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## 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.

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