Package org.apache.lucene.index

Class IndexReader

java.lang.Object
org.apache.lucene.index.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.

  • Nested Class Summary

    Nested Classes
    Modifier and Type
    Class
    Description
    static interface 
    IndexReader.CacheHelper
    A utility class that gives hooks in order to help build a cache based on the data that is contained in this index.
    static final class 
    IndexReader.CacheKey
    A cache key identifying a resource that is being cached on.
    static interface 
    IndexReader.ClosedListener
    A listener that is called when a resource gets closed.

  • Method Summary

    Modifier and Type
    Method
    Description
    final void
    close()
    Closes files associated with this index.
    final void
    decRef()
    Expert: decreases the refCount of this IndexReader instance.
    abstract int
    docFreq(Term term)
    Returns the number of documents containing the term.
    protected abstract void
    doClose()
    Implements close.
    final Document
    document(int docID)
    Returns the stored fields of the nth Document in this index.
    final Document
    document(int docID, Set<String> fieldsToLoad)
    Like document(int) but only loads the specified fields.
    abstract void
    document(int docID, StoredFieldVisitor visitor)
    Expert: visits the fields of a stored document, for custom processing/loading of each field.
    protected final void
    ensureOpen()
    Throws AlreadyClosedException if this IndexReader or any of its child readers is closed, otherwise returns.
    final boolean
    equals(Object obj)
    abstract IndexReaderContext
    getContext()
    Expert: Returns the root IndexReaderContext for this IndexReader's sub-reader tree.
    abstract int
    getDocCount(String field)
    Returns the number of documents that have at least one term for this field.
    abstract IndexReader.CacheHelper
    getReaderCacheHelper()
    Optional method: Return a IndexReader.CacheHelper that can be used to cache based on the content of this reader.
    final int
    getRefCount()
    Expert: returns the current refCount for this reader
    abstract long
    getSumDocFreq(String field)
    Returns the sum of TermsEnum.docFreq() for all terms in this field.
    abstract long
    getSumTotalTermFreq(String field)
    Returns the sum of TermsEnum.totalTermFreq() for all terms in this field.
    final Terms
    getTermVector(int docID, String field)
    Retrieve term vector for this document and field, or null if term vectors were not indexed.
    abstract Fields
    getTermVectors(int docID)
    Retrieve term vectors for this document, or null if term vectors were not indexed.
    boolean
    hasDeletions()
    Returns true if any documents have been deleted.
    final int
    hashCode()
    final void
    incRef()
    Expert: increments the refCount of this IndexReader instance.
    final List<LeafReaderContext>
    leaves()
    Returns the reader's leaves, or itself if this reader is atomic.
    abstract int
    maxDoc()
    Returns one greater than the largest possible document number.
    protected void
    notifyReaderClosedListeners()
    For test framework use only.
    final int
    numDeletedDocs()
    Returns the number of deleted documents.
    abstract int
    numDocs()
    Returns the number of documents in this index.
    final void
    registerParentReader(IndexReader reader)
    Expert: This method is called by IndexReaders which wrap other readers (e.g.
    abstract long
    totalTermFreq(Term term)
    Returns the total number of occurrences of term across all documents (the sum of the freq() for each doc that has this term).
    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.

    Methods inherited from class java.lang.Object

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

  • Method Details

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

    • notifyReaderClosedListeners

      protected void notifyReaderClosedListeners() throws IOException
      For test framework use only.
      Throws:
      IOException
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    • 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:

    • 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

      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:

    • ensureOpen

      protected final void ensureOpen() throws AlreadyClosedException
      Throws AlreadyClosedException if this IndexReader or any of its child readers is closed, otherwise returns.
      Throws:
      AlreadyClosedException

    • equals

      public final boolean equals(Object obj)

      IndexReader subclasses are not allowed to implement equals/hashCode, so methods are declared final.

      Overrides:
      equals in class Object

    • hashCode

      public final int hashCode()

      IndexReader subclasses are not allowed to implement equals/hashCode, so methods are declared final.

      Overrides:
      hashCode in class Object

    • 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.

      NOTE: This operation may run in O(maxDoc). Implementations that can't return this number in constant-time should cache it.

    • 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.

      NOTE: This operation may run in O(maxDoc).

    • document

      public abstract void document(int docID, StoredFieldVisitor visitor) throws IOException
      Expert: visits the fields of a stored document, for custom processing/loading of each field. If you simply want to load all fields, use document(int). If you want to load a subset, use DocumentStoredFieldVisitor.
      Throws:
      IOException

    • 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 MultiBits.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

    • document

      public final Document document(int docID, Set<String> fieldsToLoad) throws IOException
      Like document(int) but only loads the specified fields. Note that this is simply sugar for DocumentStoredFieldVisitor(Set).
      Throws:
      IOException

    • 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 AutoCloseable
      Specified by:
      close in interface Closeable
      Throws:
      IOException - if there is a low-level IO error

    • doClose

      protected abstract void doClose() throws IOException
      Implements close.
      Throws:
      IOException

    • 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.

    • leaves

      public final List<LeafReaderContext> leaves()
      Returns the reader's leaves, or itself if this reader is atomic. This is a convenience method calling this.getContext().leaves().
      See Also:

    • getReaderCacheHelper

      public abstract IndexReader.CacheHelper getReaderCacheHelper()
      Optional method: Return a IndexReader.CacheHelper that can be used to cache based on the content of this reader. Two readers that have different data or different sets of deleted documents will be considered different.

      A return value of null indicates that this reader is not suited for caching, which is typically the case for short-lived wrappers that alter the content of the wrapped reader.

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

    • 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:

    • 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). Note that, like other term measures, this measure does not take deleted documents into account.
      Throws:
      IOException

    • getSumDocFreq

      public abstract long getSumDocFreq(String field) throws IOException
      Returns the sum of TermsEnum.docFreq() for all terms in this field. Note that, just like other term measures, this measure does not take deleted documents into account.
      Throws:
      IOException
      See Also:

    • getDocCount

      public abstract int getDocCount(String field) throws IOException
      Returns the number of documents that have at least one term for this field. Note that, just like other term measures, this measure does not take deleted documents into account.
      Throws:
      IOException
      See Also:

    • getSumTotalTermFreq

      public abstract long getSumTotalTermFreq(String field) throws IOException
      Returns the sum of TermsEnum.totalTermFreq() for all terms in this field. Note that, just like other term measures, this measure does not take deleted documents into account.
      Throws:
      IOException
      See Also: