Recursive folder walks using NIO.2 API

FileVisitor Interface

The FileVisitor interface allows us to (recursively) traverse file structures - folders, sub-folders, and files.

Every method of this interface can return four possible results (instances of the FileVisitResult enum):

• FileVisitResult.CONTINUE: This means that the traversal process will continue.

• FileVisitResult.SKIP_SIBLINGS: This means that the traversal process will continue without visiting the siblings (files or folders) of that particular Path

• FileVisitResult.SKIP_SUBTREE: This means that the traversal process will continue without visiting the rest of the tree entries.

• FileVisitResult.TERMINATE: This means that the traversal process should stop.

This FileVisitor interface has four methods:

• visitFile(): The method is invoked for a file. The method should return a FileVisitResult.CONTINUE result or a FileVisitResult.TERMINATE result. The method receives a reference to the file (a Path object) and to the BasicFileAttributes object associated with the Path.

• preVisitDirectory(): This method is invoked for a directory before visiting its children. The method returns FileVisitResult.CONTINUE if we want its children to be visited or FileVisitResult.SKIP_SUBTREE if we want the process to stop. If we’re going to skip visiting the siblings of the directory, we need to return FileVisitResult.SKIP_SIBLINGS.

• postVisitDirectory(): This method is invoked after we visit all the children of a directory (including other folders and their descendants).

• visitFileFailed(): This method is invoked if a file (or folder) cannot be accessed.

In practice, it is also possible to use the SimpleFileVisitor class if we want to traverse only the directories.

Once we have created the “recursive-walking-mechanism” by implementing FileVisitor interface, or by extending the SimpleFileVisitor class, we can start the recursive process by calling the walkFileTree() method .

Example: Writing an application that searches for files bigger than a pre-defined size

In this example, we will implement a FileVisitor that walks a folder and logs to System.out all files that are bigger than a specific size.

The first step is to write the FileVisitor:

class FileSizeVisitor implements FileVisitor {

	private Long size;

	public FileSizeVisitor(Long size) {
		this.size = size;
	}

	/**
	 * This is triggered before visiting a directory.
	 */
	@Override
	public FileVisitResult preVisitDirectory(Path path,
			BasicFileAttributes attrs) throws IOException {
		return FileVisitResult.CONTINUE;
	}

	/**
	 * This is triggered when we visit a file.
	 */
	@Override
	public FileVisitResult visitFile(Path path, BasicFileAttributes attrs)
			throws IOException {

		long fileSize = attrs.size() / 1024;

		if (fileSize >= this.size) {
			System.out.println("File bigger than " + this.size + "KB  found: "
					+ path);
		}

		return FileVisitResult.CONTINUE;
	}

	/**
	 * This is triggered if we cannot visit a Path We log the fact we cannot
	 * visit a specified path .
	 */
	@Override
	public FileVisitResult visitFileFailed(Path path, IOException exc)
			throws IOException {
		// We print the error
		System.err.println("ERROR: Cannot visit path: " + path);
		// We continue the folder walk
		return FileVisitResult.CONTINUE;
	}

	/**
	 * This is triggered after we finish visiting a specified folder.
	 */
	@Override
	public FileVisitResult postVisitDirectory(Path path, IOException exc)
			throws IOException {
		// We continue the folder walk
		return FileVisitResult.CONTINUE;
	}
} 

The main method:

Path homeFolder = Paths.get("C:\\");
FileVisitor fileVisitor = new FileSizeVisitor(new Long(5000));
try {
	Files.walkFileTree(homeFolder, fileVisitor);
} catch (IOException e) {
	e.printStackTrace();
}

The sample output:

File bigger than 5000  found: C:\\Windows\\System32\\IME\\imekr8\\applets\\mshwkorrIME.dll
File bigger than 5000  found: C:\\Windows\\System32\\IME\\IMETC10\\applets\\MSHWCHTRIME.dll
File bigger than 5000  found: C:\\Windows\\System32\\korwbrkr.lex
ERROR: Cannot visit path: C:\\Windows\\System32\\LogFiles\\WMI\\RtBackup

Example: Writing a file search application based on a criteria

We can extend the example from above and create a more general approach.

The idea is to write an abstract implementation of the FileVisitor interface that contains an abstract method criteria(Path, BasicFileAttributes).

Later we can use anonymous classes to define a new behavior of our visitors, specifying only the criteria and avoiding writing the boiler-plate code necessary to implement a FileVisitor.

We will name our FileVisitor implementation FileSearchByCriteriaVisitor:

abstract class FileSearchByCriteriaVisitor implements FileVisitor

This class will have two instance variables called results and failedVisits:

private List results = new LinkedList();
private List failedVisits = new LinkedList();
public List getResults() {
	return this.results;
}
public List getFailedVisits() {
	return this.failedVisits;
}

The “criteria(Path, BasicFileAttributes)” mentioned before will be used like this:

// This method will be later implemented
protected abstract Boolean criteria(Path path, BasicFileAttributes attrs);
@Override
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
		throws IOException {
	// Everytime we visit a file we check if that particular file matches a
	// criteria .
	if (criteria(file, attrs)) {
		// If the file matches the criteria we add it as a result
		results.add(file);
	}
	return FileVisitResult.CONTINUE;
}

Everytime we implement a new FileSearchByCriteriaVisitor we must supply an implementation for the abstract method defined before.

Example on how to use the FileSearchByCriteriaVisitor:

// Search all the files bigger than size
final long size = 5000; // KB
// Defining a new visitor criteria
FileSearchByCriteriaVisitor sizeVisitor = new FileSearchByCriteriaVisitor() {
	@Override
	protected Boolean criteria(Path path, BasicFileAttributes attrs) {
		if (attrs.size() / 1024 >= size) {
			return true;
		}
		return false;
	}
};
// Walk don't run
Files.walkFileTree(Paths.get("C:\\\\"), sizeVisitor);
// Evaluate results
evaluate(sizeVisitor.getResults());

The results and the paths with errors are available in sizeVisitor.getResults() and sizeVisitor.getFailedVisits().