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## org.apache.lucene.index
Class IndexReader

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

All Implemented Interfaces:
  Closeable, Cloneable

Direct Known Subclasses:
  FilterIndexReader, MultiReader, ParallelReader, SegmentReader

    public abstract class IndexReader

extends Object
implements Cloneable, Closeable

IndexReader is an abstract class, providing an interface for accessing an index. Search of an index is done entirely through this abstract interface, so that any subclass which implements it is searchable.

Concrete subclasses of IndexReader are usually constructed with a call to one of the static `open()` methods, e.g. `open(Directory, boolean)`.

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.

An IndexReader can be opened on a directory for which an IndexWriter is opened already, but it cannot be used to delete documents from the index then.

NOTE: for backwards API compatibility, several methods are not listed as abstract, but have no useful implementations in this base class and instead always throw UnsupportedOperationException. Subclasses are strongly encouraged to override these methods, but in many cases may not need to.

NOTE: as of 2.4, it's possible to open a read-only IndexReader using the static open methods that accept the boolean readOnly parameter. Such a reader has better concurrency as it's not necessary to synchronize on the isDeleted method. You must specify false if you want to make changes with the resulting IndexReader.

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
`static interface` | `IndexReader.ReaderClosedListener` A custom listener that's invoked when the IndexReader is closed.

