org.apache.lucene.store

Class FSDirectory

  • All Implemented Interfaces:
    Closeable, AutoCloseable
    Direct Known Subclasses:
    MMapDirectory, NativeUnixDirectory, NIOFSDirectory, RAFDirectory, SimpleFSDirectory, WindowsDirectory


    public abstract class FSDirectory
    extends BaseDirectory
    Base class for Directory implementations that store index files in the file system. There are currently three core subclasses:
    • SimpleFSDirectory is a straightforward implementation using Files.newByteChannel. However, it has poor concurrent performance (multiple threads will bottleneck) as it synchronizes when multiple threads read from the same file.
    • NIOFSDirectory uses java.nio's FileChannel's positional io when reading to avoid synchronization when reading from the same file. Unfortunately, due to a Windows-only Sun JRE bug this is a poor choice for Windows, but on all other platforms this is the preferred choice. Applications using Thread.interrupt() or Future.cancel(boolean) should use RAFDirectory instead. See NIOFSDirectory java doc for details.
    • MMapDirectory uses memory-mapped IO when reading. This is a good choice if you have plenty of virtual memory relative to your index size, eg if you are running on a 64 bit JRE, or you are running on a 32 bit JRE but your index sizes are small enough to fit into the virtual memory space. Java has currently the limitation of not being able to unmap files from user code. The files are unmapped, when GC releases the byte buffers. Due to this bug in Sun's JRE, MMapDirectory's IndexInput.close() is unable to close the underlying OS file handle. Only when GC finally collects the underlying objects, which could be quite some time later, will the file handle be closed. This will consume additional transient disk usage: on Windows, attempts to delete or overwrite the files will result in an exception; on other platforms, which typically have a "delete on last close" semantics, while such operations will succeed, the bytes are still consuming space on disk. For many applications this limitation is not a problem (e.g. if you have plenty of disk space, and you don't rely on overwriting files on Windows) but it's still an important limitation to be aware of. This class supplies a (possibly dangerous) workaround mentioned in the bug report, which may fail on non-Sun JVMs.

    Unfortunately, because of system peculiarities, there is no single overall best implementation. Therefore, we've added the open(java.nio.file.Path) method, to allow Lucene to choose the best FSDirectory implementation given your environment, and the known limitations of each implementation. For users who have no reason to prefer a specific implementation, it's best to simply use open(java.nio.file.Path). For all others, you should instantiate the desired implementation directly.

    NOTE: Accessing one of the above subclasses either directly or indirectly from a thread while it's interrupted can close the underlying channel immediately if at the same time the thread is blocked on IO. The channel will remain closed and subsequent access to the index will throw a ClosedChannelException. Applications using Thread.interrupt() or Future.cancel(boolean) should use the slower legacy RAFDirectory from the misc Lucene module instead.

    The locking implementation is by default NativeFSLockFactory, but can be changed by passing in a custom LockFactory instance.

    See Also:
    Directory
    • Constructor Detail

      • FSDirectory

        protected FSDirectory(Path path,
                              LockFactory lockFactory)
                       throws IOException
        Create a new FSDirectory for the named location (ctor for subclasses). The directory is created at the named location if it does not yet exist.

        FSDirectory resolves the given Path to a canonical / real path to ensure it can correctly lock the index directory and no other process can interfere with changing possible symlinks to the index directory inbetween. If you want to use symlinks and change them dynamically, close all IndexWriters and create a new FSDirecory instance.

        Parameters:
        path - the path of the directory
        lockFactory - the lock factory to use, or null for the default (NativeFSLockFactory);
        Throws:
        IOException - if there is a low-level I/O error
    • Method Detail

      • open

        public static FSDirectory open(Path path)
                                throws IOException
        Creates an FSDirectory instance, trying to pick the best implementation given the current environment. The directory returned uses the NativeFSLockFactory. The directory is created at the named location if it does not yet exist.

        FSDirectory resolves the given Path when calling this method to a canonical / real path to ensure it can correctly lock the index directory and no other process can interfere with changing possible symlinks to the index directory inbetween. If you want to use symlinks and change them dynamically, close all IndexWriters and create a new FSDirecory instance.

        Currently this returns MMapDirectory for Linux, MacOSX, Solaris, and Windows 64-bit JREs, NIOFSDirectory for other non-Windows JREs, and SimpleFSDirectory for other JREs on Windows. It is highly recommended that you consult the implementation's documentation for your platform before using this method.

        NOTE: this method may suddenly change which implementation is returned from release to release, in the event that higher performance defaults become possible; if the precise implementation is important to your application, please instantiate it directly, instead. For optimal performance you should consider using MMapDirectory on 64 bit JVMs.

        See above

        Throws:
        IOException
      • listAll

        public static String[] listAll(Path dir)
                                throws IOException
        Lists all files (including subdirectories) in the directory.
        Throws:
        IOException - if there was an I/O error during listing
      • fileLength

        public long fileLength(String name)
                        throws IOException
        Description copied from class: Directory
        Returns the length of a file in the directory. This method follows the following contract:
        Specified by:
        fileLength in class Directory
        Parameters:
        name - the name of the file for which to return the length.
        Throws:
        IOException - if there was an IO error while retrieving the file's length.
      • sync

        public void sync(Collection<String> names)
                  throws IOException
        Description copied from class: Directory
        Ensure that any writes to these files are moved to stable storage. Lucene uses this to properly commit changes to the index, to prevent a machine/OS crash from corrupting the index.
        NOTE: Clients may call this method for same files over and over again, so some impls might optimize for that. For other impls the operation can be a noop, for various reasons.
        Specified by:
        sync in class Directory
        Throws:
        IOException
      • rename

        public void rename(String source,
                           String dest)
                    throws IOException
        Description copied from class: Directory
        Renames source to dest as an atomic operation, where dest does not yet exist in the directory.

        Notes: This method is used by IndexWriter to publish commits. It is ok if this operation is not truly atomic, for example both source and dest can be visible temporarily. It is just important that the contents of dest appear atomically, or an exception is thrown.

        Specified by:
        rename in class Directory
        Throws:
        IOException
      • getDirectory

        public Path getDirectory()
        Returns:
        the underlying filesystem directory
      • checkPendingDeletions

        public boolean checkPendingDeletions()
                                      throws IOException
        Tries to delete any pending deleted files, and returns true if there are still files that could not be deleted.
        Throws:
        IOException
      • deletePendingFiles

        public void deletePendingFiles()
                                throws IOException
        Try to delete any pending files that we had previously tried to delete but failed because we are on Windows and the files were still held open.
        Throws:
        IOException