Package org.apache.lucene.facet.taxonomy

Class TaxonomyReader

  • All Implemented Interfaces:
    java.io.Closeable, java.lang.AutoCloseable
    Direct Known Subclasses:
    DirectoryTaxonomyReader
    public abstract class TaxonomyReader
extends java.lang.Object
implements java.io.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.

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description
      static class  TaxonomyReader.ChildrenIterator
      An iterator over a category's children.

    • Field Summary

      Fields 
      Modifier and Type Field Description
      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

      Constructors 
      Constructor Description
      TaxonomyReader()
      Sole constructor.

    • 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(FacetLabel) 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()
        Sole constructor.

    • Method Detail

      • openIfChanged

        public static <T extends TaxonomyReader> T openIfChanged​(T oldTaxoReader)
                                                  throws java.io.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:
        java.io.IOException

      • close

        public final void close()
                 throws java.io.IOException
        Specified by:
        close in interface java.lang.AutoCloseable
        Specified by:
        close in interface java.io.Closeable
        Throws:
        java.io.IOException

      • decRef

        public final void decRef()
                  throws java.io.IOException
        Expert: decreases the refCount of this TaxonomyReader instance. If the refCount drops to 0 this taxonomy reader is closed.
        Throws:
        java.io.IOException

      • getParallelTaxonomyArrays

        public abstract ParallelTaxonomyArrays getParallelTaxonomyArrays()
                                                          throws java.io.IOException
        Returns a ParallelTaxonomyArrays object which can be used to efficiently traverse the taxonomy tree.
        Throws:
        java.io.IOException

      • getChildren

        public TaxonomyReader.ChildrenIterator getChildren​(int ordinal)
                                            throws java.io.IOException
        Returns an iterator over the children of the given ordinal.
        Throws:
        java.io.IOException

      • getCommitUserData

        public abstract java.util.Map<java.lang.String,​java.lang.String> getCommitUserData()
                                                                                  throws java.io.IOException
        Retrieve user committed data.
        Throws:
        java.io.IOException
        See Also:
        TaxonomyWriter.setCommitData(Map)

      • getOrdinal

        public abstract int getOrdinal​(FacetLabel categoryPath)
                        throws java.io.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:
        java.io.IOException

      • getOrdinal

        public int getOrdinal​(java.lang.String dim,
                      java.lang.String[] path)
               throws java.io.IOException
        Returns ordinal for the dim + path.
        Throws:
        java.io.IOException

      • getPath

        public abstract FacetLabel getPath​(int ordinal)
                            throws java.io.IOException
        Returns the path name of the category with the given ordinal.
        Throws:
        java.io.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.