Field Summary
`protected boolean` | `hasChanges`

Constructor Summary
`protected` | `IndexReader()`

Method Summary
`protected void` | `acquireWriteLock()` Deprecated. Write support will be removed in Lucene 4.0.
`void` | `addReaderClosedListener(IndexReader.ReaderClosedListener listener)` Expert: adds a `IndexReader.ReaderClosedListener`.
`Object` | `clone()` Efficiently clones the IndexReader (sharing most internal state).
`IndexReader` | `clone(boolean openReadOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `clone()` instead.
`void` | `close()` Closes files associated with this index.
`protected void` | `commit()` Deprecated. Write support will be removed in Lucene 4.0.
`void` | `commit(Map<String,String> commitUserData)` Deprecated. Write support will be removed in Lucene 4.0.
`void` | `decRef()` Expert: decreases the refCount of this IndexReader instance.
`void` | `deleteDocument(int docNum)` Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead
`int` | `deleteDocuments(Term term)` Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead
`Directory` | `directory()` Returns the directory associated with this index.
`abstract int` | `docFreq(Term t)` Returns the number of documents containing the term `t`.
`protected abstract void` | `doClose()` Implements close.
`protected abstract void` | `doCommit(Map<String,String> commitUserData)` Deprecated. Write support will be removed in Lucene 4.0.
`Document` | `document(int n)` Returns the stored fields of the `n`th `Document` in this index.
`abstract Document` | `document(int n, FieldSelector fieldSelector)` Get the `Document` at the `n` th position.
`protected abstract void` | `doDelete(int docNum)` Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead
`protected IndexReader` | `doOpenIfChanged()` If the index has changed since it was opened, open and return a new reader; else, return `null`.
`protected IndexReader` | `doOpenIfChanged(boolean openReadOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `doOpenIfChanged()` instead
`protected IndexReader` | `doOpenIfChanged(IndexCommit commit)` If the index has changed since it was opened, open and return a new reader; else, return `null`.
`protected IndexReader` | `doOpenIfChanged(IndexWriter writer, boolean applyAllDeletes)` If the index has changed since it was opened, open and return a new reader; else, return `null`.
`protected abstract void` | `doSetNorm(int doc, String field, byte value)` Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.
`protected abstract void` | `doUndeleteAll()` Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.
`protected void` | `ensureOpen()`
`void` | `flush()` Deprecated. Write support will be removed in Lucene 4.0.
`void` | `flush(Map<String,String> commitUserData)` Deprecated. Write support will be removed in Lucene 4.0.
`Map<String,String>` | `getCommitUserData()` Deprecated. Call `getIndexCommit()` and then call `IndexCommit.getUserData()`.
`static Map<String,String>` | `getCommitUserData(Directory directory)` Deprecated. Call `getIndexCommit()` on an open IndexReader, and then call `IndexCommit.getUserData()`.
`Object` | `getCoreCacheKey()` Expert
`static long` | `getCurrentVersion(Directory directory)` Deprecated. Use `getVersion()` on an opened IndexReader.
`Object` | `getDeletesCacheKey()` Expert.
`abstract FieldInfos` | `getFieldInfos()` Get the `FieldInfos` describing all fields in this reader.
`IndexCommit` | `getIndexCommit()` Expert: return the IndexCommit that this reader has opened.
`int` | `getRefCount()` Expert: returns the current refCount for this reader
`IndexReader[]` | `getSequentialSubReaders()` Expert: returns the sequential sub readers that this reader is logically composed of.
`abstract TermFreqVector` | `getTermFreqVector(int docNumber, String field)` Return a term frequency vector for the specified document and field.
`abstract void` | `getTermFreqVector(int docNumber, String field, TermVectorMapper mapper)` Load the Term Vector into a user-defined data structure instead of relying on the parallel arrays of the `TermFreqVector`.
`abstract void` | `getTermFreqVector(int docNumber, TermVectorMapper mapper)` Map all the term vectors for all fields in a Document
`abstract TermFreqVector[]` | `getTermFreqVectors(int docNumber)` Return an array of term frequency vectors for the specified document.
`int` | `getTermInfosIndexDivisor()` For IndexReader implementations that use TermInfosReader to read terms, this returns the current indexDivisor as specified when the reader was opened.
`long` | `getUniqueTermCount()` Returns the number of unique terms (across all fields) in this reader.
`long` | `getVersion()` Version number when this IndexReader was opened.
`abstract boolean` | `hasDeletions()` Returns true if any documents have been deleted
`boolean` | `hasNorms(String field)` Returns true if there are norms stored for this field.
`void` | `incRef()` Expert: increments the refCount of this IndexReader instance.
`static boolean` | `indexExists(Directory directory)` Returns `true` if an index exists at the specified directory.
`boolean` | `isCurrent()` Check whether any new changes have occurred to the index since this reader was opened.
`abstract boolean` | `isDeleted(int n)` Returns true if document n has been deleted
`boolean` | `isOptimized()` Deprecated. Check segment count using `getSequentialSubReaders()` instead.
`static long` | `lastModified(Directory directory2)` Deprecated. If you need to track commit time of an index, you can store it in the commit data (see `IndexWriter.commit(Map)`
`static Collection<IndexCommit>` | `listCommits(Directory dir)` Returns all commit points that exist in the Directory.
`abstract int` | `maxDoc()` Returns one greater than the largest possible document number.
`abstract byte[]` | `norms(String field)` Returns the byte-encoded normalization factor for the named field of every document.
`abstract void` | `norms(String field, byte[] bytes, int offset)` Reads the byte-encoded normalization factor for the named field of every document.
`int` | `numDeletedDocs()` Returns the number of deleted documents.
`abstract int` | `numDocs()` Returns the number of documents in this index.
`static IndexReader` | `open(Directory directory)` Returns a IndexReader reading the index in the given Directory, with readOnly=true.
`static IndexReader` | `open(Directory directory, boolean readOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory)` instead
`static IndexReader` | `open(Directory directory, IndexDeletionPolicy deletionPolicy, boolean readOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory)` instead
`static IndexReader` | `open(Directory directory, IndexDeletionPolicy deletionPolicy, boolean readOnly, int termInfosIndexDivisor)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory,int)` instead
`static IndexReader` | `open(Directory directory, int termInfosIndexDivisor)` Expert: Returns a IndexReader reading the index in the given Director and given termInfosIndexDivisor
`static IndexReader` | `open(IndexCommit commit)` Expert: returns an IndexReader reading the index in the given `IndexCommit`.
`static IndexReader` | `open(IndexCommit commit, boolean readOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit)` instead
`static IndexReader` | `open(IndexCommit commit, IndexDeletionPolicy deletionPolicy, boolean readOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit)` instead
`static IndexReader` | `open(IndexCommit commit, IndexDeletionPolicy deletionPolicy, boolean readOnly, int termInfosIndexDivisor)` Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit,int)` instead
`static IndexReader` | `open(IndexCommit commit, int termInfosIndexDivisor)` Expert: returns an IndexReader reading the index in the given `IndexCommit` and termInfosIndexDivisor.
`static IndexReader` | `open(IndexWriter writer, boolean applyAllDeletes)` Open a near real time IndexReader from the `IndexWriter`.
`static IndexReader` | `openIfChanged(IndexReader oldReader)` If the index has changed since the provided reader was opened, open and return a new reader; else, return null.
`static IndexReader` | `openIfChanged(IndexReader oldReader, boolean readOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `openIfChanged(IndexReader)` instead
`static IndexReader` | `openIfChanged(IndexReader oldReader, IndexCommit commit)` If the IndexCommit differs from what the provided reader is searching, or the provided reader is not already read-only, open and return a new `readOnly=true` reader; else, return null.
`static IndexReader` | `openIfChanged(IndexReader oldReader, IndexWriter writer, boolean applyAllDeletes)` Expert: If there changes (committed or not) in the `IndexWriter` versus what the provided reader is searching, then open and return a new read-only IndexReader searching both committed and uncommitted changes from the writer; else, return null (though, the current implementation never returns null).
`void` | `removeReaderClosedListener(IndexReader.ReaderClosedListener listener)` Expert: remove a previously added `IndexReader.ReaderClosedListener`.
`IndexReader` | `reopen()` Deprecated. Use `openIfChanged(IndexReader)` instead
`IndexReader` | `reopen(boolean openReadOnly)` Deprecated. Write support will be removed in Lucene 4.0. Use `openIfChanged(IndexReader)` instead
`IndexReader` | `reopen(IndexCommit commit)` Deprecated. Use `openIfChanged(IndexReader,IndexCommit)` instead
`IndexReader` | `reopen(IndexWriter writer, boolean applyAllDeletes)` Deprecated. Use `openIfChanged(IndexReader,IndexWriter,boolean)` instead
`void` | `setNorm(int doc, String field, byte value)` Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.
`void` | `setNorm(int doc, String field, float value)` Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.
`abstract TermDocs` | `termDocs()` Returns an unpositioned `TermDocs` enumerator.
`TermDocs` | `termDocs(Term term)` Returns an enumeration of all the documents which contain `term`.
`abstract TermPositions` | `termPositions()` Returns an unpositioned `TermPositions` enumerator.
`TermPositions` | `termPositions(Term term)` Returns an enumeration of all the documents which contain `term`.
`abstract TermEnum` | `terms()` Returns an enumeration of all the terms in the index.
`abstract TermEnum` | `terms(Term t)` Returns an enumeration of all terms starting at a given term.
`String` | `toString()`
`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`.
`void` | `undeleteAll()` Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

Methods inherited from class java.lang.Object
`equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Field Detail

### hasChanges

    protected boolean hasChanges

Constructor Detail

### IndexReader

    protected IndexReader()

Method Detail

### addReaderClosedListener

    public final void addReaderClosedListener(IndexReader.ReaderClosedListener listener)

  Expert: adds a `IndexReader.ReaderClosedListener`. The provided listener will be invoked when this reader is closed.

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

### removeReaderClosedListener

    public final void removeReaderClosedListener(IndexReader.ReaderClosedListener listener)

  Expert: remove a previously added `IndexReader.ReaderClosedListener`.

  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 been 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()`

### toString

    public String toString()

  Overrides:
    `toString` in class `Object`

### decRef

    public final void decRef()
                      throws IOException

  Expert: decreases the refCount of this IndexReader instance. If the refCount drops to 0, then pending changes (if any) are committed to the index and this reader is closed. If an exception is hit, the refCount is unchanged.

