list files or directories recursively in high efficiency

Discussion in 'Java' started by Ken, Dec 2, 2005.

  1. Ken

    Ken Guest

    could someone here show me a demo how to write these kind of code that
    list files or directories recursively in high efficiency just like
    native program's speed?
    Does the new feature NIO in JDK 1.4 could help?
    Ken, Dec 2, 2005
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  2. Roedy Green

    Roedy Green Guest

    On 1 Dec 2005 23:36:33 -0800, "Ken" <> wrote, quoted
    or indirectly quoted someone who said :

    >could someone here show me a demo how to write these kind of code that
    >list files or directories recursively in high efficiency just like
    >native program's speed?

    I too have found File.list is pretty slow, even when Jet compiled.

    Here is a method that uses two filters, one for directories and one
    for files to find junk to get rid of:

    * Delete junk files in given fully qualified directory, and all
    * uses current definition of junk. Wipes out files matching
    definition of
    * junk, not dirs.
    * @param pDir
    * fully qualified directory. Would normally be some
    * directory.
    * @param pRecursive
    * true if should also clean subdirs of this dir.
    public static void cleanDir ( File pDir, boolean pRecursive )
    if ( pDir == null ) return;

    // clean out child dirs
    if ( pRecursive )
    // all immediate child subdirs of this dir.
    String[] allDirs = pDir.list( mDirFilter );
    if ( allDirs != null )
    for ( int i = 0; i < allDirs.length; i++ )
    cleanDir( new File( pDir, allDirs[ i ] ), true );
    // just junk files in this dir, not dirs themselves.
    String[] allFiles = pDir.list( mJunkFilter );
    if ( allFiles != null )
    for ( int i = 0; i < allFiles.length; i++ )
    deleteFile( new File( pDir, allFiles[ i ] ) );
    I don't know WHY File.list is so slow. Before embarking on replacing
    it with JNI check out its code to see if it doing anything obviously
    stupid or overly generic that you could improve on.

    I use 4NT which every once in while builds directory tree and stores
    it as a flat file. It can find directories in a fraction of a second.
    That approach might be suitable for some problems.

    In the Replicator, I build an structure to look up files where
    segments used to create names are stored as an array of interned
    String constants to represent each name. This conserves RAM.

    Canadian Mind Products, Roedy Green. Java custom programming, consulting and coaching.
    Roedy Green, Dec 2, 2005
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