org.apache.commons.io

Class DirectoryWalker

public abstract class DirectoryWalker extends Object

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

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

The following sections describe:

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:

Implementations need to provide:

Two possible scenarios are envisaged for cancellation:

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;
    }

    private void 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 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
    }
  }
 

Since: Commons IO 1.3

Version: $Revision: 424748 $

Nested Class Summary
static classDirectoryWalker.CancelException
CancelException is thrown in DirectoryWalker to cancel the current processing.
Field Summary
intdepthLimit
The limit on the directory depth to walk.
FileFilterfilter
The file filter to use to filter files and directories.
Constructor Summary
protected DirectoryWalker()
Construct an instance with no filtering and unlimited depth.
protected DirectoryWalker(FileFilter filter, int depthLimit)
Construct an instance with a filter and limit the depth navigated to.
protected DirectoryWalker(IOFileFilter directoryFilter, IOFileFilter fileFilter, int depthLimit)
Construct an instance with a directory and a file filter and an optional limit on the depth navigated to.
Method Summary
protected voidcheckIfCancelled(File file, int depth, Collection results)
Checks whether the walk has been cancelled by calling DirectoryWalker, throwing a CancelException if it has.
protected voidhandleCancelled(File startDirectory, Collection results, DirectoryWalker.CancelException cancel)
Overridable callback method invoked when the operation is cancelled.
protected booleanhandleDirectory(File directory, int depth, Collection results)
Overridable callback method invoked to determine if a directory should be processed.
protected voidhandleDirectoryEnd(File directory, int depth, Collection results)
Overridable callback method invoked at the end of processing each directory.
protected voidhandleDirectoryStart(File directory, int depth, Collection results)
Overridable callback method invoked at the start of processing each directory.
protected voidhandleEnd(Collection results)
Overridable callback method invoked at the end of processing.
protected voidhandleFile(File file, int depth, Collection results)
Overridable callback method invoked for each (non-directory) file.
protected booleanhandleIsCancelled(File file, int depth, Collection results)
Overridable callback method invoked to determine if the entire walk operation should be immediately cancelled.
protected voidhandleRestricted(File directory, int depth, Collection results)
Overridable callback method invoked for each restricted directory.
protected voidhandleStart(File startDirectory, Collection results)
Overridable callback method invoked at the start of processing.
protected voidwalk(File startDirectory, Collection results)
Internal method that walks the directory hierarchy in a depth-first manner.
voidwalk(File directory, int depth, Collection results)
Main recursive method to examine the directory hierarchy.

Field Detail

depthLimit

private final int depthLimit
The limit on the directory depth to walk.

filter

private final FileFilter filter
The file filter to use to filter files and directories.

Constructor Detail

DirectoryWalker

protected DirectoryWalker()
Construct an instance with no filtering and unlimited depth.

DirectoryWalker

protected DirectoryWalker(FileFilter filter, int depthLimit)
Construct an instance with a filter and limit the depth navigated to.

The filter controls which files and directories will be navigated to as part of the walk. The FileFilterUtils class is useful for combining various filters together. A null filter means that no filtering should occur and all files and directories will be visited.

Parameters: filter the filter to apply, null means visit all files depthLimit controls how deep the hierarchy is navigated to (less than 0 means unlimited)

DirectoryWalker

protected DirectoryWalker(IOFileFilter directoryFilter, IOFileFilter fileFilter, int depthLimit)
Construct an instance with a directory and a file filter and an optional limit on the depth navigated to.

The filters control which files and directories will be navigated to as part of the walk. This constructor uses makeDirectoryOnly and makeFileOnly internally to combine the filters. A null filter means that no filtering should occur.

Parameters: directoryFilter the filter to apply to directories, null means visit all directories fileFilter the filter to apply to files, null means visit all files depthLimit controls how deep the hierarchy is navigated to (less than 0 means unlimited)

Method Detail

checkIfCancelled

protected final void checkIfCancelled(File file, int depth, Collection results)
Checks whether the walk has been cancelled by calling DirectoryWalker, throwing a CancelException if it has.