  Throws:
    `IOException` - in case an IOException occurs in commit() or doClose()
  See Also:
    `incRef()`

### ensureOpen

    protected final void ensureOpen()
                             throws AlreadyClosedException

  Throws:
    `AlreadyClosedException` - if this IndexReader is closed

### open

    public static IndexReader open(Directory directory)
                            throws CorruptIndexException,
                                   IOException

  Returns a IndexReader reading the index in the given Directory, with readOnly=true.

  Parameters:
    `directory` - the index directory
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(Directory directory,
                                              boolean readOnly)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory)` instead

  Returns an IndexReader reading the index in the given Directory. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `directory` - the index directory
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    public static IndexReader open(IndexWriter writer,
                                   boolean applyAllDeletes)
                            throws CorruptIndexException,
                                   IOException

  Open a near real time IndexReader from the `IndexWriter`.

  Parameters:
    `writer` - The IndexWriter to open from
    `applyAllDeletes` - If true, all buffered deletes will be applied (made visible) in the returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter) so that they will be applied in the future. Applying deletes can be costly, so if your app can tolerate deleted documents being returned you might gain some performance by passing false.
  Returns:
    The new IndexReader
  Throws:
    `CorruptIndexException`
    `IOException` - if there is a low-level IO error
  See Also:
    `openIfChanged(IndexReader,IndexWriter,boolean)`
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### open

    public static IndexReader open(IndexCommit commit)
                            throws CorruptIndexException,
                                   IOException

  Expert: returns an IndexReader reading the index in the given `IndexCommit`.

  Parameters:
    `commit` - the commit point to open
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(IndexCommit commit,
                                              boolean readOnly)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit)` instead

  Expert: returns an IndexReader reading the index in the given `IndexCommit`. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `commit` - the commit point to open
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(Directory directory,
                                              IndexDeletionPolicy deletionPolicy,
                                              boolean readOnly)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory)` instead

  Expert: returns an IndexReader reading the index in the given Directory, with a custom `IndexDeletionPolicy`. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `directory` - the index directory
    `deletionPolicy` - a custom deletion policy (only used if you use this reader to perform deletes or to set norms); see `IndexWriter` for details.
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(Directory directory,
                                              IndexDeletionPolicy deletionPolicy,
                                              boolean readOnly,
                                              int termInfosIndexDivisor)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(Directory,int)` instead

  Expert: returns an IndexReader reading the index in the given Directory, with a custom `IndexDeletionPolicy`. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `directory` - the index directory
    `deletionPolicy` - a custom deletion policy (only used if you use this reader to perform deletes or to set norms); see `IndexWriter` for details.
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
    `termInfosIndexDivisor` - Subsamples which indexed terms are loaded into RAM. This has the same effect as `IndexWriter.setTermIndexInterval(int)` except that setting must be done at indexing time while this setting can be set per reader. When set to N, then one in every N*termIndexInterval terms in the index is loaded into memory. By setting this to a value > 1 you can reduce memory usage, at the expense of higher latency when loading a TermInfo. The default value is 1. Set this to -1 to skip loading the terms index entirely.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(IndexCommit commit,
                                              IndexDeletionPolicy deletionPolicy,
                                              boolean readOnly)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit)` instead

  Expert: returns an IndexReader reading the index in the given Directory, using a specific commit and with a custom `IndexDeletionPolicy`. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `commit` - the specific `IndexCommit` to open; see `listCommits(org.apache.lucene.store.Directory)` to list all commits in a directory
    `deletionPolicy` - a custom deletion policy (only used if you use this reader to perform deletes or to set norms); see `IndexWriter` for details.
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    @Deprecated
    public static IndexReader open(IndexCommit commit,
                                              IndexDeletionPolicy deletionPolicy,
                                              boolean readOnly,
                                              int termInfosIndexDivisor)
                            throws CorruptIndexException,
                                   IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `open(IndexCommit,int)` instead

  Expert: returns an IndexReader reading the index in the given Directory, using a specific commit and with a custom `IndexDeletionPolicy`. You should pass readOnly=true, since it gives much better concurrent performance, unless you intend to do write operations (delete documents or change norms) with the reader.

  Parameters:
    `commit` - the specific `IndexCommit` to open; see `listCommits(org.apache.lucene.store.Directory)` to list all commits in a directory
    `deletionPolicy` - a custom deletion policy (only used if you use this reader to perform deletes or to set norms); see `IndexWriter` for details.
    `readOnly` - true if no changes (deletions, norms) will be made with this IndexReader
    `termInfosIndexDivisor` - Subsamples which indexed terms are loaded into RAM. This has the same effect as `IndexWriter.setTermIndexInterval(int)` except that setting must be done at indexing time while this setting can be set per reader. When set to N, then one in every N*termIndexInterval terms in the index is loaded into memory. By setting this to a value > 1 you can reduce memory usage, at the expense of higher latency when loading a TermInfo. The default value is 1. Set this to -1 to skip loading the terms index entirely. This is only useful in advanced situations when you will only .next() through all terms; attempts to seek will hit an exception.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    public static IndexReader open(Directory directory,
                                   int termInfosIndexDivisor)
                            throws CorruptIndexException,
                                   IOException

  Expert: Returns a IndexReader reading the index in the given Director and given termInfosIndexDivisor

  Parameters:
    `directory` - the index directory
    `termInfosIndexDivisor` - Subsamples which indexed terms are loaded into RAM. This has the same effect as `IndexWriterConfig.setTermIndexInterval(int)` except that setting must be done at indexing time while this setting can be set per reader. When set to N, then one in every N*termIndexInterval terms in the index is loaded into memory. By setting this to a value > 1 you can reduce memory usage, at the expense of higher latency when loading a TermInfo. The default value is 1. Set this to -1 to skip loading the terms index entirely.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### open

    public static IndexReader open(IndexCommit commit,
                                   int termInfosIndexDivisor)
                            throws CorruptIndexException,
                                   IOException

  Expert: returns an IndexReader reading the index in the given `IndexCommit` and termInfosIndexDivisor.

