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## org.apache.lucene.index
Class IndexWriter

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

All Implemented Interfaces:
  Closeable, TwoPhaseCommit

    public class IndexWriter

extends Object
implements Closeable, TwoPhaseCommit

An `IndexWriter` creates and maintains an index.

The `create` argument to the `constructor` determines whether a new index is created, or whether an existing index is opened. Note that you can open an index with `create=true` even while readers are using the index. The old readers will continue to search the "point in time" snapshot they had opened, and won't see the newly created index until they re-open. There are also `constructors` with no `create` argument which will create a new index if there is not already an index at the provided path and otherwise open the existing index.

In either case, documents are added with `addDocument` and removed with `deleteDocuments(Term)` or `deleteDocuments(Query)`. A document can be updated with `updateDocument` (which just deletes and then adds the entire document). When finished adding, deleting and updating documents, `close` should be called.

These changes are buffered in memory and periodically flushed to the `Directory` (during the above method calls). A flush is triggered when there are enough buffered deletes (see `setMaxBufferedDeleteTerms(int)`) or enough added documents since the last flush, whichever is sooner. For the added documents, flushing is triggered either by RAM usage of the documents (see `setRAMBufferSizeMB(double)`) or the number of added documents. The default is to flush when RAM usage hits 16 MB. For best indexing speed you should flush by RAM usage with a large RAM buffer. Note that flushing just moves the internal buffered state in IndexWriter into the index, but these changes are not visible to IndexReader until either `commit()` or `close()` is called. A flush may also trigger one or more segment merges which by default run with a background thread so as not to block the addDocument calls (see below for changing the `MergeScheduler`).

Opening an `IndexWriter` creates a lock file for the directory in use. Trying to open another `IndexWriter` on the same directory will lead to a `LockObtainFailedException`. The `LockObtainFailedException` is also thrown if an IndexReader on the same directory is used to delete documents from the index.

Expert: `IndexWriter` allows an optional `IndexDeletionPolicy` implementation to be specified. You can use this to control when prior commits are deleted from the index. The default policy is `KeepOnlyLastCommitDeletionPolicy` which removes all prior commits as soon as a new commit is done (this matches behavior before 2.2). Creating your own policy can allow you to explicitly keep previous "point in time" commits alive in the index for some time, to allow readers to refresh to the new commit without having the old commit deleted out from under them. This is necessary on filesystems like NFS that do not support "delete on last close" semantics, which Lucene's "point in time" search normally relies on.

Expert: `IndexWriter` allows you to separately change the `MergePolicy` and the `MergeScheduler`. The `MergePolicy` is invoked whenever there are changes to the segments in the index. Its role is to select which merges to do, if any, and return a `MergePolicy.MergeSpecification` describing the merges. The default is `LogByteSizeMergePolicy`. Then, the `MergeScheduler` is invoked with the requested merges and it decides when and how to run the merges. The default is `ConcurrentMergeScheduler`.

NOTE: if you hit an OutOfMemoryError then IndexWriter will quietly record this fact and block all future segment commits. This is a defensive measure in case any internal state (buffered documents and deletions) were corrupted. Any subsequent calls to `commit()` will throw an IllegalStateException. The only course of action is to call `close()`, which internally will call `rollback()`, to undo any changes to the index since the last commit. You can also just call `rollback()` directly.

NOTE: `IndexWriter` 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 `IndexWriter` instance as this may cause deadlock; use your own (non-Lucene) objects instead.

NOTE: If you call `Thread.interrupt()` on a thread that's within IndexWriter, IndexWriter will try to catch this (eg, if it's in a wait() or Thread.sleep()), and will then throw the unchecked exception `ThreadInterruptedException` and clear the interrupt status on the thread.

Nested Class Summary
`static class` | `IndexWriter.IndexReaderWarmer` If `getReader()` has been called (ie, this writer is in near real-time mode), then after a merge completes, this class can be invoked to warm the reader on the newly merged segment, before the merge commits.
`static class` | `IndexWriter.MaxFieldLength` Deprecated. use `LimitTokenCountAnalyzer` instead.

Field Summary
`static int` | `DEFAULT_MAX_BUFFERED_DELETE_TERMS` Deprecated. use `IndexWriterConfig.DEFAULT_MAX_BUFFERED_DELETE_TERMS` instead
`static int` | `DEFAULT_MAX_BUFFERED_DOCS` Deprecated. use `IndexWriterConfig.DEFAULT_MAX_BUFFERED_DOCS` instead.
`static int` | `DEFAULT_MAX_FIELD_LENGTH` Deprecated. see `IndexWriterConfig`
`static double` | `DEFAULT_RAM_BUFFER_SIZE_MB` Deprecated. use `IndexWriterConfig.DEFAULT_RAM_BUFFER_SIZE_MB` instead.
`static int` | `DEFAULT_TERM_INDEX_INTERVAL` Deprecated. use `IndexWriterConfig.DEFAULT_TERM_INDEX_INTERVAL` instead.
`static int` | `DISABLE_AUTO_FLUSH` Deprecated. use `IndexWriterConfig.DISABLE_AUTO_FLUSH` instead
`static int` | `MAX_TERM_LENGTH` Absolute hard maximum length for a term.
`static String` | `WRITE_LOCK_NAME` Name of the write lock in the index.
`static long` | `WRITE_LOCK_TIMEOUT` Deprecated. use `IndexWriterConfig.WRITE_LOCK_TIMEOUT` instead

Constructor Summary
`IndexWriter(Directory d, Analyzer a, boolean create, IndexDeletionPolicy deletionPolicy, IndexWriter.MaxFieldLength mfl)` Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead
`IndexWriter(Directory d, Analyzer a, boolean create, IndexWriter.MaxFieldLength mfl)` Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead
`IndexWriter(Directory d, Analyzer a, IndexDeletionPolicy deletionPolicy, IndexWriter.MaxFieldLength mfl)` Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead
`IndexWriter(Directory d, Analyzer a, IndexDeletionPolicy deletionPolicy, IndexWriter.MaxFieldLength mfl, IndexCommit commit)` Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead
`IndexWriter(Directory d, Analyzer a, IndexWriter.MaxFieldLength mfl)` Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead
`IndexWriter(Directory d, IndexWriterConfig conf)` Constructs a new IndexWriter per the settings given in `conf`.

Method Summary
`void` | `addDocument(Document doc)` Adds a document to this index.
`void` | `addDocument(Document doc, Analyzer analyzer)` Adds a document to this index, using the provided analyzer instead of the value of `getAnalyzer()`.
`void` | `addDocuments(Collection<Document> docs)` Atomically adds a block of documents with sequentially assigned document IDs, such that an external reader will see all or none of the documents.
`void` | `addDocuments(Collection<Document> docs, Analyzer analyzer)` Atomically adds a block of documents, analyzed using the provided analyzer, with sequentially assigned document IDs, such that an external reader will see all or none of the documents.
`void` | `addIndexes(Directory... dirs)` Adds all segments from an array of indexes into this index.
`void` | `addIndexes(IndexReader... readers)` Merges the provided indexes into this index.
`void` | `addIndexesNoOptimize(Directory... dirs)` Deprecated. use `addIndexes(Directory...)` instead
`void` | `close()` Commits all changes to an index and closes all associated files.
`void` | `close(boolean waitForMerges)` Closes the index with or without waiting for currently running merges to finish.
`void` | `commit()` Commits all pending changes (added & deleted documents, segment merges, added indexes, etc.) to the index, and syncs all referenced index files, such that a reader will see the changes and the index updates will survive an OS or machine crash or power loss.
`void` | `commit(Map<String,String> commitUserData)` Commits all changes to the index, specifying a commitUserData Map (String -> String).
`void` | `deleteAll()` Delete all documents in the index.
`void` | `deleteDocuments(Query... queries)` Deletes the document(s) matching any of the provided queries.
`void` | `deleteDocuments(Query query)` Deletes the document(s) matching the provided query.
`void` | `deleteDocuments(Term... terms)` Deletes the document(s) containing any of the terms.
`void` | `deleteDocuments(Term term)` Deletes the document(s) containing `term`.
`void` | `deleteUnusedFiles()` Expert: remove any index files that are no longer used.
`protected void` | `doAfterFlush()` A hook for extending classes to execute operations after pending added and deleted documents have been flushed to the Directory but before the change is committed (new segments_N file written).
`protected void` | `doBeforeFlush()` A hook for extending classes to execute operations before pending added and deleted documents are flushed to the Directory.
`protected void` | `ensureOpen()`
`protected void` | `ensureOpen(boolean includePendingClose)` Used internally to throw an `AlreadyClosedException` if this IndexWriter has been closed.
`void` | `expungeDeletes()` Deprecated.
`void` | `expungeDeletes(boolean doWait)` Deprecated.
`protected void` | `flush(boolean triggerMerge, boolean applyAllDeletes)` Flush all in-memory buffered updates (adds and deletes) to the Directory.
`protected void` | `flush(boolean triggerMerge, boolean flushDocStores, boolean flushDeletes)` NOTE: flushDocStores is ignored now (hardwired to true); this method is only here for backwards compatibility
`void` | `forceMerge(int maxNumSegments)` Forces merge policy to merge segments until there's <= maxNumSegments.
`void` | `forceMerge(int maxNumSegments, boolean doWait)` Just like `forceMerge(int)`, except you can specify whether the call should block until all merging completes.
`void` | `forceMergeDeletes()` Forces merging of all segments that have deleted documents.
`void` | `forceMergeDeletes(boolean doWait)` Just like `forceMergeDeletes()`, except you can specify whether the call should block until the operation completes.
`Analyzer` | `getAnalyzer()` Returns the analyzer used by this index.
`IndexWriterConfig` | `getConfig()` Returns the private `IndexWriterConfig`, cloned from the `IndexWriterConfig` passed to `IndexWriter(Directory, IndexWriterConfig)`.
`static PrintStream` | `getDefaultInfoStream()` Returns the current default infoStream for newly instantiated IndexWriters.
`static long` | `getDefaultWriteLockTimeout()` Deprecated. use `IndexWriterConfig.getDefaultWriteLockTimeout()` instead
`Directory` | `getDirectory()` Returns the Directory used by this index.
`PrintStream` | `getInfoStream()` Returns the current infoStream in use by this writer.
`int` | `getMaxBufferedDeleteTerms()` Deprecated. use `IndexWriterConfig.getMaxBufferedDeleteTerms()` instead
`int` | `getMaxBufferedDocs()` Deprecated. use `IndexWriterConfig.getMaxBufferedDocs()` instead.
`int` | `getMaxFieldLength()` Deprecated. use `LimitTokenCountAnalyzer` to limit number of tokens.
`int` | `getMaxMergeDocs()` Deprecated. use `LogMergePolicy.getMaxMergeDocs()` directly.
`IndexWriter.IndexReaderWarmer` | `getMergedSegmentWarmer()` Deprecated. use `IndexWriterConfig.getMergedSegmentWarmer()` instead.
`int` | `getMergeFactor()` Deprecated. use `LogMergePolicy.getMergeFactor()` directly.
`MergePolicy` | `getMergePolicy()` Deprecated. use `IndexWriterConfig.getMergePolicy()` instead
`MergeScheduler` | `getMergeScheduler()` Deprecated. use `IndexWriterConfig.getMergeScheduler()` instead
`Collection<SegmentInfo>` | `getMergingSegments()` Expert: to be used by a `MergePolicy` to avoid selecting merges for segments already being merged.
`MergePolicy.OneMerge` | `getNextMerge()` Expert: the `MergeScheduler` calls this method to retrieve the next merge requested by the MergePolicy
`PayloadProcessorProvider` | `getPayloadProcessorProvider()` Returns the `PayloadProcessorProvider` that is used during segment merges to process payloads.
`double` | `getRAMBufferSizeMB()` Deprecated. use `IndexWriterConfig.getRAMBufferSizeMB()` instead.
`IndexReader` | `getReader()` Deprecated. Please use `IndexReader.open(IndexWriter,boolean)` instead.
`IndexReader` | `getReader(int termInfosIndexDivisor)` Deprecated. Please use `IndexReader.open(IndexWriter,boolean)` instead. Furthermore, this method cannot guarantee the reader (and its sub-readers) will be opened with the termInfosIndexDivisor setting because some of them may have already been opened according to `IndexWriterConfig.setReaderTermsIndexDivisor(int)`. You should set the requested termInfosIndexDivisor through `IndexWriterConfig.setReaderTermsIndexDivisor(int)` and use `getReader()`.
`int` | `getReaderTermsIndexDivisor()` Deprecated. use `IndexWriterConfig.getReaderTermsIndexDivisor()` instead.
`Similarity` | `getSimilarity()` Deprecated. use `IndexWriterConfig.getSimilarity()` instead
`int` | `getTermIndexInterval()` Deprecated. use `IndexWriterConfig.getTermIndexInterval()`
`boolean` | `getUseCompoundFile()` Deprecated. use `LogMergePolicy.getUseCompoundFile()`
`long` | `getWriteLockTimeout()` Deprecated. use `IndexWriterConfig.getWriteLockTimeout()`
`boolean` | `hasDeletions()`
`static boolean` | `isLocked(Directory directory)` Returns `true` iff the index in the named directory is currently locked.
`int` | `maxDoc()` Returns total number of docs in this index, including docs not yet flushed (still in the RAM buffer), not counting deletions.
`void` | `maybeMerge()` Expert: asks the mergePolicy whether any merges are necessary now and if so, runs the requested merges and then iterate (test again if merges are needed) until no more merges are returned by the mergePolicy.
`void` | `merge(MergePolicy.OneMerge merge)` Merges the indicated segments, replacing them in the stack with a single segment.
`void` | `message(String message)` Prints a message to the infoStream (if non-null), prefixed with the identifying information for this writer and the thread that's calling it.
`int` | `numDeletedDocs(SegmentInfo info)` Obtain the number of deleted docs for a pooled reader.
`int` | `numDocs()` Returns total number of docs in this index, including docs not yet flushed (still in the RAM buffer), and including deletions.
`int` | `numRamDocs()` Expert: Return the number of documents currently buffered in RAM.
`void` | `optimize()` Deprecated.
`void` | `optimize(boolean doWait)` Deprecated.
`void` | `optimize(int maxNumSegments)` Deprecated.
`void` | `prepareCommit()` Expert: prepare for commit.
`void` | `prepareCommit(Map<String,String> commitUserData)` Expert: prepare for commit, specifying commitUserData Map (String -> String).
`long` | `ramSizeInBytes()` Expert: Return the total size of all index files currently cached in memory.
`void` | `rollback()` Close the `IndexWriter` without committing any changes that have occurred since the last commit (or since it was opened, if commit hasn't been called).
`String` | `segString()`
`String` | `segString(Iterable<SegmentInfo> infos)`
`String` | `segString(SegmentInfo info)`
`static void` | `setDefaultInfoStream(PrintStream infoStream)` If non-null, this will be the default infoStream used by a newly instantiated IndexWriter.
`static void` | `setDefaultWriteLockTimeout(long writeLockTimeout)` Deprecated. use `IndexWriterConfig.setDefaultWriteLockTimeout(long)` instead
`void` | `setInfoStream(PrintStream infoStream)` If non-null, information about merges, deletes and a message when maxFieldLength is reached will be printed to this.
`void` | `setMaxBufferedDeleteTerms(int maxBufferedDeleteTerms)` Deprecated. use `IndexWriterConfig.setMaxBufferedDeleteTerms(int)` instead.
`void` | `setMaxBufferedDocs(int maxBufferedDocs)` Deprecated. use `IndexWriterConfig.setMaxBufferedDocs(int)` instead.
`void` | `setMaxFieldLength(int maxFieldLength)` Deprecated. use `LimitTokenCountAnalyzer` instead. Note that the behvaior slightly changed - the analyzer limits the number of tokens per token stream created, while this setting limits the total number of tokens to index. This only matters if you index many multi-valued fields though.
`void` | `setMaxMergeDocs(int maxMergeDocs)` Deprecated. use `LogMergePolicy.setMaxMergeDocs(int)` directly.
`void` | `setMergedSegmentWarmer(IndexWriter.IndexReaderWarmer warmer)` Deprecated. use `IndexWriterConfig.setMergedSegmentWarmer(org.apache.lucene.index.IndexWriter.IndexReaderWarmer)` instead.
`void` | `setMergeFactor(int mergeFactor)` Deprecated. use `LogMergePolicy.setMergeFactor(int)` directly.
`void` | `setMergePolicy(MergePolicy mp)` Deprecated. use `IndexWriterConfig.setMergePolicy(MergePolicy)` instead.
`void` | `setMergeScheduler(MergeScheduler mergeScheduler)` Deprecated. use `IndexWriterConfig.setMergeScheduler(MergeScheduler)` instead
`void` | `setPayloadProcessorProvider(PayloadProcessorProvider pcp)` Sets the `PayloadProcessorProvider` to use when merging payloads.
`void` | `setRAMBufferSizeMB(double mb)` Deprecated. use `IndexWriterConfig.setRAMBufferSizeMB(double)` instead.
`void` | `setReaderTermsIndexDivisor(int divisor)` Deprecated. use `IndexWriterConfig.setReaderTermsIndexDivisor(int)` instead.
`void` | `setSimilarity(Similarity similarity)` Deprecated. use `IndexWriterConfig.setSimilarity(Similarity)` instead
`void` | `setTermIndexInterval(int interval)` Deprecated. use `IndexWriterConfig.setTermIndexInterval(int)`
`void` | `setUseCompoundFile(boolean value)` Deprecated. use `LogMergePolicy.setUseCompoundFile(boolean)`.
`void` | `setWriteLockTimeout(long writeLockTimeout)` Deprecated. use `IndexWriterConfig.setWriteLockTimeout(long)` instead
`static void` | `unlock(Directory directory)` Forcibly unlocks the index in the named directory.
`void` | `updateDocument(Term term, Document doc)` Updates a document by first deleting the document(s) containing `term` and then adding the new document.
`void` | `updateDocument(Term term, Document doc, Analyzer analyzer)` Updates a document by first deleting the document(s) containing `term` and then adding the new document.
`void` | `updateDocuments(Term delTerm, Collection<Document> docs)` Atomically deletes documents matching the provided delTerm and adds a block of documents with sequentially assigned document IDs, such that an external reader will see all or none of the documents.
`void` | `updateDocuments(Term delTerm, Collection<Document> docs, Analyzer analyzer)` Atomically deletes documents matching the provided delTerm and adds a block of documents, analyzed using the provided analyzer, with sequentially assigned document IDs, such that an external reader will see all or none of the documents.
`boolean` | `verbose()` Returns true if verbosing is enabled (i.e., infoStream != null).
`void` | `waitForMerges()` Wait for any currently outstanding merges to finish.

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

Field Detail

### WRITE_LOCK_TIMEOUT

    @Deprecated
    public static long WRITE_LOCK_TIMEOUT

  Deprecated. use `IndexWriterConfig.WRITE_LOCK_TIMEOUT` instead
  Default value for the write lock timeout (1,000).

  See Also:
    `setDefaultWriteLockTimeout(long)`

### WRITE_LOCK_NAME

    public static final String WRITE_LOCK_NAME

  Name of the write lock in the index.

  See Also:
    Constant Field Values

### DISABLE_AUTO_FLUSH

    @Deprecated
    public static final int DISABLE_AUTO_FLUSH

  Deprecated. use `IndexWriterConfig.DISABLE_AUTO_FLUSH` instead
  Value to denote a flush trigger is disabled

  See Also:
    Constant Field Values

### DEFAULT_MAX_BUFFERED_DOCS

    @Deprecated
    public static final int DEFAULT_MAX_BUFFERED_DOCS

  Deprecated. use `IndexWriterConfig.DEFAULT_MAX_BUFFERED_DOCS` instead.
  Disabled by default (because IndexWriter flushes by RAM usage by default). Change using `setMaxBufferedDocs(int)`.

  See Also:
    Constant Field Values

### DEFAULT_RAM_BUFFER_SIZE_MB

    @Deprecated
    public static final double DEFAULT_RAM_BUFFER_SIZE_MB

  Deprecated. use `IndexWriterConfig.DEFAULT_RAM_BUFFER_SIZE_MB` instead.
  Default value is 16 MB (which means flush when buffered docs consume 16 MB RAM). Change using `setRAMBufferSizeMB(double)`.

  See Also:
    Constant Field Values

### DEFAULT_MAX_BUFFERED_DELETE_TERMS

    @Deprecated
    public static final int DEFAULT_MAX_BUFFERED_DELETE_TERMS

  Deprecated. use `IndexWriterConfig.DEFAULT_MAX_BUFFERED_DELETE_TERMS` instead
  Disabled by default (because IndexWriter flushes by RAM usage by default). Change using `setMaxBufferedDeleteTerms(int)`.

  See Also:
    Constant Field Values

### DEFAULT_MAX_FIELD_LENGTH

    @Deprecated
    public static final int DEFAULT_MAX_FIELD_LENGTH

  Deprecated. see `IndexWriterConfig`
  Default value is 10,000. Change using `setMaxFieldLength(int)`.

### DEFAULT_TERM_INDEX_INTERVAL

    @Deprecated
    public static final int DEFAULT_TERM_INDEX_INTERVAL

  Deprecated. use `IndexWriterConfig.DEFAULT_TERM_INDEX_INTERVAL` instead.
  Default value is 128. Change using `setTermIndexInterval(int)`.

  See Also:
    Constant Field Values

### MAX_TERM_LENGTH

    public static final int MAX_TERM_LENGTH

  Absolute hard maximum length for a term. If a term arrives from the analyzer longer than this length, it is skipped and a message is printed to infoStream, if set (see `setInfoStream(java.io.PrintStream)`).

  See Also:
    Constant Field Values

Constructor Detail

### IndexWriter

    @Deprecated
    public IndexWriter(Directory d,
                                  Analyzer a,
                                  boolean create,
                                  IndexWriter.MaxFieldLength mfl)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead

  Constructs an IndexWriter for the index in `d`. Text will be analyzed with `a`. If `create` is true, then a new, empty index will be created in `d`, replacing the index already there, if any.

  Parameters:
    `d` - the index directory
    `a` - the analyzer to use
    `create` - `true` to create the index or overwrite the existing one; `false` to append to the existing index
    `mfl` - Maximum field length in number of terms/tokens: LIMITED, UNLIMITED, or user-specified via the MaxFieldLength constructor.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to, or if it does not exist and `create` is `false` or if there is any other low-level IO error

### IndexWriter

    @Deprecated
    public IndexWriter(Directory d,
                                  Analyzer a,
                                  IndexWriter.MaxFieldLength mfl)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead

  Constructs an IndexWriter for the index in `d`, first creating it if it does not already exist. Text will be analyzed with `a`.

  Parameters:
    `d` - the index directory
    `a` - the analyzer to use
    `mfl` - Maximum field length in number of terms/tokens: LIMITED, UNLIMITED, or user-specified via the MaxFieldLength constructor.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to or if there is any other low-level IO error

### IndexWriter

    @Deprecated
    public IndexWriter(Directory d,
                                  Analyzer a,
                                  IndexDeletionPolicy deletionPolicy,
                                  IndexWriter.MaxFieldLength mfl)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead

  Expert: constructs an IndexWriter with a custom `IndexDeletionPolicy`, for the index in `d`, first creating it if it does not already exist. Text will be analyzed with `a`.

  Parameters:
    `d` - the index directory
    `a` - the analyzer to use
    `deletionPolicy` - see above
    `mfl` - whether or not to limit field lengths
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to or if there is any other low-level IO error

### IndexWriter

    @Deprecated
    public IndexWriter(Directory d,
                                  Analyzer a,
                                  boolean create,
                                  IndexDeletionPolicy deletionPolicy,
                                  IndexWriter.MaxFieldLength mfl)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead

  Expert: constructs an IndexWriter with a custom `IndexDeletionPolicy`, for the index in `d`. Text will be analyzed with `a`. If `create` is true, then a new, empty index will be created in `d`, replacing the index already there, if any.

  Parameters:
    `d` - the index directory
    `a` - the analyzer to use
    `create` - `true` to create the index or overwrite the existing one; `false` to append to the existing index
    `deletionPolicy` - see above
    `mfl` - `IndexWriter.MaxFieldLength`, whether or not to limit field lengths. Value is in number of terms/tokens
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to, or if it does not exist and `create` is `false` or if there is any other low-level IO error

### IndexWriter

    @Deprecated
    public IndexWriter(Directory d,
                                  Analyzer a,
                                  IndexDeletionPolicy deletionPolicy,
                                  IndexWriter.MaxFieldLength mfl,
                                  IndexCommit commit)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Deprecated. use `IndexWriter(Directory, IndexWriterConfig)` instead

  Expert: constructs an IndexWriter on specific commit point, with a custom `IndexDeletionPolicy`, for the index in `d`. Text will be analyzed with `a`.

  This is only meaningful if you've used a `IndexDeletionPolicy` in that past that keeps more than just the last commit.

  This operation is similar to `rollback()`, except that method can only rollback what's been done with the current instance of IndexWriter since its last commit, whereas this method can rollback to an arbitrary commit point from the past, assuming the `IndexDeletionPolicy` has preserved past commits.

