Package org.apache.lucene.store

# Class Directory

java.lang.Object
org.apache.lucene.store.Directory

All Implemented Interfaces:
  `Closeable`, `AutoCloseable`

Direct Known Subclasses:
  `BaseDirectory`, `CompoundDirectory`, `FileSwitchDirectory`, `FilterDirectory`

public abstract class Directory extends Object implements Closeable
A `Directory` provides an abstraction layer for storing a list of files. A directory contains only files (no sub-folder hierarchy).

Implementing classes must comply with the following:

- A file in a directory can be created (`createOutput(java.lang.String, org.apache.lucene.store.IOContext)`), appended to, then closed.
- A file open for writing may not be available for read access until the corresponding `IndexOutput` is closed.
- Once a file is created it must only be opened for input (`openInput(java.lang.String, org.apache.lucene.store.IOContext)`), or deleted (`deleteFile(java.lang.String)`). Calling `createOutput(java.lang.String, org.apache.lucene.store.IOContext)` on an existing file must throw `FileAlreadyExistsException`.

NOTE: If your application requires external synchronization, you should not synchronize on the `Directory` implementation instance as this may cause deadlock; use your own (non-Lucene) objects instead.

See Also:
  - `FSDirectory`
  - `ByteBuffersDirectory`
  - `FilterDirectory`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `Directory()`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `abstract void`
  `close()`
  Closes the directory.
  `void`
  `copyFrom(Directory from, String src, String dest, IOContext context)`
  Copies an existing `src` file from directory `from` to a non-existent file `dest` in this directory.
  `abstract IndexOutput`
  `createOutput(String name, IOContext context)`
  Creates a new, empty file in the directory and returns an `IndexOutput` instance for appending data to this file.
  `abstract IndexOutput`
  `createTempOutput(String prefix, String suffix, IOContext context)`
  Creates a new, empty, temporary file in the directory and returns an `IndexOutput` instance for appending data to this file.
  `abstract void`
  `deleteFile(String name)`
  Removes an existing file in the directory.
  `protected void`
  `ensureOpen()`
  Ensures this directory is still open.
  `abstract long`
  `fileLength(String name)`
  Returns the byte length of a file in the directory.
  `abstract Set<String>`
  `getPendingDeletions()`
  Returns a set of files currently pending deletion in this directory.
  `protected static String`
  `getTempFileName(String prefix, String suffix, long counter)`
  Creates a file name for a temporary file.
  `abstract String[]`
  `listAll()`
  Returns names of all files stored in this directory.
  `abstract Lock`
  `obtainLock(String name)`
  Acquires and returns a `Lock` for a file with the given name.
  `ChecksumIndexInput`
  `openChecksumInput(String name)`
  Opens a checksum-computing stream for reading an existing file.
  `abstract IndexInput`
  `openInput(String name, IOContext context)`
  Opens a stream for reading an existing file.
  `abstract void`
  `rename(String source, String dest)`
  Renames `source` file to `dest` file where `dest` must not already exist in the directory.
  `abstract void`
  `sync(Collection<String> names)`
  Ensures that any writes to these files are moved to stable storage (made durable).
  `abstract void`
  `syncMetaData()`
  Ensures that directory metadata, such as recent file renames, are moved to stable storage.
  `String`
  `toString()`

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

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

- ## Constructor Details

  - ### Directory

    public Directory()

- ## Method Details

  - ### listAll

    public abstract String[] listAll() throws IOException
    Returns names of all files stored in this directory. The output must be in sorted (UTF-16, java's `String.compareTo(java.lang.String)`) order.
    Throws:
      `IOException` - in case of I/O error

  - ### deleteFile

    public abstract void deleteFile(String name) throws IOException
    Removes an existing file in the directory.

    This method must throw either `NoSuchFileException` or `FileNotFoundException` if `name` points to a non-existing file.

    Parameters:
      `name` - the name of an existing file.
    Throws:
      `IOException` - in case of I/O error

  - ### fileLength

    public abstract long fileLength(String name) throws IOException
    Returns the byte length of a file in the directory.

    This method must throw either `NoSuchFileException` or `FileNotFoundException` if `name` points to a non-existing file.