  Parameters:
    `commit` - the commit point to open
    `termInfosIndexDivisor` - Subsamples which indexed terms are loaded into RAM. This has the same effect as `IndexWriterConfig.setTermIndexInterval(int)` except that setting must be done at indexing time while this setting can be set per reader. When set to N, then one in every N*termIndexInterval terms in the index is loaded into memory. By setting this to a value > 1 you can reduce memory usage, at the expense of higher latency when loading a TermInfo. The default value is 1. Set this to -1 to skip loading the terms index entirely.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### openIfChanged

    public static IndexReader openIfChanged(IndexReader oldReader)
                                     throws IOException

  If the index has changed since the provided reader was opened, open and return a new reader; else, return null. The new reader, if not null, will be the same type of reader as the previous one, ie an NRT reader will open a new NRT reader, a MultiReader will open a new MultiReader, etc.

  This method is typically far less costly than opening a fully new `IndexReader` as it shares resources (for example sub-readers) with the provided `IndexReader`, when possible.

  The provided reader is not closed (you are responsible for doing so); if a new reader is returned you also must eventually close it. Be sure to never close a reader while other threads are still using it; see `SearcherManager` to simplify managing this.

  If a new reader is returned, it's safe to make changes (deletions, norms) with it. All shared mutable state with the old reader uses "copy on write" semantics to ensure the changes are not seen by other readers.

  Returns:
    null if there are no changes; else, a new IndexReader instance which you must eventually close
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### openIfChanged

    @Deprecated
    public static IndexReader openIfChanged(IndexReader oldReader,
                                                       boolean readOnly)
                                     throws IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `openIfChanged(IndexReader)` instead

  If the index has changed since the provided reader was opened, open and return a new reader, with the specified `readOnly`; else, return null.

  Throws:
    `IOException`
  See Also:
    `openIfChanged(IndexReader)`

### openIfChanged

    public static IndexReader openIfChanged(IndexReader oldReader,
                                            IndexCommit commit)
                                     throws IOException

  If the IndexCommit differs from what the provided reader is searching, or the provided reader is not already read-only, open and return a new `readOnly=true` reader; else, return null.

  Throws:
    `IOException`
  See Also:
    `openIfChanged(IndexReader)`

### openIfChanged

    public static IndexReader openIfChanged(IndexReader oldReader,
                                            IndexWriter writer,
                                            boolean applyAllDeletes)
                                     throws IOException

  Expert: If there changes (committed or not) in the `IndexWriter` versus what the provided reader is searching, then open and return a new read-only IndexReader searching both committed and uncommitted changes from the writer; else, return null (though, the current implementation never returns null).

  This provides "near real-time" searching, in that changes made during an `IndexWriter` session can be quickly made available for searching without closing the writer nor calling `IndexWriter.commit()`.

  It's near real-time because there is no hard guarantee on how quickly you can get a new reader after making changes with IndexWriter. You'll have to experiment in your situation to determine if it's fast enough. As this is a new and experimental feature, please report back on your findings so we can learn, improve and iterate.

  The very first time this method is called, this writer instance will make every effort to pool the readers that it opens for doing merges, applying deletes, etc. This means additional resources (RAM, file descriptors, CPU time) will be consumed.

  For lower latency on reopening a reader, you should call `IndexWriterConfig.setMergedSegmentWarmer(org.apache.lucene.index.IndexWriter.IndexReaderWarmer)` to pre-warm a newly merged segment before it's committed to the index. This is important for minimizing index-to-search delay after a large merge.

  If an addIndexes* call is running in another thread, then this reader will only search those segments from the foreign index that have been successfully copied over, so far.

  NOTE: Once the writer is closed, any outstanding readers may continue to be used. However, if you attempt to reopen any of those readers, you'll hit an `AlreadyClosedException`.

  Parameters:
    `writer` - The IndexWriter to open from
    `applyAllDeletes` - If true, all buffered deletes will be applied (made visible) in the returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter) so that they will be applied in the future. Applying deletes can be costly, so if your app can tolerate deleted documents being returned you might gain some performance by passing false.
  Returns:
    IndexReader that covers entire index plus all changes made so far by this IndexWriter instance, or null if there are no new changes
  Throws:
    `IOException`
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### reopen

    @Deprecated
    public IndexReader reopen()
                       throws CorruptIndexException,
                              IOException

  Deprecated. Use `openIfChanged(IndexReader)` instead

  Refreshes an IndexReader if the index has changed since this instance was (re)opened.

  Opening an IndexReader is an expensive operation. This method can be used to refresh an existing IndexReader to reduce these costs. This method tries to only load segments that have changed or were created after the IndexReader was (re)opened.

  If the index has not changed since this instance was (re)opened, then this call is a NOOP and returns this instance. Otherwise, a new instance is returned. The old instance is not closed and remains usable.

  If the reader is reopened, even though they share resources internally, it's safe to make changes (deletions, norms) with the new reader. All shared mutable state obeys "copy on write" semantics to ensure the changes are not seen by other readers.

  You can determine whether a reader was actually reopened by comparing the old instance with the instance returned by this method:

       IndexReader reader = ...
       ...
       IndexReader newReader = r.reopen();
       if (newReader != reader) {
       ...     // reader was reopened
         reader.close();
       }
       reader = newReader;
       ...

  Be sure to synchronize that code so that other threads, if present, can never use reader after it has been closed and before it's switched to newReader.

  NOTE: If this reader is a near real-time reader (obtained from `IndexWriter.getReader()`, reopen() will simply call writer.getReader() again for you, though this may change in the future.

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

### reopen

    @Deprecated
    public IndexReader reopen(boolean openReadOnly)
                       throws CorruptIndexException,
                              IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `openIfChanged(IndexReader)` instead

  Just like `reopen()`, except you can change the readOnly of the original reader. If the index is unchanged but readOnly is different then a new reader will be returned.