  Parameters:
    `d` - the index directory
    `a` - the analyzer to use
    `deletionPolicy` - see above
    `mfl` - whether or not to limit field lengths, value is in number of terms/tokens. See `IndexWriter.MaxFieldLength`.
    `commit` - which commit to open
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to, or if it does not exist and `create` is `false` or if there is any other low-level IO error

### IndexWriter

    public IndexWriter(Directory d,
                       IndexWriterConfig conf)
                throws CorruptIndexException,
                       LockObtainFailedException,
                       IOException

  Constructs a new IndexWriter per the settings given in `conf`. Note that the passed in `IndexWriterConfig` is privately cloned; if you need to make subsequent "live" changes to the configuration use `getConfig()`.

  Parameters:
    `d` - the index directory. The index is either created or appended according `conf.getOpenMode()`.
    `conf` - the configuration settings according to which IndexWriter should be initialized.
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `LockObtainFailedException` - if another writer has this index open (`write.lock` could not be obtained)
    `IOException` - if the directory cannot be read/written to, or if it does not exist and `conf.getOpenMode()` is `OpenMode.APPEND` or if there is any other low-level IO error

Method Detail

### getReader

    @Deprecated
    public IndexReader getReader()
                          throws IOException

  Deprecated. Please use `IndexReader.open(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} and then using `IndexReader.open(org.apache.lucene.store.Directory)` to open a new reader. But the turarnound 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 `IndexReader.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 `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`.

  Returns:
    IndexReader that covers entire index plus all changes made so far by this IndexWriter instance
  Throws:
    `IOException`
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### getReader

    @Deprecated
    public IndexReader getReader(int termInfosIndexDivisor)
                          throws IOException

  Deprecated. Please use `IndexReader.open(IndexWriter,boolean)` instead. Furthermore, this method cannot guarantee the reader (and its sub-readers) will be opened with the termInfosIndexDivisor setting because some of them may have already been opened according to `IndexWriterConfig.setReaderTermsIndexDivisor(int)`. You should set the requested termInfosIndexDivisor through `IndexWriterConfig.setReaderTermsIndexDivisor(int)` and use `getReader()`.

  Expert: like `getReader()`, except you can specify which termInfosIndexDivisor should be used for any newly opened readers.

  Parameters:
    `termInfosIndexDivisor` - Subsamples which indexed terms are loaded into RAM. This has the same effect as `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:
    `IOException`

### numDeletedDocs

    public int numDeletedDocs(SegmentInfo info)
                       throws IOException

  Obtain the number of deleted docs for a pooled reader. If the reader isn't being pooled, the segmentInfo's delCount is returned.

  Throws:
    `IOException`

### ensureOpen

    protected final void ensureOpen(boolean includePendingClose)
                             throws AlreadyClosedException

  Used internally to throw an `AlreadyClosedException` if this IndexWriter has been closed.

  Throws:
    `AlreadyClosedException` - if this IndexWriter is closed

### ensureOpen

    protected final void ensureOpen()
                             throws AlreadyClosedException

  Throws:
    `AlreadyClosedException`

### message

    public void message(String message)

  Prints a message to the infoStream (if non-null), prefixed with the identifying information for this writer and the thread that's calling it.

### getUseCompoundFile

    @Deprecated
    public boolean getUseCompoundFile()

  Deprecated. use `LogMergePolicy.getUseCompoundFile()`

  Get the current setting of whether newly flushed segments will use the compound file format. Note that this just returns the value previously set with setUseCompoundFile(boolean), or the default value (true). You cannot use this to query the status of previously flushed segments.

  Note that this method is a convenience method: it just calls mergePolicy.getUseCompoundFile as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

  See Also:
    `setUseCompoundFile(boolean)`

### setUseCompoundFile

    @Deprecated
    public void setUseCompoundFile(boolean value)

  Deprecated. use `LogMergePolicy.setUseCompoundFile(boolean)`.

  Setting to turn on usage of a compound file. When on, multiple files for each segment are merged into a single file when a new segment is flushed.

  Note that this method is a convenience method: it just calls mergePolicy.setUseCompoundFile as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

### setSimilarity

    @Deprecated
    public void setSimilarity(Similarity similarity)

  Deprecated. use `IndexWriterConfig.setSimilarity(Similarity)` instead

  Expert: Set the Similarity implementation used by this IndexWriter.

  See Also:
    `Similarity.setDefault(Similarity)`

### getSimilarity

    @Deprecated
    public Similarity getSimilarity()

  Deprecated. use `IndexWriterConfig.getSimilarity()` instead

  Expert: Return the Similarity implementation used by this IndexWriter.

  This defaults to the current value of `Similarity.getDefault()`.

### setTermIndexInterval

    @Deprecated
    public void setTermIndexInterval(int interval)

  Deprecated. use `IndexWriterConfig.setTermIndexInterval(int)`

  Expert: Set the interval between indexed terms. Large values cause less memory to be used by IndexReader, but slow random-access to terms. Small values cause more memory to be used by an IndexReader, and speed random-access to terms. This parameter determines the amount of computation required per query term, regardless of the number of documents that contain that term. In particular, it is the maximum number of other terms that must be scanned before a term is located and its frequency and position information may be processed. In a large index with user-entered query terms, query processing time is likely to be dominated not by term lookup but rather by the processing of frequency and positional data. In a small index or when many uncommon query terms are generated (e.g., by wildcard queries) term lookup may become a dominant cost. In particular, `numUniqueTerms/interval` terms are read into memory by an IndexReader, and, on average, `interval/2` terms must be scanned for each random term access.

  See Also:
    `DEFAULT_TERM_INDEX_INTERVAL`

### getTermIndexInterval

    @Deprecated
    public int getTermIndexInterval()

  Deprecated. use `IndexWriterConfig.getTermIndexInterval()`

  Expert: Return the interval between indexed terms.

  See Also:
    `setTermIndexInterval(int)`

### getConfig

    public IndexWriterConfig getConfig()

  Returns the private `IndexWriterConfig`, cloned from the `IndexWriterConfig` passed to `IndexWriter(Directory, IndexWriterConfig)`.

  NOTE: some settings may be changed on the returned `IndexWriterConfig`, and will take effect in the current IndexWriter instance. See the javadocs for the specific setters in `IndexWriterConfig` for details.

### setMergePolicy

    @Deprecated
    public void setMergePolicy(MergePolicy mp)

  Deprecated. use `IndexWriterConfig.setMergePolicy(MergePolicy)` instead.

  Expert: set the merge policy used by this writer.

### getMergePolicy

    @Deprecated
    public MergePolicy getMergePolicy()

  Deprecated. use `IndexWriterConfig.getMergePolicy()` instead

  Expert: returns the current MergePolicy in use by this writer.

  See Also:
    `setMergePolicy(org.apache.lucene.index.MergePolicy)`

### setMergeScheduler

    @Deprecated
    public void setMergeScheduler(MergeScheduler mergeScheduler)
                           throws CorruptIndexException,
                                  IOException

  Deprecated. use `IndexWriterConfig.setMergeScheduler(MergeScheduler)` instead

  Expert: set the merge scheduler used by this writer.

  Throws:
    `CorruptIndexException`
    `IOException`

### getMergeScheduler

    @Deprecated
    public MergeScheduler getMergeScheduler()

  Deprecated. use `IndexWriterConfig.getMergeScheduler()` instead

  Expert: returns the current MergeScheduler in use by this writer.

  See Also:
    `setMergeScheduler(MergeScheduler)`

### setMaxMergeDocs

    @Deprecated
    public void setMaxMergeDocs(int maxMergeDocs)

  Deprecated. use `LogMergePolicy.setMaxMergeDocs(int)` directly.

  Determines the largest segment (measured by document count) that may be merged with other segments. Small values (e.g., less than 10,000) are best for interactive indexing, as this limits the length of pauses while indexing to a few seconds. Larger values are best for batched indexing and speedier searches.

  The default value is `Integer.MAX_VALUE`.

  Note that this method is a convenience method: it just calls mergePolicy.setMaxMergeDocs as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

  The default merge policy (`LogByteSizeMergePolicy`) also allows you to set this limit by net size (in MB) of the segment, using `LogByteSizeMergePolicy.setMaxMergeMB(double)`.

### getMaxMergeDocs

    @Deprecated
    public int getMaxMergeDocs()

  Deprecated. use `LogMergePolicy.getMaxMergeDocs()` directly.

  Returns the largest segment (measured by document count) that may be merged with other segments.

