org.apache.lucene.index

Class IndexReader

  • All Implemented Interfaces:
    Closeable, AutoCloseable
    Direct Known Subclasses:
    CompositeReader, LeafReader


    public abstract class IndexReader
    extends Object
    implements Closeable
    IndexReader is an abstract class, providing an interface for accessing a point-in-time view of an index. Any changes made to the index via IndexWriter will not be visible until a new IndexReader is opened. It's best to use DirectoryReader.open(IndexWriter) to obtain an IndexReader, if your IndexWriter is in-process. When you need to re-open to see changes to the index, it's best to use DirectoryReader.openIfChanged(DirectoryReader) since the new reader will share resources with the previous one when possible. Search of an index is done entirely through this abstract interface, so that any subclass which implements it is searchable.

    There are two different types of IndexReaders:

    • LeafReader: These indexes do not consist of several sub-readers, they are atomic. They support retrieval of stored fields, doc values, terms, and postings.
    • CompositeReader: Instances (like DirectoryReader) of this reader can only be used to get stored fields from the underlying LeafReaders, but it is not possible to directly retrieve postings. To do that, get the sub-readers via CompositeReader.getSequentialSubReaders().

    IndexReader instances for indexes on disk are usually constructed with a call to one of the static DirectoryReader.open() methods, e.g. DirectoryReader.open(org.apache.lucene.store.Directory). DirectoryReader implements the CompositeReader interface, it is not possible to directly get postings.

    For efficiency, in this API documents are often referred to via document numbers, non-negative integers which each name a unique document in the index. These document numbers are ephemeral -- they may change as documents are added to and deleted from an index. Clients should thus not rely on a given document having the same number between sessions.

    NOTE: IndexReader instances are completely thread safe, meaning multiple threads can call any of its methods, concurrently. If your application requires external synchronization, you should not synchronize on the IndexReader instance; use your own (non-Lucene) objects instead.

    • Method Detail

      • registerParentReader

        public final void registerParentReader(IndexReader reader)
        Expert: This method is called by IndexReaders which wrap other readers (e.g. CompositeReader or FilterLeafReader) to register the parent at the child (this reader) on construction of the parent. When this reader is closed, it will mark all registered parents as closed, too. The references to parent readers are weak only, so they can be GCed once they are no longer in use.
      • getRefCount

        public final int getRefCount()
        Expert: returns the current refCount for this reader
      • incRef

        public final void incRef()
        Expert: increments the refCount of this IndexReader instance. RefCounts are used to determine when a 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. Note that close() simply calls decRef(), which means that the IndexReader will not really be closed until decRef() has been called for all outstanding references.
        See Also:
        decRef(), tryIncRef()
      • tryIncRef

        public final boolean tryIncRef()
        Expert: increments the refCount of this IndexReader instance only if the IndexReader has not been closed yet and returns true iff the refCount was successfully incremented, otherwise false. If this method returns false the reader is either already closed or is currently being closed. Either way this reader instance shouldn't be used by an application unless true is returned.

        RefCounts are used to determine when a 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. Note that close() simply calls decRef(), which means that the IndexReader will not really be closed until decRef() has been called for all outstanding references.

        See Also:
        decRef(), incRef()
      • decRef

        public final void decRef()
                          throws IOException
        Expert: decreases the refCount of this IndexReader instance. If the refCount drops to 0, then this reader is closed. If an exception is hit, the refCount is unchanged.
        Throws:
        IOException - in case an IOException occurs in doClose()
        See Also:
        incRef()
      • getTermVectors

        public abstract Fields getTermVectors(int docID)
                                       throws IOException
        Retrieve term vectors for this document, or null if term vectors were not indexed. The returned Fields instance acts like a single-document inverted index (the docID will be 0).
        Throws:
        IOException
      • getTermVector