  Throws:
    `CorruptIndexException`
    `IOException`

### reopen

    @Deprecated
    public IndexReader reopen(IndexCommit commit)
                       throws CorruptIndexException,
                              IOException

  Deprecated. Use `openIfChanged(IndexReader,IndexCommit)` instead

  Expert: reopen this reader on a specific commit point. This always returns a readOnly reader. If the specified commit point matches what this reader is already on, and this reader is already readOnly, then this same instance is returned; if it is not already readOnly, a readOnly clone is returned.

  Throws:
    `CorruptIndexException`
    `IOException`

### reopen

    @Deprecated
    public IndexReader reopen(IndexWriter writer,
                                         boolean applyAllDeletes)
                       throws CorruptIndexException,
                              IOException

  Deprecated. Use `openIfChanged(IndexReader,IndexWriter,boolean)` instead

  Expert: returns a readonly reader, covering all committed as well as un-committed changes to the index. This provides "near real-time" searching, in that changes made during an IndexWriter session can be quickly made available for searching without closing the writer nor calling `commit()`.

  Note that this is functionally equivalent to calling {#flush} (an internal IndexWriter operation) and then using `open(org.apache.lucene.store.Directory)` to open a new reader. But the turnaround time of this method should be faster since it avoids the potentially costly `commit()`.

  You must close the `IndexReader` returned by this method once you are done using it.

  It's near real-time because there is no hard guarantee on how quickly you can get a new reader after making changes with IndexWriter. You'll have to experiment in your situation to determine if it's fast enough. As this is a new and experimental feature, please report back on your findings so we can learn, improve and iterate.

  The resulting reader supports `reopen()`, but that call will simply forward back to this method (though this may change in the future).

  The very first time this method is called, this writer instance will make every effort to pool the readers that it opens for doing merges, applying deletes, etc. This means additional resources (RAM, file descriptors, CPU time) will be consumed.

  For lower latency on reopening a reader, you should call `IndexWriterConfig.setMergedSegmentWarmer(org.apache.lucene.index.IndexWriter.IndexReaderWarmer)` to pre-warm a newly merged segment before it's committed to the index. This is important for minimizing index-to-search delay after a large merge.

  If an addIndexes* call is running in another thread, then this reader will only search those segments from the foreign index that have been successfully copied over, so far

  .

  NOTE: Once the writer is closed, any outstanding readers may continue to be used. However, if you attempt to reopen any of those readers, you'll hit an `AlreadyClosedException`.

  Parameters:
    `writer` - The IndexWriter to open from
    `applyAllDeletes` - If true, all buffered deletes will be applied (made visible) in the returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter) so that they will be applied in the future. Applying deletes can be costly, so if your app can tolerate deleted documents being returned you might gain some performance by passing false.
  Returns:
    IndexReader that covers entire index plus all changes made so far by this IndexWriter instance
  Throws:
    `IOException`
    `CorruptIndexException`
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### doOpenIfChanged

    protected IndexReader doOpenIfChanged()
                                   throws CorruptIndexException,
                                          IOException

  If the index has changed since it was opened, open and return a new reader; else, return `null`.

  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `openIfChanged(IndexReader)`

### doOpenIfChanged

    @Deprecated
    protected IndexReader doOpenIfChanged(boolean openReadOnly)
                                   throws CorruptIndexException,
                                          IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `doOpenIfChanged()` instead

  If the index has changed since it was opened, open and return a new reader; else, return `null`.

  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `openIfChanged(IndexReader, boolean)`

### doOpenIfChanged

    protected IndexReader doOpenIfChanged(IndexCommit commit)
                                   throws CorruptIndexException,
                                          IOException

  If the index has changed since it was opened, open and return a new reader; else, return `null`.

  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `openIfChanged(IndexReader, IndexCommit)`

### doOpenIfChanged

    protected IndexReader doOpenIfChanged(IndexWriter writer,
                                          boolean applyAllDeletes)
                                   throws CorruptIndexException,
                                          IOException

  If the index has changed since it was opened, open and return a new reader; else, return `null`.

  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `openIfChanged(IndexReader, IndexWriter, boolean)`

### clone

    public Object clone()

  Efficiently clones the IndexReader (sharing most internal state).

  On cloning a reader with pending changes (deletions, norms), the original reader transfers its write lock to the cloned reader. This means only the cloned reader may make further changes to the index, and commit the changes to the index on close, but the old reader still reflects all changes made up until it was cloned.

  Like `openIfChanged(IndexReader)`, it's safe to make changes to either the original or the cloned reader: all shared mutable state obeys "copy on write" semantics to ensure the changes are not seen by other readers.

  Overrides:
    `clone` in class `Object`

### clone

    @Deprecated
    public IndexReader clone(boolean openReadOnly)
                      throws CorruptIndexException,
                             IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `clone()` instead.

  Clones the IndexReader and optionally changes readOnly. A readOnly reader cannot open a writeable reader.

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

### directory

    public Directory directory()

  Returns the directory associated with this index. The Default implementation returns the directory specified by subclasses when delegating to the IndexReader(Directory) constructor, or throws an UnsupportedOperationException if one was not specified.

  Throws:
    `UnsupportedOperationException` - if no directory

### lastModified

    @Deprecated
    public static long lastModified(Directory directory2)
                             throws CorruptIndexException,
                                    IOException

  Deprecated. If you need to track commit time of an index, you can store it in the commit data (see `IndexWriter.commit(Map)`

  Returns the time the index in the named directory was last modified. Do not use this to check whether the reader is still up-to-date, use `isCurrent()` instead.

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

### getCurrentVersion

    @Deprecated
    public static long getCurrentVersion(Directory directory)
                                  throws CorruptIndexException,
                                         IOException

  Deprecated. Use `getVersion()` on an opened IndexReader.

  Reads version number from segments files. The version number is initialized with a timestamp and then increased by one for each change of the index.

  Parameters:
    `directory` - where the index resides.
  Returns:
    version number.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### getCommitUserData

    @Deprecated
    public static Map<String,String> getCommitUserData(Directory directory)
                                                throws CorruptIndexException,
                                                       IOException

  Deprecated. Call `getIndexCommit()` on an open IndexReader, and then call `IndexCommit.getUserData()`.