  Note that this method is a convenience method: it just calls mergePolicy.getMaxMergeDocs as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

  See Also:
    `setMaxMergeDocs(int)`

### setMaxFieldLength

    @Deprecated
    public void setMaxFieldLength(int maxFieldLength)

  Deprecated. use `LimitTokenCountAnalyzer` instead. Note that the behvaior slightly changed - the analyzer limits the number of tokens per token stream created, while this setting limits the total number of tokens to index. This only matters if you index many multi-valued fields though.

  The maximum number of terms that will be indexed for a single field in a document. This limits the amount of memory required for indexing, so that collections with very large files will not crash the indexing process by running out of memory. This setting refers to the number of running terms, not to the number of different terms.

  Note: this silently truncates large documents, excluding from the index all terms that occur further in the document. If you know your source documents are large, be sure to set this value high enough to accomodate the expected size. If you set it to Integer.MAX_VALUE, then the only limit is your memory, but you should anticipate an OutOfMemoryError.

  By default, no more than `DEFAULT_MAX_FIELD_LENGTH` terms will be indexed for a field.

### getMaxFieldLength

    @Deprecated
    public int getMaxFieldLength()

  Deprecated. use `LimitTokenCountAnalyzer` to limit number of tokens.

  Returns the maximum number of terms that will be indexed for a single field in a document.

  See Also:
    `setMaxFieldLength(int)`

### setReaderTermsIndexDivisor

    @Deprecated
    public void setReaderTermsIndexDivisor(int divisor)

  Deprecated. use `IndexWriterConfig.setReaderTermsIndexDivisor(int)` instead.

### getReaderTermsIndexDivisor

    @Deprecated
    public int getReaderTermsIndexDivisor()

  Deprecated. use `IndexWriterConfig.getReaderTermsIndexDivisor()` instead.

### setMaxBufferedDocs

    @Deprecated
    public void setMaxBufferedDocs(int maxBufferedDocs)

  Deprecated. use `IndexWriterConfig.setMaxBufferedDocs(int)` instead.

  Determines the minimal number of documents required before the buffered in-memory documents are flushed as a new Segment. Large values generally gives faster indexing.

  When this is set, the writer will flush every maxBufferedDocs added documents. Pass in `DISABLE_AUTO_FLUSH` to prevent triggering a flush due to number of buffered documents. Note that if flushing by RAM usage is also enabled, then the flush will be triggered by whichever comes first.

  Disabled by default (writer flushes by RAM usage).

  Throws:
    `IllegalArgumentException` - if maxBufferedDocs is enabled but smaller than 2, or it disables maxBufferedDocs when ramBufferSize is already disabled
  See Also:
    `setRAMBufferSizeMB(double)`

### getMaxBufferedDocs

    @Deprecated
    public int getMaxBufferedDocs()

  Deprecated. use `IndexWriterConfig.getMaxBufferedDocs()` instead.

  Returns the number of buffered added documents that will trigger a flush if enabled.

  See Also:
    `setMaxBufferedDocs(int)`

### setRAMBufferSizeMB

    @Deprecated
    public void setRAMBufferSizeMB(double mb)

  Deprecated. use `IndexWriterConfig.setRAMBufferSizeMB(double)` instead.

  Determines the amount of RAM that may be used for buffering added documents and deletions before they are flushed to the Directory. Generally for faster indexing performance it's best to flush by RAM usage instead of document count and use as large a RAM buffer as you can.

  When this is set, the writer will flush whenever buffered documents and deletions use this much RAM. Pass in `DISABLE_AUTO_FLUSH` to prevent triggering a flush due to RAM usage. Note that if flushing by document count is also enabled, then the flush will be triggered by whichever comes first.

  NOTE: the account of RAM usage for pending deletions is only approximate. Specifically, if you delete by Query, Lucene currently has no way to measure the RAM usage if individual Queries so the accounting will under-estimate and you should compensate by either calling commit() periodically yourself, or by using `setMaxBufferedDeleteTerms(int)` to flush by count instead of RAM usage (each buffered delete Query counts as one).

  NOTE: because IndexWriter uses `int`s when managing its internal storage, the absolute maximum value for this setting is somewhat less than 2048 MB. The precise limit depends on various factors, such as how large your documents are, how many fields have norms, etc., so it's best to set this value comfortably under 2048.

  The default value is `DEFAULT_RAM_BUFFER_SIZE_MB`.

  Throws:
    `IllegalArgumentException` - if ramBufferSize is enabled but non-positive, or it disables ramBufferSize when maxBufferedDocs is already disabled

### getRAMBufferSizeMB

    @Deprecated
    public double getRAMBufferSizeMB()

  Deprecated. use `IndexWriterConfig.getRAMBufferSizeMB()` instead.

  Returns the value set by `setRAMBufferSizeMB(double)` if enabled.

### setMaxBufferedDeleteTerms

    @Deprecated
    public void setMaxBufferedDeleteTerms(int maxBufferedDeleteTerms)

  Deprecated. use `IndexWriterConfig.setMaxBufferedDeleteTerms(int)` instead.

  Determines the minimal number of delete terms required before the buffered in-memory delete terms are applied and flushed. If there are documents buffered in memory at the time, they are merged and a new segment is created.

  Disabled by default (writer flushes by RAM usage).

  Throws:
    `IllegalArgumentException` - if maxBufferedDeleteTerms is enabled but smaller than 1
  See Also:
    `setRAMBufferSizeMB(double)`

### getMaxBufferedDeleteTerms

    @Deprecated
    public int getMaxBufferedDeleteTerms()

  Deprecated. use `IndexWriterConfig.getMaxBufferedDeleteTerms()` instead

  Returns the number of buffered deleted terms that will trigger a flush if enabled.

  See Also:
    `setMaxBufferedDeleteTerms(int)`

### setMergeFactor

    @Deprecated
    public void setMergeFactor(int mergeFactor)

  Deprecated. use `LogMergePolicy.setMergeFactor(int)` directly.

  Determines how often segment indices are merged by addDocument(). With smaller values, less RAM is used while indexing, and searches on unoptimized indices are faster, but indexing speed is slower. With larger values, more RAM is used during indexing, and while searches on unoptimized indices are slower, indexing is faster. Thus larger values (> 10) are best for batch index creation, and smaller values (< 10) for indices that are interactively maintained.

  Note that this method is a convenience method: it just calls mergePolicy.setMergeFactor as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

  This must never be less than 2. The default value is 10.

### getMergeFactor

    @Deprecated
    public int getMergeFactor()

  Deprecated. use `LogMergePolicy.getMergeFactor()` directly.

  Returns the number of segments that are merged at once and also controls the total number of segments allowed to accumulate in the index.

  Note that this method is a convenience method: it just calls mergePolicy.getMergeFactor as long as mergePolicy is an instance of `LogMergePolicy`. Otherwise an IllegalArgumentException is thrown.

  See Also:
    `setMergeFactor(int)`

### setDefaultInfoStream

    public static void setDefaultInfoStream(PrintStream infoStream)

  If non-null, this will be the default infoStream used by a newly instantiated IndexWriter.

  See Also:
    `setInfoStream(java.io.PrintStream)`

### getDefaultInfoStream

    public static PrintStream getDefaultInfoStream()

  Returns the current default infoStream for newly instantiated IndexWriters.

  See Also:
    `setDefaultInfoStream(java.io.PrintStream)`

### setInfoStream

    public void setInfoStream(PrintStream infoStream)
                       throws IOException

  If non-null, information about merges, deletes and a message when maxFieldLength is reached will be printed to this.

  Throws:
    `IOException`

### getInfoStream

    public PrintStream getInfoStream()

  Returns the current infoStream in use by this writer.

  See Also:
    `setInfoStream(java.io.PrintStream)`

### verbose

    public boolean verbose()

  Returns true if verbosing is enabled (i.e., infoStream != null).

### setWriteLockTimeout

    @Deprecated
    public void setWriteLockTimeout(long writeLockTimeout)

  Deprecated. use `IndexWriterConfig.setWriteLockTimeout(long)` instead

  Sets the maximum time to wait for a write lock (in milliseconds) for this instance of IndexWriter. @see

  See Also:
    `to change the default value for all instances of IndexWriter.`

### getWriteLockTimeout

    @Deprecated
    public long getWriteLockTimeout()

  Deprecated. use `IndexWriterConfig.getWriteLockTimeout()`

  Returns allowed timeout when acquiring the write lock.

  See Also:
    `setWriteLockTimeout(long)`

### setDefaultWriteLockTimeout

    @Deprecated
    public static void setDefaultWriteLockTimeout(long writeLockTimeout)

  Deprecated. use `IndexWriterConfig.setDefaultWriteLockTimeout(long)` instead

  Sets the default (for any instance of IndexWriter) maximum time to wait for a write lock (in milliseconds).