        public final Terms getTermVector(int docID,
                                         String field)
                                  throws IOException
        Retrieve term vector for this document and field, or null if term vectors were not indexed. The returned Fields instance acts like a single-document inverted index (the docID will be 0).
        Throws:
        IOException
      • numDocs

        public abstract int numDocs()
        Returns the number of documents in this index.
      • maxDoc

        public abstract int maxDoc()
        Returns one greater than the largest possible document number. This may be used to, e.g., determine how big to allocate an array which will have an element for every document number in an index.
      • numDeletedDocs

        public final int numDeletedDocs()
        Returns the number of deleted documents.
      • document

        public final Document document(int docID)
                                throws IOException
        Returns the stored fields of the nth Document in this index. This is just sugar for using DocumentStoredFieldVisitor.

        NOTE: for performance reasons, this method does not check if the requested document is deleted, and therefore asking for a deleted document may yield unspecified results. Usually this is not required, however you can test if the doc is deleted by checking the Bits returned from MultiFields.getLiveDocs(org.apache.lucene.index.IndexReader). NOTE: only the content of a field is returned, if that field was stored during indexing. Metadata like boost, omitNorm, IndexOptions, tokenized, etc., are not preserved.

        Throws:
        CorruptIndexException - if the index is corrupt
        IOException - if there is a low-level IO error
      • hasDeletions

        public boolean hasDeletions()
        Returns true if any documents have been deleted. Implementers should consider overriding this method if maxDoc() or numDocs() are not constant-time operations.
      • close

        public final void close()
                         throws IOException
        Closes files associated with this index. Also saves any new deletions to disk. No other methods should be called after this has been called.
        Specified by:
        close in interface Closeable
        Specified by:
        close in interface AutoCloseable
        Throws:
        IOException - if there is a low-level IO error
      • getContext

        public abstract IndexReaderContext getContext()
        Expert: Returns the root IndexReaderContext for this IndexReader's sub-reader tree.

        Iff this reader is composed of sub readers, i.e. this reader being a composite reader, this method returns a CompositeReaderContext holding the reader's direct children as well as a view of the reader tree's atomic leaf contexts. All sub- IndexReaderContext instances referenced from this readers top-level context are private to this reader and are not shared with another context tree. For example, IndexSearcher uses this API to drive searching by one atomic leaf reader at a time. If this reader is not composed of child readers, this method returns an LeafReaderContext.

        Note: Any of the sub-CompositeReaderContext instances referenced from this top-level context do not support CompositeReaderContext.leaves(). Only the top-level context maintains the convenience leaf-view for performance reasons.

      • getCoreCacheKey

        public Object getCoreCacheKey()
        Expert: Returns a key for this IndexReader, so CachingWrapperFilter can find it again. This key must not have equals()/hashCode() methods, so "equals" means "identical".
      • getCombinedCoreAndDeletesKey

        public Object getCombinedCoreAndDeletesKey()
        Expert: Returns a key for this IndexReader that also includes deletions, so CachingWrapperFilter can find it again. This key must not have equals()/hashCode() methods, so "equals" means "identical".
      • docFreq

        public abstract int docFreq(Term term)
                             throws IOException
        Returns the number of documents containing the term. This method returns 0 if the term or field does not exists. This method does not take into account deleted documents that have not yet been merged away.
        Throws:
        IOException
        See Also:
        TermsEnum.docFreq()
      • totalTermFreq

        public abstract long totalTermFreq(Term term)
                                    throws IOException
        Returns the total number of occurrences of term across all documents (the sum of the freq() for each doc that has this term). This will be -1 if the codec doesn't support this measure. Note that, like other term measures, this measure does not take deleted documents into account.
        Throws:
        IOException
      • getDocCount

        public abstract int getDocCount(String field)
                                 throws IOException
        Returns the number of documents that have at least one term for this field, or -1 if this measure isn't stored by the codec. Note that, just like other term measures, this measure does not take deleted documents into account.
        Throws:
        IOException
        See Also:
        Terms.getDocCount()