org.apache.lucene.index

## Class DirectoryReader

- java.lang.Object
- - org.apache.lucene.index.IndexReader
  - - org.apache.lucene.index.CompositeReader
    - - org.apache.lucene.index.BaseCompositeReader<AtomicReader>
      - - org.apache.lucene.index.DirectoryReader

- All Implemented Interfaces:
    Closeable, AutoCloseable

      public abstract class DirectoryReader
      extends BaseCompositeReader<AtomicReader>

  DirectoryReader is an implementation of `CompositeReader` that can read indexes in a `Directory`.

  DirectoryReader instances are usually constructed with a call to one of the static `open()` methods, e.g. `open(Directory)`.

  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/interfaces inherited from class org.apache.lucene.index.IndexReader

      `IndexReader.ReaderClosedListener`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `DEFAULT_TERMS_INDEX_DIVISOR` Default termInfosIndexDivisor.
    `protected Directory` | `directory` The index directory.

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `DirectoryReader(Directory directory, AtomicReader[] segmentReaders)` Expert: Constructs a `DirectoryReader` on the given subReaders.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `Directory` | `directory()` Returns the directory this index resides in.
    `protected abstract DirectoryReader` | `doOpenIfChanged()` Implement this method to support `openIfChanged(DirectoryReader)`.
    `protected abstract DirectoryReader` | `doOpenIfChanged(IndexCommit commit)` Implement this method to support `openIfChanged(DirectoryReader,IndexCommit)`.
    `protected abstract DirectoryReader` | `doOpenIfChanged(IndexWriter writer, boolean applyAllDeletes)` Implement this method to support `openIfChanged(DirectoryReader,IndexWriter,boolean)`.
    `abstract IndexCommit` | `getIndexCommit()` Expert: return the IndexCommit that this reader has opened.
    `abstract long` | `getVersion()` Version number when this IndexReader was opened.
    `static boolean` | `indexExists(Directory directory)` Returns `true` if an index exists at the specified directory.
    `abstract boolean` | `isCurrent()` Check whether any new changes have occurred to the index since this reader was opened.
    `static List<IndexCommit>` | `listCommits(Directory dir)` Returns all commit points that exist in the Directory.
    `static DirectoryReader` | `open(Directory directory)` Returns a IndexReader reading the index in the given Directory
    `static DirectoryReader` | `open(Directory directory, int termInfosIndexDivisor)` Expert: Returns a IndexReader reading the index in the given Directory with the given termInfosIndexDivisor.
    `static DirectoryReader` | `open(IndexCommit commit)` Expert: returns an IndexReader reading the index in the given `IndexCommit`.
    `static DirectoryReader` | `open(IndexCommit commit, int termInfosIndexDivisor)` Expert: returns an IndexReader reading the index in the given `IndexCommit` and termInfosIndexDivisor.
    `static DirectoryReader` | `open(IndexWriter writer, boolean applyAllDeletes)` Open a near real time IndexReader from the `IndexWriter`.
    `static DirectoryReader` | `openIfChanged(DirectoryReader oldReader)` If the index has changed since the provided reader was opened, open and return a new reader; else, return null.
    `static DirectoryReader` | `openIfChanged(DirectoryReader oldReader, IndexCommit commit)` If the IndexCommit differs from what the provided reader is searching, open and return a new reader; else, return null.
    `static DirectoryReader` | `openIfChanged(DirectoryReader 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 IndexReader searching both committed and uncommitted changes from the writer; else, return null (though, the current implementation never returns null).

    - ### Methods inherited from class org.apache.lucene.index.BaseCompositeReader

      `docFreq, document, getSequentialSubReaders, getTermVectors, hasDeletions, maxDoc, numDocs, readerBase, readerIndex, totalTermFreq`

    - ### Methods inherited from class org.apache.lucene.index.CompositeReader

      `getContext, toString`

    - ### Methods inherited from class org.apache.lucene.index.IndexReader

      `addReaderClosedListener, close, decRef, doClose, document, document, ensureOpen, equals, getCombinedCoreAndDeletesKey, getCoreCacheKey, getRefCount, getTermVector, hashCode, incRef, leaves, numDeletedDocs, registerParentReader, removeReaderClosedListener, tryIncRef`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### DEFAULT_TERMS_INDEX_DIVISOR

          public static final int DEFAULT_TERMS_INDEX_DIVISOR

      Default termInfosIndexDivisor.
      See Also:
        Constant Field Values

    - #### directory

          protected final Directory directory

      The index directory.

  - ### Constructor Detail

    - #### DirectoryReader

          protected DirectoryReader(Directory directory,
                         AtomicReader[] segmentReaders)

      Expert: Constructs a `DirectoryReader` on the given subReaders.
      Parameters:
        `segmentReaders` - the wrapped atomic index segment readers. This array is returned by `BaseCompositeReader.getSequentialSubReaders()` and used to resolve the correct subreader for docID-based methods. Please note: This array is not cloned and not protected for modification outside of this reader. Subclasses of `DirectoryReader` should take care to not allow modification of this internal array, e.g. `doOpenIfChanged()`.