### getDefaultWriteLockTimeout

    @Deprecated
    public static long getDefaultWriteLockTimeout()

  Deprecated. use `IndexWriterConfig.getDefaultWriteLockTimeout()` instead

  Returns default write lock timeout for newly instantiated IndexWriters.

  See Also:
    `setDefaultWriteLockTimeout(long)`

### close

    public void close()
               throws CorruptIndexException,
                      IOException

  Commits all changes to an index and closes all associated files. Note that this may be a costly operation, so, try to re-use a single writer instead of closing and opening a new one. See `commit()` for caveats about write caching done by some IO devices.

  If an Exception is hit during close, eg due to disk full or some other reason, then both the on-disk index and the internal state of the IndexWriter instance will be consistent. However, the close will not be complete even though part of it (flushing buffered documents) may have succeeded, so the write lock will still be held.

  If you can correct the underlying cause (eg free up some disk space) then you can call close() again. Failing that, if you want to force the write lock to be released (dangerous, because you may then lose buffered docs in the IndexWriter instance) then you can do something like this:

       try {
         writer.close();
       } finally {
         if (IndexWriter.isLocked(directory)) {
           IndexWriter.unlock(directory);
         }
       }

  after which, you must be certain not to use the writer instance anymore.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer, again. See above for details.

  Specified by:
    `close` in interface `Closeable`
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### close

    public void close(boolean waitForMerges)
               throws CorruptIndexException,
                      IOException

  Closes the index with or without waiting for currently running merges to finish. This is only meaningful when using a MergeScheduler that runs merges in background threads.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer, again. See above for details.

  NOTE: it is dangerous to always call close(false), especially when IndexWriter is not open for very long, because this can result in "merge starvation" whereby long merges will never have a chance to finish. This will cause too many segments in your index over time.

  Parameters:
    `waitForMerges` - if true, this call will block until all merges complete; else, it will ask all running merges to abort, wait until those merges have finished (which should be at most a few seconds), and then return.
  Throws:
    `CorruptIndexException`
    `IOException`

### getDirectory

    public Directory getDirectory()

  Returns the Directory used by this index.

### getAnalyzer

    public Analyzer getAnalyzer()

  Returns the analyzer used by this index.

### maxDoc

    public int maxDoc()

  Returns total number of docs in this index, including docs not yet flushed (still in the RAM buffer), not counting deletions.

  See Also:
    `numDocs()`

### numDocs

    public int numDocs()
                throws IOException

  Returns total number of docs in this index, including docs not yet flushed (still in the RAM buffer), and including deletions. NOTE: buffered deletions are not counted. If you really need these to be counted you should call `commit()` first.

  Throws:
    `IOException`
  See Also:
    `numDocs()`

### hasDeletions

    public boolean hasDeletions()
                         throws IOException

  Throws:
    `IOException`

### addDocument

    public void addDocument(Document doc)
                     throws CorruptIndexException,
                            IOException

  Adds a document to this index. If the document contains more than `setMaxFieldLength(int)` terms for a given field, the remainder are discarded.

  Note that if an Exception is hit (for example disk full) then the index will be consistent, but this document may not have been added. Furthermore, it's possible the index will have one segment in non-compound format even when using compound files (when a merge has partially succeeded).

  This method periodically flushes pending documents to the Directory (see above), and also periodically triggers segment merges in the index according to the `MergePolicy` in use.

  Merges temporarily consume space in the directory. The amount of space required is up to 1X the size of all segments being merged, when no readers/searchers are open against the index, and up to 2X the size of all segments being merged when readers/searchers are open against the index (see `forceMerge(int)` for details). The sequence of primitive merge operations performed is governed by the merge policy.

  Note that each term in the document can be no longer than 16383 characters, otherwise an IllegalArgumentException will be thrown.

  Note that it's possible to create an invalid Unicode string in java if a UTF16 surrogate pair is malformed. In this case, the invalid characters are silently replaced with the Unicode replacement character U+FFFD.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

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

### addDocument

    public void addDocument(Document doc,
                            Analyzer analyzer)
                     throws CorruptIndexException,
                            IOException

  Adds a document to this index, using the provided analyzer instead of the value of `getAnalyzer()`. If the document contains more than `setMaxFieldLength(int)` terms for a given field, the remainder are discarded.

  See `addDocument(Document)` for details on index and IndexWriter state after an Exception, and flushing/merging temporary free space requirements.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

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

### addDocuments

    public void addDocuments(Collection<Document> docs)
                      throws CorruptIndexException,
                             IOException

  Atomically adds a block of documents with sequentially assigned document IDs, such that an external reader will see all or none of the documents.

  WARNING: the index does not currently record which documents were added as a block. Today this is fine, because merging will preserve a block. The order of documents within a segment will be preserved, even when child documents within a block are deleted. Most search features (like result grouping and block joining) require you to mark documents; when these documents are deleted these search features will not work as expected. Obviously adding documents to an existing block will require you the reindex the entire block.

  However it's possible that in the future Lucene may merge more aggressively re-order documents (for example, perhaps to obtain better index compression), in which case you may need to fully re-index your documents at that time.

  See `addDocument(Document)` for details on index and IndexWriter state after an Exception, and flushing/merging temporary free space requirements.

  NOTE: tools that do offline splitting of an index (for example, IndexSplitter in contrib) or re-sorting of documents (for example, IndexSorter in contrib) are not aware of these atomically added documents and will likely break them up. Use such tools at your own risk!

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### addDocuments

    public void addDocuments(Collection<Document> docs,
                             Analyzer analyzer)
                      throws CorruptIndexException,
                             IOException

  Atomically adds a block of documents, analyzed using the provided analyzer, with sequentially assigned document IDs, such that an external reader will see all or none of the documents.

  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### updateDocuments

    public void updateDocuments(Term delTerm,
                                Collection<Document> docs)
                         throws CorruptIndexException,
                                IOException

  Atomically deletes documents matching the provided delTerm and adds a block of documents with sequentially assigned document IDs, such that an external reader will see all or none of the documents. See `addDocuments(Collection)`.

  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### updateDocuments

    public void updateDocuments(Term delTerm,
                                Collection<Document> docs,
                                Analyzer analyzer)
                         throws CorruptIndexException,
                                IOException

  Atomically deletes documents matching the provided delTerm and adds a block of documents, analyzed using the provided analyzer, with sequentially assigned document IDs, such that an external reader will see all or none of the documents. See `addDocuments(Collection)`.

  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  WARNING: This API is experimental and might change in incompatible ways in the next release.

### deleteDocuments

    public void deleteDocuments(Term term)
                         throws CorruptIndexException,
                                IOException

  Deletes the document(s) containing `term`.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `term` - the term to identify the documents to be deleted
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### deleteDocuments

    public void deleteDocuments(Term... terms)
                         throws CorruptIndexException,
                                IOException

  Deletes the document(s) containing any of the terms. All deletes are flushed at the same time.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `terms` - array of terms to identify the documents to be deleted
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### deleteDocuments

    public void deleteDocuments(Query query)
                         throws CorruptIndexException,
                                IOException

  Deletes the document(s) matching the provided query.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `query` - the query to identify the documents to be deleted
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### deleteDocuments

    public void deleteDocuments(Query... queries)
                         throws CorruptIndexException,
                                IOException

  Deletes the document(s) matching any of the provided queries. All deletes are flushed at the same time.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `queries` - array of queries to identify the documents to be deleted
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### updateDocument

    public void updateDocument(Term term,
                               Document doc)
                        throws CorruptIndexException,
                               IOException

  Updates a document by first deleting the document(s) containing `term` and then adding the new document. The delete and then add are atomic as seen by a reader on the same index (flush may happen only after the add).

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `term` - the term to identify the document(s) to be deleted
    `doc` - the document to be added
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### updateDocument

    public void updateDocument(Term term,
                               Document doc,
                               Analyzer analyzer)
                        throws CorruptIndexException,
                               IOException

  Updates a document by first deleting the document(s) containing `term` and then adding the new document. The delete and then add are atomic as seen by a reader on the same index (flush may happen only after the add).

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Parameters:
    `term` - the term to identify the document(s) to be deleted
    `doc` - the document to be added
    `analyzer` - the analyzer to use when analyzing the document
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error

### optimize

    @Deprecated
    public void optimize()
                  throws CorruptIndexException,
                         IOException

  Deprecated.

  This method has been deprecated, as it is horribly inefficient and very rarely justified. Lucene's multi-segment search performance has improved over time, and the default TieredMergePolicy now targets segments with deletions.

