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 and 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 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

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `close()` Closes files associated with this index.
    `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.
    `Document` | `document(int docID)` Returns the stored fields of the `n`th `Document` in this index.
    `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 void` | `ensureOpen()` Throws AlreadyClosedException if this IndexReader or any of its child readers is closed, otherwise returns.
    `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.
    `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.
    `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.
    `int` | `hashCode()`
    `void` | `incRef()` Expert: increments the refCount of this IndexReader instance.
    `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.
    `int` | `numDeletedDocs()` Returns the number of deleted documents.
    `abstract int` | `numDocs()` Returns the number of documents in this index.
    `void` | `registerParentReader(IndexReader reader)` Expert: This method is called by `IndexReader`s 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).
    `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 Detail

    - #### registerParentReader

          public final void registerParentReader(IndexReader reader)

      Expert: This method is called by `IndexReader`s 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.

    - #### 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()`

    - #### 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.

    - #### 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 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 `n`th `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.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 `Closeable`
      Specified by:
        `close` in interface `AutoCloseable`
      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:
        `IndexReaderContext.leaves()`

    - #### 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:
        `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). 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:
        `Terms.getSumDocFreq()`

    - #### 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:
        `Terms.getDocCount()`

    - #### 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:
        `Terms.getSumTotalTermFreq()`

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