    Parameters:
      `name` - the name of an existing file.
    Throws:
      `IOException` - in case of I/O error

  - ### createOutput

    public abstract IndexOutput createOutput(String name, IOContext context) throws IOException
    Creates a new, empty file in the directory and returns an `IndexOutput` instance for appending data to this file.

    This method must throw `FileAlreadyExistsException` if the file already exists.

    Parameters:
      `name` - the name of the file to create.
    Throws:
      `IOException` - in case of I/O error

  - ### createTempOutput

    public abstract IndexOutput createTempOutput(String prefix, String suffix, IOContext context) throws IOException
    Creates a new, empty, temporary file in the directory and returns an `IndexOutput` instance for appending data to this file.

    The temporary file name (accessible via `IndexOutput.getName()`) will start with `prefix`, end with `suffix` and have a reserved file extension `.tmp`.

    Throws:
      `IOException`

  - ### sync

    public abstract void sync(Collection<String> names) throws IOException
    Ensures that any writes to these files are moved to stable storage (made durable).

    Lucene uses this to properly commit changes to the index, to prevent a machine/OS crash from corrupting the index.

    Throws:
      `IOException`
    See Also:
      - `syncMetaData()`

  - ### syncMetaData

    public abstract void syncMetaData() throws IOException
    Ensures that directory metadata, such as recent file renames, are moved to stable storage.
    Throws:
      `IOException`
    See Also:
      - `sync(Collection)`

  - ### rename

    public abstract void rename(String source, String dest) throws IOException
    Renames `source` file to `dest` file where `dest` must not already exist in the directory.

    It is permitted for this operation to not be truly atomic, for example both `source` and `dest` can be visible temporarily in `listAll()`. However, the implementation of this method must ensure the content of `dest` appears as the entire `source` atomically. So once `dest` is visible for readers, the entire content of previous `source` is visible.

    This method is used by IndexWriter to publish commits.

    Throws:
      `IOException`

  - ### openInput

    public abstract IndexInput openInput(String name, IOContext context) throws IOException
    Opens a stream for reading an existing file.

    This method must throw either `NoSuchFileException` or `FileNotFoundException` if `name` points to a non-existing file.

    Parameters:
      `name` - the name of an existing file.
    Throws:
      `IOException` - in case of I/O error

  - ### openChecksumInput

    public ChecksumIndexInput openChecksumInput(String name) throws IOException
    Opens a checksum-computing stream for reading an existing file.

    This method must throw either `NoSuchFileException` or `FileNotFoundException` if `name` points to a non-existing file.

    Parameters:
      `name` - the name of an existing file.
    Throws:
      `IOException` - in case of I/O error

  - ### obtainLock

    public abstract Lock obtainLock(String name) throws IOException
    Acquires and returns a `Lock` for a file with the given name.
    Parameters:
      `name` - the name of the lock file
    Throws:
      `LockObtainFailedException` - (optional specific exception) if the lock could not be obtained because it is currently held elsewhere.
      `IOException` - if any i/o error occurs attempting to gain the lock

  - ### close

    public abstract void close() throws IOException
    Closes the directory.
    Specified by:
      `close` in interface `AutoCloseable`
    Specified by:
      `close` in interface `Closeable`
    Throws:
      `IOException`

  - ### copyFrom

    public void copyFrom(Directory from, String src, String dest, IOContext context) throws IOException
    Copies an existing `src` file from directory `from` to a non-existent file `dest` in this directory. The given IOContext is only used for opening the destination file.
    Throws:
      `IOException`

  - ### toString

    public String toString()
    Overrides:
      `toString` in class `Object`

  - ### ensureOpen

    protected void ensureOpen() throws AlreadyClosedException
    Ensures this directory is still open.
    Throws:
      `AlreadyClosedException` - if this directory is closed.

  - ### getPendingDeletions

    public abstract Set<String> getPendingDeletions() throws IOException
    Returns a set of files currently pending deletion in this directory.
    Throws:
      `IOException`
    NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

  - ### getTempFileName

    protected static String getTempFileName(String prefix, String suffix, long counter)
    Creates a file name for a temporary file. The name will start with `prefix`, end with `suffix` and have a reserved file extension `.tmp`.
    See Also:
      - `createTempOutput(String, String, IOContext)`