public abstract class DirectoryWalker<T>
extends Object

Abstract class that walks through a directory hierarchy and provides subclasses with convenient hooks to add specific behavior.

This class operates with a FileFilter and maximum depth to limit the files and directories visited. Commons IO supplies many common filter implementations in the filefilter package.

The following sections describe:

• 1. Example Implementation - example FileCleaner implementation.
• 2. Filter Example - using FileFilter(s) with DirectoryWalker.
• 3. Cancellation - how to implement cancellation behavior.

1. Example Implementation

There are many possible extensions, for example, to delete all files and '.svn' directories, and return a list of deleted files:

  public class FileCleaner extends DirectoryWalker {

    public FileCleaner() {
      super();
    }

    public List clean(File startDirectory) {
      List results = new ArrayList();
      walk(startDirectory, results);
      return results;
    }

    protected boolean handleDirectory(File directory, int depth, Collection results) {
      // delete svn directories and then skip
      if (".svn".equals(directory.getName())) {
        directory.delete();
        return false;
      } else {
        return true;
      }

    }

    protected void handleFile(File file, int depth, Collection results) {
      // delete file and add to list of deleted
      file.delete();
      results.add(file);
    }
  }
 

2. Filter Example

Choosing which directories and files to process can be a key aspect of using this class. This information can be setup in three ways, via three different constructors.

The first option is to visit all directories and files. This is achieved via the no-args constructor.

The second constructor option is to supply a single FileFilter that describes the files and directories to visit. Care must be taken with this option as the same filter is used for both directories and files.

For example, if you wanted all directories which are not hidden and files which end in ".txt":

  public class FooDirectoryWalker extends DirectoryWalker {
    public FooDirectoryWalker(FileFilter filter) {
      super(filter, -1);
    }
  }

  // Build up the filters and create the walker
    // Create a filter for Non-hidden directories
    IOFileFilter fooDirFilter =
        FileFilterUtils.andFileFilter(FileFilterUtils.directoryFileFilter,
                                      HiddenFileFilter.VISIBLE);

    // Create a filter for Files ending in ".txt"
    IOFileFilter fooFileFilter =
        FileFilterUtils.andFileFilter(FileFilterUtils.fileFileFilter,
                                      FileFilterUtils.suffixFileFilter(".txt"));

    // Combine the directory and file filters using an OR condition
    java.io.FileFilter fooFilter =
        FileFilterUtils.orFileFilter(fooDirFilter, fooFileFilter);

    // Use the filter to construct a DirectoryWalker implementation
    FooDirectoryWalker walker = new FooDirectoryWalker(fooFilter);
 

The third constructor option is to specify separate filters, one for directories and one for files. These are combined internally to form the correct FileFilter, something which is very easy to get wrong when attempted manually, particularly when trying to express constructs like 'any file in directories named docs'.

For example, if you wanted all directories which are not hidden and files which end in ".txt":

  public class FooDirectoryWalker extends DirectoryWalker {
    public FooDirectoryWalker(IOFileFilter dirFilter, IOFileFilter fileFilter) {
      super(dirFilter, fileFilter, -1);
    }
  }

  // Use the filters to construct the walker
  FooDirectoryWalker walker = new FooDirectoryWalker(
    HiddenFileFilter.VISIBLE,
    FileFilterUtils.suffixFileFilter(".txt"),
  );
 

This is much simpler than the previous example, and is why it is the preferred option for filtering.

3. Cancellation

The DirectoryWalker contains some of the logic required for cancel processing. Subclasses must complete the implementation.

What DirectoryWalker does provide for cancellation is:

• DirectoryWalker.CancelException which can be thrown in any of the lifecycle methods to stop processing.
• The walk() method traps thrown DirectoryWalker.CancelException and calls the handleCancelled() method, providing a place for custom cancel processing.

Implementations need to provide:

• The decision logic on whether to cancel processing or not.
• Constructing and throwing a DirectoryWalker.CancelException.
• Custom cancel processing in the handleCancelled() method.

Two possible scenarios are envisaged for cancellation:

• 3.1 External / Multi-threaded - cancellation being decided/initiated by an external process.
• 3.2 Internal - cancellation being decided/initiated from within a DirectoryWalker implementation.

The following sections provide example implementations for these two different scenarios.

3.1 External / Multi-threaded

This example provides a public cancel() method that can be called by another thread to stop the processing. A typical example use-case would be a cancel button on a GUI. Calling this method sets a volatile flag to ensure it will work properly in a multi-threaded environment. The flag is returned by the handleIsCancelled() method, which will cause the walk to stop immediately. The handleCancelled() method will be the next, and last, callback method received once cancellation has occurred.

  public class FooDirectoryWalker extends DirectoryWalker {

    private volatile boolean cancelled = false;

    public void cancel() {
        cancelled = true;
    }

    protected boolean handleIsCancelled(File file, int depth, Collection results) {
        return cancelled;
    }

    protected void handleCancelled(File startDirectory, Collection results, CancelException cancel) {
        // implement processing required when a cancellation occurs
    }
  }
 

3.2 Internal

This shows an example of how internal cancellation processing could be implemented. Note the decision logic and throwing a DirectoryWalker.CancelException could be implemented in any of the lifecycle methods.

  public class BarDirectoryWalker extends DirectoryWalker {

    protected boolean handleDirectory(File directory, int depth, Collection results) throws IOException {
        // cancel if hidden directory
        if (directory.isHidden()) {
            throw new CancelException(file, depth);
        }
        return true;
    }

    protected void handleFile(File file, int depth, Collection results) throws IOException {
        // cancel if read-only file
        if (!file.canWrite()) {
            throw new CancelException(file, depth);
        }
        results.add(file);
    }

    protected void handleCancelled(File startDirectory, Collection results, CancelException cancel) {
        // implement processing required when a cancellation occurs
    }
  }