  Throws:
    `CorruptIndexException`
    `IOException`

### optimize

    @Deprecated
    public void optimize(int maxNumSegments)
                  throws CorruptIndexException,
                         IOException

  Deprecated.

  This method has been deprecated, as it is horribly inefficient and very rarely justified. Lucene's multi-segment search performance has improved over time, and the default TieredMergePolicy now targets segments with deletions.

  Throws:
    `CorruptIndexException`
    `IOException`

### optimize

    @Deprecated
    public void optimize(boolean doWait)
                  throws CorruptIndexException,
                         IOException

  Deprecated.

  This method has been deprecated, as it is horribly inefficient and very rarely justified. Lucene's multi-segment search performance has improved over time, and the default TieredMergePolicy now targets segments with deletions.

  Throws:
    `CorruptIndexException`
    `IOException`

### forceMerge

    public void forceMerge(int maxNumSegments)
                    throws CorruptIndexException,
                           IOException

  Forces merge policy to merge segments until there's <= maxNumSegments. The actual merges to be executed are determined by the `MergePolicy`.

  This is a horribly costly operation, especially when you pass a small `maxNumSegments`; usually you should only call this if the index is static (will no longer be changed).

  Note that this requires up to 2X the index size free space in your Directory (3X if you're using compound file format). For example, if your index size is 10 MB then you need up to 20 MB free for this to complete (30 MB if you're using compound file format). Also, it's best to call `commit()` afterwards, to allow IndexWriter to free up disk space.

  If some but not all readers re-open while merging is underway, this will cause > 2X temporary space to be consumed as those new readers will then hold open the temporary segments at that time. It is best not to re-open readers while merging is running.

  The actual temporary usage could be much less than these figures (it depends on many factors).

  In general, once the this completes, the total size of the index will be less than the size of the starting index. It could be quite a bit smaller (if there were many pending deletes) or just slightly smaller.

  If an Exception is hit, for example due to disk full, the index will not be corrupt and no documents will have been lost. However, it may have been partially merged (some segments were merged but not all), and it's possible that one of the segments in the index will be in non-compound format even when using compound file format. This will occur when the Exception is hit during conversion of the segment into compound format.

  This call will merge those segments present in the index when the call started. If other threads are still adding documents and flushing segments, those newly created segments will not be merged unless you call forceMerge again.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  NOTE: if you call `close(boolean)` with `false`, which aborts all running merges, then any thread still running this method might hit a `MergePolicy.MergeAbortedException`.

  Parameters:
    `maxNumSegments` - maximum number of segments left in the index after merging finishes
  Throws:
    `CorruptIndexException` - if the index is corrupt
    `IOException` - if there is a low-level IO error
  See Also:
    `MergePolicy.findMerges(org.apache.lucene.index.SegmentInfos)`

### forceMerge

    public void forceMerge(int maxNumSegments,
                           boolean doWait)
                    throws CorruptIndexException,
                           IOException

  Just like `forceMerge(int)`, except you can specify whether the call should block until all merging completes. This is only meaningful with a `MergeScheduler` that is able to run merges in background threads.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Throws:
    `CorruptIndexException`
    `IOException`

### expungeDeletes

    @Deprecated
    public void expungeDeletes(boolean doWait)
                        throws CorruptIndexException,
                               IOException

  Deprecated.

  This method has been deprecated, as it is horribly inefficient and very rarely justified. Lucene's multi-segment search performance has improved over time, and the default TieredMergePolicy now targets segments with deletions.

  Throws:
    `CorruptIndexException`
    `IOException`

### forceMergeDeletes

    public void forceMergeDeletes(boolean doWait)
                           throws CorruptIndexException,
                                  IOException

  Just like `forceMergeDeletes()`, except you can specify whether the call should block until the operation completes. This is only meaningful with a `MergeScheduler` that is able to run merges in background threads.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  NOTE: if you call `close(boolean)` with `false`, which aborts all running merges, then any thread still running this method might hit a `MergePolicy.MergeAbortedException`.

  Throws:
    `CorruptIndexException`
    `IOException`

### expungeDeletes

    @Deprecated
    public void expungeDeletes()
                        throws CorruptIndexException,
                               IOException

  Deprecated.

  This method has been deprecated, as it is horribly inefficient and very rarely justified. Lucene's multi-segment search performance has improved over time, and the default TieredMergePolicy now targets segments with deletions.

  Throws:
    `CorruptIndexException`
    `IOException`

### forceMergeDeletes

    public void forceMergeDeletes()
                           throws CorruptIndexException,
                                  IOException

  Forces merging of all segments that have deleted documents. The actual merges to be executed are determined by the `MergePolicy`. For example, the default `TieredMergePolicy` will only pick a segment if the percentage of deleted docs is over 10%.

  This is often a horribly costly operation; rarely is it warranted.

  To see how many deletions you have pending in your index, call `IndexReader.numDeletedDocs()`.

  NOTE: this method first flushes a new segment (if there are indexed documents), and applies all buffered deletes.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Throws:
    `CorruptIndexException`
    `IOException`

### maybeMerge

    public final void maybeMerge()
                          throws CorruptIndexException,
                                 IOException

  Expert: asks the mergePolicy whether any merges are necessary now and if so, runs the requested merges and then iterate (test again if merges are needed) until no more merges are returned by the mergePolicy. Explicit calls to maybeMerge() are usually not necessary. The most common case is when merge policy parameters have changed.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Throws:
    `CorruptIndexException`
    `IOException`

### getMergingSegments

    public Collection<SegmentInfo> getMergingSegments()

  Expert: to be used by a `MergePolicy` to avoid selecting merges for segments already being merged. The returned collection is not cloned, and thus is only safe to access if you hold IndexWriter's lock (which you do when IndexWriter invokes the MergePolicy).

  Do not alter the returned collection!

### getNextMerge

    public MergePolicy.OneMerge getNextMerge()

  Expert: the `MergeScheduler` calls this method to retrieve the next merge requested by the MergePolicy

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

### rollback

    public void rollback()
                  throws IOException

  Close the `IndexWriter` without committing any changes that have occurred since the last commit (or since it was opened, if commit hasn't been called). This removes any temporary files that had been created, after which the state of the index will be the same as it was when commit() was last called or when this writer was first opened. This also clears a previous call to `prepareCommit()`.

  Specified by:
    `rollback` in interface `TwoPhaseCommit`
  Throws:
    `IOException` - if there is a low-level IO error

### deleteAll

    public void deleteAll()
                   throws IOException

  Delete all documents in the index.

  This method will drop all buffered documents and will remove all segments from the index. This change will not be visible until a `commit()` has been called. This method can be rolled back using `rollback()`.

  NOTE: this method is much faster than using deleteDocuments( new MatchAllDocsQuery() ).

  NOTE: this method will forcefully abort all merges in progress. If other threads are running `forceMerge(int)`, `addIndexes(IndexReader[])` or `forceMergeDeletes(boolean)` methods, they may receive `MergePolicy.MergeAbortedException`s.

  Throws:
    `IOException`

### waitForMerges

    public void waitForMerges()

  Wait for any currently outstanding merges to finish.

  It is guaranteed that any merges started prior to calling this method will have completed once this method completes.

### addIndexesNoOptimize

    @Deprecated
    public void addIndexesNoOptimize(Directory... dirs)
                              throws CorruptIndexException,
                                     IOException

  Deprecated. use `addIndexes(Directory...)` instead

  Throws:
    `CorruptIndexException`
    `IOException`

### addIndexes

    public void addIndexes(Directory... dirs)
                    throws CorruptIndexException,
                           IOException

  Adds all segments from an array of indexes into this index.

  This may be used to parallelize batch indexing. A large document collection can be broken into sub-collections. Each sub-collection can be indexed in parallel, on a different thread, process or machine. The complete index can then be created by merging sub-collection indexes with this method.

  NOTE: the index in each `Directory` must not be changed (opened by a writer) while this method is running. This method does not acquire a write lock in each input Directory, so it is up to the caller to enforce this.

  This method is transactional in how Exceptions are handled: it does not commit a new segments_N file until all indexes are added. This means if an Exception occurs (for example disk full), then either no indexes will have been added or they all will have been.

  Note that this requires temporary free space in the `Directory` up to 2X the sum of all input indexes (including the starting index). If readers/searchers are open against the starting index, then temporary free space required will be higher by the size of the starting index (see `forceMerge(int)` for details).

  NOTE: this method only copies the segments of the incomning indexes and does not merge them. Therefore deleted documents are not removed and the new segments are not merged with the existing ones.