  Reads commitUserData, previously passed to `IndexWriter.commit(Map)`, from current index segments file. This will return null if `IndexWriter.commit(Map)` has never been called for this index.

  Parameters:
    `directory` - where the index resides.
  Returns:
    commit userData.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### getVersion

    public long getVersion()

  Version number when this IndexReader was opened. Not implemented in the IndexReader base class.

  If this reader is based on a Directory (ie, was created by calling `open(org.apache.lucene.store.Directory)`, or `openIfChanged(org.apache.lucene.index.IndexReader)` on a reader based on a Directory), then this method returns the version recorded in the commit that the reader opened. This version is advanced every time `IndexWriter.commit()` is called.

  Throws:
    `UnsupportedOperationException` - unless overridden in subclass

### getCommitUserData

    @Deprecated
    public Map<String,String> getCommitUserData()

  Deprecated. Call `getIndexCommit()` and then call `IndexCommit.getUserData()`.

  Retrieve the String userData optionally passed to IndexWriter#commit. This will return null if `IndexWriter.commit(Map)` has never been called for this index.

### isCurrent

    public boolean isCurrent()
                      throws CorruptIndexException,
                             IOException

  Check whether any new changes have occurred to the index since this reader was opened.

  If this reader is based on a Directory (ie, was created by calling `open(org.apache.lucene.store.Directory)`, or `openIfChanged(org.apache.lucene.index.IndexReader)` on a reader based on a Directory), then this method checks if any further commits (see `IndexWriter.commit()` have occurred in that directory).

  If instead this reader is a near real-time reader (ie, obtained by a call to `IndexWriter.getReader()`, or by calling `openIfChanged(org.apache.lucene.index.IndexReader)` on a near real-time reader), then this method checks if either a new commmit has occurred, or any new uncommitted changes have taken place via the writer. Note that even if the writer has only performed merging, this method will still return false.

  In any event, if this returns false, you should call `openIfChanged(org.apache.lucene.index.IndexReader)` to get a new reader that sees the changes.

  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
    `UnsupportedOperationException` - unless overridden in subclass

### isOptimized

    @Deprecated
    public boolean isOptimized()

  Deprecated. Check segment count using `getSequentialSubReaders()` instead.

### getTermFreqVectors

    public abstract TermFreqVector[] getTermFreqVectors(int docNumber)
                                                 throws IOException

  Return an array of term frequency vectors for the specified document. The array contains a vector for each vectorized field in the document. Each vector contains terms and frequencies for all terms in a given vectorized field. If no such fields existed, the method returns null. The term vectors that are returned may either be of type `TermFreqVector` or of type `TermPositionVector` if positions or offsets have been stored.

  Parameters:
    `docNumber` - document for which term frequency vectors are returned
  Returns:
    array of term frequency vectors. May be null if no term vectors have been stored for the specified document.
  Throws:
    `IOException` - if index cannot be accessed
  See Also:
    `Field.TermVector`

### getTermFreqVector

    public abstract TermFreqVector getTermFreqVector(int docNumber,
                                                     String field)
                                              throws IOException

  Return a term frequency vector for the specified document and field. The returned vector contains terms and frequencies for the terms in the specified field of this document, if the field had the storeTermVector flag set. If termvectors had been stored with positions or offsets, a `TermPositionVector` is returned.

  Parameters:
    `docNumber` - document for which the term frequency vector is returned
    `field` - field for which the term frequency vector is returned.
  Returns:
    term frequency vector May be null if field does not exist in the specified document or term vector was not stored.
  Throws:
    `IOException` - if index cannot be accessed
  See Also:
    `Field.TermVector`

### getTermFreqVector

    public abstract void getTermFreqVector(int docNumber,
                                           String field,
                                           TermVectorMapper mapper)
                                    throws IOException

  Load the Term Vector into a user-defined data structure instead of relying on the parallel arrays of the `TermFreqVector`.

  Parameters:
    `docNumber` - The number of the document to load the vector for
    `field` - The name of the field to load
    `mapper` - The `TermVectorMapper` to process the vector. Must not be null
  Throws:
    `IOException` - if term vectors cannot be accessed or if they do not exist on the field and doc. specified.

### getTermFreqVector

    public abstract void getTermFreqVector(int docNumber,
                                           TermVectorMapper mapper)
                                    throws IOException

  Map all the term vectors for all fields in a Document

  Parameters:
    `docNumber` - The number of the document to load the vector for
    `mapper` - The `TermVectorMapper` to process the vector. Must not be null
  Throws:
    `IOException` - if term vectors cannot be accessed or if they do not exist on the field and doc. specified.

### indexExists

    public static boolean indexExists(Directory directory)
                               throws IOException

  Returns `true` if an index exists at the specified directory.

  Parameters:
    `directory` - the directory to check for an index
  Returns:
    `true` if an index exists; `false` otherwise
  Throws:
    `IOException` - if there is a problem with accessing the index

### 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 n)
                            throws CorruptIndexException,
                                   IOException

  Returns the stored fields of the `n`th `Document` in this index.

  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 call `isDeleted(int)` with the requested document ID to verify the document is not deleted.

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

### document

    public abstract Document document(int n,
                                      FieldSelector fieldSelector)
                               throws CorruptIndexException,
                                      IOException

  Get the `Document` at the `n` th position. The `FieldSelector` may be used to determine what `Field`s to load and how they should be loaded. NOTE: If this Reader (more specifically, the underlying `FieldsReader`) is closed before the lazy `Field` is loaded an exception may be thrown. If you want the value of a lazy `Field` to be available after closing you must explicitly load it or fetch the Document again with a new loader.

  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 call `isDeleted(int)` with the requested document ID to verify the document is not deleted.

