the ordinal of the category given as a path. The ordinal is the category's serial number, an integer which starts with 0 and grows as more categories are added (note that once a category is added, it can never be deleted).

Returns:

the category's ordinal or INVALID_ORDINAL if the category wasn't foun.

Throws:

IOException

getPath

public abstract CategoryPath getPath(int ordinal)
                              throws IOException

Returns the path name of the category with the given ordinal.

Throws:

IOException

getRefCount

public final int getRefCount()

Returns the current refCount for this taxonomy reader.

getSize

public abstract int getSize()

Returns the number of categories in the taxonomy. Note that the number of categories returned is often slightly higher than the number of categories inserted into the taxonomy; This is because when a category is added to the taxonomy, its ancestors are also added automatically (including the root, which always get ordinal 0).

incRef

public final void incRef()

Expert: increments the refCount of this TaxonomyReader instance. RefCounts can be used to determine when a taxonomy reader can be closed safely, i.e. as soon as there are no more references. Be sure to always call a corresponding decRef(), in a finally clause; otherwise the reader may never be closed.

tryIncRef

public final boolean tryIncRef()

Expert: increments the refCount of this TaxonomyReader instance only if it has not been closed yet. Returns true on success.