Writers of subclasses should not normally call this method as it is called automatically by the walk of the tree. However, sometimes a single method, typically DirectoryWalker, may take a long time to run. In that case, you may wish to check for cancellation by calling this method.

Parameters: file the current file being processed depth the current file level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleCancelled

protected void handleCancelled(File startDirectory, Collection results, DirectoryWalker.CancelException cancel)
Overridable callback method invoked when the operation is cancelled. The file being processed when the cancellation occurred can be obtained from the exception.

This implementation just re-throws the CancelException.

Parameters: startDirectory the directory that the walk started from results the collection of result objects, may be updated cancel the exception throw to cancel further processing containing details at the point of cancellation.

Throws: IOException if an I/O Error occurs

handleDirectory

protected boolean handleDirectory(File directory, int depth, Collection results)
Overridable callback method invoked to determine if a directory should be processed.

This method returns a boolean to indicate if the directory should be examined or not. If you return false, the entire directory and any subdirectories will be skipped. Note that this functionality is in addition to the filtering by file filter.

This implementation does nothing and returns true.

Parameters: directory the current directory being processed depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Returns: true to process this directory, false to skip this directory

Throws: IOException if an I/O Error occurs

handleDirectoryEnd

protected void handleDirectoryEnd(File directory, int depth, Collection results)
Overridable callback method invoked at the end of processing each directory.

This implementation does nothing.

Parameters: directory the directory being processed depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleDirectoryStart

protected void handleDirectoryStart(File directory, int depth, Collection results)
Overridable callback method invoked at the start of processing each directory.

This implementation does nothing.

Parameters: directory the current directory being processed depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleEnd

protected void handleEnd(Collection results)
Overridable callback method invoked at the end of processing.

This implementation does nothing.

Parameters: results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleFile

protected void handleFile(File file, int depth, Collection results)
Overridable callback method invoked for each (non-directory) file.

This implementation does nothing.

Parameters: file the current file being processed depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleIsCancelled

protected boolean handleIsCancelled(File file, int depth, Collection results)
Overridable callback method invoked to determine if the entire walk operation should be immediately cancelled.

This method should be implemented by those subclasses that want to provide a public cancel() method available from another thread. The design pattern for the subclass should be as follows:

  public class FooDirectoryWalker extends DirectoryWalker {
    private volatile boolean cancelled = false;

    public void cancel() {
        cancelled = true;
    }
    private void 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
    }
  }
 

If this method returns true, then the directory walk is immediately cancelled. The next callback method will be DirectoryWalker.

This implementation returns false.

Parameters: file the file or directory being processed depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Returns: true if the walk has been cancelled

Throws: IOException if an I/O Error occurs

handleRestricted

protected void handleRestricted(File directory, int depth, Collection results)
Overridable callback method invoked for each restricted directory.

This implementation does nothing.

Parameters: directory the restricted directory depth the current directory level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

handleStart

protected void handleStart(File startDirectory, Collection results)
Overridable callback method invoked at the start of processing.

This implementation does nothing.

Parameters: startDirectory the directory to start from results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

walk

protected final void walk(File startDirectory, Collection results)
Internal method that walks the directory hierarchy in a depth-first manner.

Users of this class do not need to call this method. This method will be called automatically by another (public) method on the specific subclass.

Writers of subclasses should call this method to start the directory walk. Once called, this method will emit events as it walks the hierarchy. The event methods have the prefix handle.

Parameters: startDirectory the directory to start from, not null results the collection of result objects, may be updated

Throws: NullPointerException if the start directory is null IOException if an I/O Error occurs

walk

private void walk(File directory, int depth, Collection results)
Main recursive method to examine the directory hierarchy.

Parameters: directory the directory to examine, not null depth the directory level (starting directory = 0) results the collection of result objects, may be updated

Throws: IOException if an I/O Error occurs

Copyright (c) 2002-2011 Apache Software Foundation