  Parameters:
    `n` - Get the document at the `n`th position
    `fieldSelector` - The `FieldSelector` to use to determine what Fields should be loaded on the Document. May be null, in which case all Fields will be loaded.
  Returns:
    The stored fields of the `Document` at the nth position
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  See Also:
    `Fieldable`, `FieldSelector`, `SetBasedFieldSelector`, `LoadFirstFieldSelector`

### isDeleted

    public abstract boolean isDeleted(int n)

  Returns true if document n has been deleted

### hasDeletions

    public abstract boolean hasDeletions()

  Returns true if any documents have been deleted

### hasNorms

    public boolean hasNorms(String field)
                     throws IOException

  Returns true if there are norms stored for this field.

  Throws:
    `IOException`

### norms

    public abstract byte[] norms(String field)
                          throws IOException

  Returns the byte-encoded normalization factor for the named field of every document. This is used by the search code to score documents. Returns null if norms were not indexed for this field.

  Throws:
    `IOException`
  See Also:
    `AbstractField.setBoost(float)`

### norms

    public abstract void norms(String field,
                               byte[] bytes,
                               int offset)
                        throws IOException

  Reads the byte-encoded normalization factor for the named field of every document. This is used by the search code to score documents.

  Throws:
    `IOException`
  See Also:
    `AbstractField.setBoost(float)`

### setNorm

    @Deprecated
    public final void setNorm(int doc,
                                         String field,
                                         byte value)
                       throws StaleReaderException,
                              CorruptIndexException,
                              LockObtainFailedException,
                              IOException

  Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

  Expert: Resets the normalization factor for the named field of the named document. The norm represents the product of the field's `boost` and its `length normalization`. Thus, to preserve the length normalization values when resetting this, one should base the new value upon the old. NOTE: If this field does not index norms, then this method throws `IllegalStateException`.

  Throws:
    `StaleReaderException` - if the index has changed since this reader was opened
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if there is a low-level IO error
    `IllegalStateException` - if the field does not index norms
  See Also:
    `norms(String)`, `Similarity.decodeNormValue(byte)`

### doSetNorm

    @Deprecated
    protected abstract void doSetNorm(int doc,
                                                 String field,
                                                 byte value)
                               throws CorruptIndexException,
                                      IOException

  Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

  Implements setNorm in subclass.

  Throws:
    `CorruptIndexException`
    `IOException`

### setNorm

    @Deprecated
    public final void setNorm(int doc,
                                         String field,
                                         float value)
                       throws StaleReaderException,
                              CorruptIndexException,
                              LockObtainFailedException,
                              IOException

  Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

  Expert: Resets the normalization factor for the named field of the named document.

  Throws:
    `StaleReaderException` - if the index has changed since this reader was opened
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if there is a low-level IO error
  See Also:
    `norms(String)`, `Similarity.decodeNormValue(byte)`

### terms

    public abstract TermEnum terms()
                            throws IOException

  Returns an enumeration of all the terms in the index. The enumeration is ordered by Term.compareTo(). Each term is greater than all that precede it in the enumeration. Note that after calling terms(), `TermEnum.next()` must be called on the resulting enumeration before calling other methods such as `TermEnum.term()`.

  Throws:
    `IOException` - if there is a low-level IO error

### terms

    public abstract TermEnum terms(Term t)
                            throws IOException

  Returns an enumeration of all terms starting at a given term. If the given term does not exist, the enumeration is positioned at the first term greater than the supplied term. The enumeration is ordered by Term.compareTo(). Each term is greater than all that precede it in the enumeration.

  Throws:
    `IOException` - if there is a low-level IO error

### docFreq

    public abstract int docFreq(Term t)
                         throws IOException

  Returns the number of documents containing the term `t`.

  Throws:
    `IOException` - if there is a low-level IO error

### termDocs

    public TermDocs termDocs(Term term)
                      throws IOException

  Returns an enumeration of all the documents which contain `term`. For each document, the document number, the frequency of the term in that document is also provided, for use in search scoring. If term is null, then all non-deleted docs are returned with freq=1. Thus, this method implements the mapping:

  Term => <docNum, freq>*

  The enumeration is ordered by document number. Each document number is greater than all that precede it in the enumeration.

  Throws:
    `IOException` - if there is a low-level IO error

### termDocs

    public abstract TermDocs termDocs()
                               throws IOException

  Returns an unpositioned `TermDocs` enumerator.

  Note: the TermDocs returned is unpositioned. Before using it, ensure that you first position it with `TermDocs.seek(Term)` or `TermDocs.seek(TermEnum)`.

  Throws:
    `IOException` - if there is a low-level IO error

### termPositions

    public final TermPositions termPositions(Term term)
                                      throws IOException

  Returns an enumeration of all the documents which contain `term`. For each document, in addition to the document number and frequency of the term in that document, a list of all of the ordinal positions of the term in the document is available. Thus, this method implements the mapping:

  Term => <docNum, freq, <pos1, pos2, ... posfreq-1> >*

  This positional information facilitates phrase and proximity searching.

  The enumeration is ordered by document number. Each document number is greater than all that precede it in the enumeration.

  Throws:
    `IOException` - if there is a low-level IO error

### termPositions

    public abstract TermPositions termPositions()
                                         throws IOException

  Returns an unpositioned `TermPositions` enumerator.

  Throws:
    `IOException` - if there is a low-level IO error

### deleteDocument

    @Deprecated
    public final void deleteDocument(int docNum)
                              throws StaleReaderException,
                                     CorruptIndexException,
                                     LockObtainFailedException,
                                     IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead

  Deletes the document numbered `docNum`. Once a document is deleted it will not appear in TermDocs or TermPostitions enumerations. Attempts to read its field with the `document(int)` method will result in an error. The presence of this document may still be reflected in the `docFreq(org.apache.lucene.index.Term)` statistic, though this will be corrected eventually as the index is further modified.

  Throws:
    `StaleReaderException` - if the index has changed since this reader was opened
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if there is a low-level IO error

### doDelete

    @Deprecated
    protected abstract void doDelete(int docNum)
                              throws CorruptIndexException,
                                     IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead

  Implements deletion of the document numbered `docNum`. Applications should call `deleteDocument(int)` or `deleteDocuments(Term)`.