  - ### Method Detail

    - #### open

          public static DirectoryReader open(Directory directory)
                                      throws IOException

      Returns a IndexReader reading the index in the given Directory
      Parameters:
        `directory` - the index directory
      Throws:
        `IOException` - if there is a low-level IO error

    - #### open

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

      Expert: Returns a IndexReader reading the index in the given Directory with the 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. NOTE: divisor settings > 1 do not apply to all PostingsFormat implementations, including the default one in this release. It only makes sense for terms indexes that can efficiently re-sample terms at load time.
      Throws:
        `IOException` - if there is a low-level IO error

    - #### open

          public static DirectoryReader open(IndexWriter writer,
                             boolean applyAllDeletes)
                                      throws 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` - if the index is corrupt
        `IOException` - if there is a low-level IO error
      See Also:
        `openIfChanged(DirectoryReader,IndexWriter,boolean)`
      WARNING: This API is experimental and might change in incompatible ways in the next release.

    - #### open

          public static DirectoryReader open(IndexCommit commit)
                                      throws IOException

      Expert: returns an IndexReader reading the index in the given `IndexCommit`.
      Parameters:
        `commit` - the commit point to open
      Throws:
        `IOException` - if there is a low-level IO error

    - #### open

          public static DirectoryReader open(IndexCommit commit,
                             int termInfosIndexDivisor)
                                      throws 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. NOTE: divisor settings > 1 do not apply to all PostingsFormat implementations, including the default one in this release. It only makes sense for terms indexes that can efficiently re-sample terms at load time.
      Throws:
        `IOException` - if there is a low-level IO error

    - #### openIfChanged

          public static DirectoryReader openIfChanged(DirectoryReader 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 `DirectoryReader` as it shares resources (for example sub-readers) with the provided `DirectoryReader`, 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.

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

    - #### openIfChanged

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

      If the IndexCommit differs from what the provided reader is searching, open and return a new reader; else, return null.
      Throws:
        `IOException`
      See Also:
        `openIfChanged(DirectoryReader)`

    - #### openIfChanged

          public static DirectoryReader openIfChanged(DirectoryReader 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 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:
        DirectoryReader that covers entire index plus all changes made so far by this IndexWriter instance, or null if there are no new changes
      Throws:
        `IOException` - if there is a low-level IO error
      WARNING: This API is experimental and might change in incompatible ways in the next release.

    - #### listCommits

          public static List<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)` 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`

    - #### indexExists

          public static boolean indexExists(Directory directory)

      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

    - #### directory

          public final Directory directory()

      Returns the directory this index resides in.

    - #### doOpenIfChanged

          protected abstract DirectoryReader doOpenIfChanged()
                                                      throws IOException

      Implement this method to support `openIfChanged(DirectoryReader)`. If this reader does not support reopen, return `null`, so client code is happy. This should be consistent with `isCurrent()` (should always return `true`) if reopen is not supported.
      Returns:
        null if there are no changes; else, a new DirectoryReader instance.
      Throws:
        `IOException` - if there is a low-level IO error

    - #### doOpenIfChanged

          protected abstract DirectoryReader doOpenIfChanged(IndexCommit commit)
                                                      throws IOException

      Implement this method to support `openIfChanged(DirectoryReader,IndexCommit)`. If this reader does not support reopen from a specific `IndexCommit`, throw `UnsupportedOperationException`.
      Returns:
        null if there are no changes; else, a new DirectoryReader instance.
      Throws:
        `IOException` - if there is a low-level IO error

    - #### doOpenIfChanged

          protected abstract DirectoryReader doOpenIfChanged(IndexWriter writer,
                                        boolean applyAllDeletes)
                                                      throws IOException

      Implement this method to support `openIfChanged(DirectoryReader,IndexWriter,boolean)`. If this reader does not support reopen from `IndexWriter`, throw `UnsupportedOperationException`.
      Returns:
        null if there are no changes; else, a new DirectoryReader instance.
      Throws:
        `IOException` - if there is a low-level IO error

    - #### getVersion

          public abstract long getVersion()

      Version number when this IndexReader was opened.

      This method returns the version recorded in the commit that the reader opened. This version is advanced every time a change is made with `IndexWriter`.

    - #### isCurrent

          public abstract boolean isCurrent()
                                     throws IOException

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

      If this reader was created by calling `open(org.apache.lucene.store.Directory)`, then this method checks if any further commits (see `IndexWriter.commit()`) have occurred in the directory.

      If instead this reader is a near real-time reader (ie, obtained by a call to `open(IndexWriter,boolean)`, or by calling `openIfChanged(org.apache.lucene.index.DirectoryReader)` on a near real-time reader), then this method checks if either a new commit 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.DirectoryReader)` to get a new reader that sees the changes.

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

    - #### getIndexCommit

          public abstract IndexCommit getIndexCommit()
                                              throws IOException

      Expert: return the IndexCommit that this reader has opened.

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

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