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
FileFilterthat 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:
-
CancelException which can be thrown in any of the
lifecycle methods to stop processing.
- The
walk()
method traps thrown
CancelExceptionand 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
CancelException.
- Custom cancel processing in the
handleCancelled()
method.
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
}
}