  Throws:
    `CorruptIndexException`
    `IOException`

### deleteDocuments

    @Deprecated
    public final int deleteDocuments(Term term)
                              throws StaleReaderException,
                                     CorruptIndexException,
                                     LockObtainFailedException,
                                     IOException

  Deprecated. Write support will be removed in Lucene 4.0. Use `IndexWriter.deleteDocuments(Term)` instead

  Deletes all documents that have a given `term` indexed. This is useful if one uses a document field to hold a unique ID string for the document. Then to delete such a document, one merely constructs a term with the appropriate field and the unique ID string as its text and passes it to this method. See `deleteDocument(int)` for information about when this deletion will become effective.

  Returns:
    the number of documents deleted
  Throws:
    `StaleReaderException` - if the index has changed since this reader was opened
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if there is a low-level IO error

### undeleteAll

    @Deprecated
    public final void undeleteAll()
                           throws StaleReaderException,
                                  CorruptIndexException,
                                  LockObtainFailedException,
                                  IOException

  Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

  Undeletes all documents currently marked as deleted in this index.

  NOTE: this method can only recover documents marked for deletion but not yet removed from the index; when and how Lucene removes deleted documents is an implementation detail, subject to change from release to release. However, you can use `numDeletedDocs()` on the current IndexReader instance to see how many documents will be un-deleted.

  Throws:
    `StaleReaderException` - if the index has changed since this reader was opened
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### doUndeleteAll

    @Deprecated
    protected abstract void doUndeleteAll()
                                   throws CorruptIndexException,
                                          IOException

  Deprecated. Write support will be removed in Lucene 4.0. There will be no replacement for this method.

  Implements actual undeleteAll() in subclass.

  Throws:
    `CorruptIndexException`
    `IOException`

### acquireWriteLock

    @Deprecated
    protected void acquireWriteLock()
                             throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Does nothing by default. Subclasses that require a write lock for index modifications must implement this method.

  Throws:
    `IOException`

### flush

    @Deprecated
    public final void flush()
                     throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Throws:
    `IOException`

### flush

    @Deprecated
    public final void flush(Map<String,String> commitUserData)
                     throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Parameters:
    `commitUserData` - Opaque Map (String -> String) that's recorded into the segments file in the index, and retrievable by `IndexCommit.getUserData()`.
  Throws:
    `IOException`

### commit

    @Deprecated
    protected final void commit()
                         throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Commit changes resulting from delete, undeleteAll, or setNorm operations If an exception is hit, then either no changes or all changes will have been committed to the index (transactional semantics).

  Throws:
    `IOException` - if there is a low-level IO error

### commit

    @Deprecated
    public final void commit(Map<String,String> commitUserData)
                      throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Commit changes resulting from delete, undeleteAll, or setNorm operations If an exception is hit, then either no changes or all changes will have been committed to the index (transactional semantics).

  Throws:
    `IOException` - if there is a low-level IO error

### doCommit

    @Deprecated
    protected abstract void doCommit(Map<String,String> commitUserData)
                              throws IOException

  Deprecated. Write support will be removed in Lucene 4.0.

  Implements commit.

  Throws:
    `IOException`

### 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`
  Throws:
    `IOException` - if there is a low-level IO error

### doClose

    protected abstract void doClose()
                             throws IOException

  Implements close.

  Throws:
    `IOException`

### getFieldInfos

    public abstract FieldInfos getFieldInfos()

  Get the `FieldInfos` describing all fields in this reader. NOTE: do not make any changes to the returned FieldInfos!

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

### getIndexCommit

    public IndexCommit getIndexCommit()
                               throws IOException

  Expert: return the IndexCommit that this reader has opened. This method is only implemented by those readers that correspond to a Directory with its own segments_N file.

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

### listCommits

    public static Collection<IndexCommit> listCommits(Directory dir)
                                               throws IOException

  Returns all commit points that exist in the Directory. Normally, because the default is `KeepOnlyLastCommitDeletionPolicy`, there would be only one commit point. But if you're using a custom `IndexDeletionPolicy` then there could be many commits. Once you have a given commit, you can open a reader on it by calling `open(IndexCommit,boolean)` There must be at least one commit in the Directory, else this method throws `IndexNotFoundException`. Note that if a commit is in progress while this method is running, that commit may or may not be returned.

  Returns:
    a sorted list of `IndexCommit`s, from oldest to latest.
  Throws:
    `IOException`

### getSequentialSubReaders

    public IndexReader[] getSequentialSubReaders()

  Expert: returns the sequential sub readers that this reader is logically composed of. For example, IndexSearcher uses this API to drive searching by one sub reader at a time. If this reader is not composed of sequential child readers, it should return null. If this method returns an empty array, that means this reader is a null reader (for example a MultiReader that has no sub readers).

  NOTE: You should not try using sub-readers returned by this method to make any changes (setNorm, deleteDocument, etc.). While this might succeed for one composite reader (like MultiReader), it will most likely lead to index corruption for other readers (like DirectoryReader obtained through `open(org.apache.lucene.store.Directory)`. Use the parent reader directly.

### getCoreCacheKey

    public Object getCoreCacheKey()

  Expert

### getDeletesCacheKey

    public Object getDeletesCacheKey()

  Expert. Warning: this returns null if the reader has no deletions

### getUniqueTermCount

    public long getUniqueTermCount()
                            throws IOException

  Returns the number of unique terms (across all fields) in this reader. This method returns long, even though internally Lucene cannot handle more than 2^31 unique terms, for a possible future when this limitation is removed.

  Throws:
    `UnsupportedOperationException` - if this count cannot be easily determined (eg Multi*Readers). Instead, you should call `getSequentialSubReaders()` and ask each sub reader for its unique term count.
    `IOException`

### getTermInfosIndexDivisor

    public int getTermInfosIndexDivisor()

  For IndexReader implementations that use TermInfosReader to read terms, this returns the current indexDivisor as specified when the reader was opened.

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