  This requires this index not be among those to be added.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

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

### addIndexes

    public void addIndexes(IndexReader... readers)
                    throws CorruptIndexException,
                           IOException

  Merges the provided indexes into this index. This method is useful if you use extensions of `IndexReader`. Otherwise, using `addIndexes(Directory...)` is highly recommended for performance reasons. It uses the `MergeScheduler` and `MergePolicy` set on this writer, which may perform merges in parallel.

  The provided IndexReaders are not closed.

  NOTE: this method does not merge the current segments, only the incoming ones.

  See `addIndexes(Directory...)` for details on transactional semantics, temporary free space required in the Directory, and non-CFS segments on an Exception.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  NOTE: if you call `close(boolean)` with `false`, which aborts all running merges, then any thread still running this method might hit a `MergePolicy.MergeAbortedException`.

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

### doAfterFlush

    protected void doAfterFlush()
                         throws IOException

  A hook for extending classes to execute operations after pending added and deleted documents have been flushed to the Directory but before the change is committed (new segments_N file written).

  Throws:
    `IOException`

### doBeforeFlush

    protected void doBeforeFlush()
                          throws IOException

  A hook for extending classes to execute operations before pending added and deleted documents are flushed to the Directory.

  Throws:
    `IOException`

### prepareCommit

    public final void prepareCommit()
                             throws CorruptIndexException,
                                    IOException

  Expert: prepare for commit.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Specified by:
    `prepareCommit` in interface `TwoPhaseCommit`
  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `prepareCommit(Map)`

### prepareCommit

    public final void prepareCommit(Map<String,String> commitUserData)
                             throws CorruptIndexException,
                                    IOException

  Expert: prepare for commit, specifying commitUserData Map (String -> String). This does the first phase of 2-phase commit. This method does all steps necessary to commit changes since this writer was opened: flushes pending added and deleted docs, syncs the index files, writes most of next segments_N file. After calling this you must call either `commit()` to finish the commit, or `rollback()` to revert the commit and undo all changes done since the writer was opened.

  You can also just call `commit(Map)` directly without prepareCommit first in which case that method will internally call prepareCommit.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Specified by:
    `prepareCommit` in interface `TwoPhaseCommit`
  Parameters:
    `commitUserData` - Opaque Map (String->String) that's recorded into the segments file in the index, and retrievable by `IndexCommit.getUserData()`. Note that when IndexWriter commits itself during `close()`, the commitUserData is unchanged (just carried over from the prior commit). If this is null then the previous commitUserData is kept. Also, the commitUserData will only "stick" if there are actually changes in the index to commit.
  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `TwoPhaseCommit.prepareCommit()`

### commit

    public final void commit()
                      throws CorruptIndexException,
                             IOException

  Commits all pending changes (added & deleted documents, segment merges, added indexes, etc.) to the index, and syncs all referenced index files, such that a reader will see the changes and the index updates will survive an OS or machine crash or power loss. Note that this does not wait for any running background merges to finish. This may be a costly operation, so you should test the cost in your application and do it only when really necessary.

  Note that this operation calls Directory.sync on the index files. That call should not return until the file contents & metadata are on stable storage. For FSDirectory, this calls the OS's fsync. But, beware: some hardware devices may in fact cache writes even during fsync, and return before the bits are actually on stable storage, to give the appearance of faster performance. If you have such a device, and it does not have a battery backup (for example) then on power loss it may still lose data. Lucene cannot guarantee consistency on such devices.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Specified by:
    `commit` in interface `TwoPhaseCommit`
  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `prepareCommit()`, `commit(Map)`

### commit

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

  Commits all changes to the index, specifying a commitUserData Map (String -> String). This just calls `prepareCommit(Map)` (if you didn't already call it) and then `finishCommit()`.

  NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.

  Specified by:
    `commit` in interface `TwoPhaseCommit`
  Throws:
    `CorruptIndexException`
    `IOException`
  See Also:
    `TwoPhaseCommit.commit()`, `TwoPhaseCommit.prepareCommit(Map)`

### flush

    protected final void flush(boolean triggerMerge,
                               boolean flushDocStores,
                               boolean flushDeletes)
                        throws CorruptIndexException,
                               IOException

  NOTE: flushDocStores is ignored now (hardwired to true); this method is only here for backwards compatibility

  Throws:
    `CorruptIndexException`
    `IOException`

### flush

    protected final void flush(boolean triggerMerge,
                               boolean applyAllDeletes)
                        throws CorruptIndexException,
                               IOException

  Flush all in-memory buffered updates (adds and deletes) to the Directory.

  Parameters:
    `triggerMerge` - if true, we may merge segments (if deletes or docs were flushed) if necessary
    `applyAllDeletes` - whether pending deletes should also
  Throws:
    `CorruptIndexException`
    `IOException`

### ramSizeInBytes

    public final long ramSizeInBytes()

  Expert: Return the total size of all index files currently cached in memory. Useful for size management with flushRamDocs()

### numRamDocs

    public final int numRamDocs()

  Expert: Return the number of documents currently buffered in RAM.

### merge

    public void merge(MergePolicy.OneMerge merge)
               throws CorruptIndexException,
                      IOException

  Merges the indicated segments, replacing them in the stack with a single segment.

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

### segString

    public String segString()
                     throws IOException

  Throws:
    `IOException`
  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

### segString

    public String segString(Iterable<SegmentInfo> infos)
                     throws IOException

  Throws:
    `IOException`
  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

### segString

    public String segString(SegmentInfo info)
                     throws IOException

  Throws:
    `IOException`
  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

### isLocked

    public static boolean isLocked(Directory directory)
                            throws IOException

  Returns `true` iff the index in the named directory is currently locked.

  Parameters:
    `directory` - the directory to check for a lock
  Throws:
    `IOException` - if there is a low-level IO error

### unlock

    public static void unlock(Directory directory)
                       throws IOException

  Forcibly unlocks the index in the named directory.

  Caution: this should only be used by failure recovery code, when it is known that no other process nor thread is in fact currently accessing this index.

  Throws:
    `IOException`

### setMergedSegmentWarmer

    @Deprecated
    public void setMergedSegmentWarmer(IndexWriter.IndexReaderWarmer warmer)

  Deprecated. use `IndexWriterConfig.setMergedSegmentWarmer(org.apache.lucene.index.IndexWriter.IndexReaderWarmer)` instead.

  Set the merged segment warmer. See `IndexWriter.IndexReaderWarmer`.

### getMergedSegmentWarmer

    @Deprecated
    public IndexWriter.IndexReaderWarmer getMergedSegmentWarmer()

  Deprecated. use `IndexWriterConfig.getMergedSegmentWarmer()` instead.

  Returns the current merged segment warmer. See `IndexWriter.IndexReaderWarmer`.

### deleteUnusedFiles

    public void deleteUnusedFiles()
                           throws IOException

  Expert: remove any index files that are no longer used.

  IndexWriter normally deletes unused files itself, during indexing. However, on Windows, which disallows deletion of open files, if there is a reader open on the index then those files cannot be deleted. This is fine, because IndexWriter will periodically retry the deletion.

  However, IndexWriter doesn't try that often: only on open, close, flushing a new segment, and finishing a merge. If you don't do any of these actions with your IndexWriter, you'll see the unused files linger. If that's a problem, call this method to delete them (once you've closed the open readers that were preventing their deletion).

  In addition, you can call this method to delete unreferenced index commits. This might be useful if you are using an `IndexDeletionPolicy` which holds onto index commits until some criteria are met, but those commits are no longer needed. Otherwise, those commits will be deleted the next time commit() is called.

  Throws:
    `IOException`

### setPayloadProcessorProvider

    public void setPayloadProcessorProvider(PayloadProcessorProvider pcp)

  Sets the `PayloadProcessorProvider` to use when merging payloads. Note that the given `pcp` will be invoked for every segment that is merged, not only external ones that are given through `addIndexes(org.apache.lucene.store.Directory...)`. If you want only the payloads of the external segments to be processed, you can return `null` whenever a `PayloadProcessorProvider.ReaderPayloadProcessor` is requested for the `Directory` of the `IndexWriter`.

  The default is `null` which means payloads are processed normally (copied) during segment merges. You can also unset it by passing `null`.

  NOTE: the set `PayloadProcessorProvider` will be in effect immediately, potentially for already running merges too. If you want to be sure it is used for further operations only, such as `addIndexes(org.apache.lucene.store.Directory...)` or `forceMerge(int)`, you can call `waitForMerges()` before.

### getPayloadProcessorProvider

    public PayloadProcessorProvider getPayloadProcessorProvider()

  Returns the `PayloadProcessorProvider` that is used during segment merges to process payloads.

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