If you have ever used a shell script, you have most likely used pattern matching to locate files — in fact, you have probably used it extensively. If you haven't used it, pattern matching uses special characters to create a pattern and then file names can be compared against that pattern. For example, in most shell scripts, the asterisk, *, matches any number of characters, so the following command:

% ls *.html

lists all the files in the current directory that end in .html.

The java.nio.file package provides programmatic support for this very useful feature. Each file system implementation provides a PathMatcher. You can get a file system's PathMatcher using the getPathMatcher(String) method in the FileSystem class. The following line of code fetches the path matcher for the default file system:

String pattern = ...;
PathMatcher matcher = FileSystems.getDefault().getPathMatcher("glob:" + pattern);

The string argument passed to getPathMatcher specifies the syntax flavor and the pattern to be matched. This example specifies glob syntax. If you are unfamiliar with glob syntax, see What is a Glob?

Glob syntax is easy to use and very flexible but, if you prefer, you can also use regular expressions, or regex, syntax. For further information on regex, see the Regular Expressions lesson. Some file system implementations may support other syntaxes.

If you want to use some other form of string-based pattern matching, you can create your own PathMatcher class. The examples in this lesson use glob syntax.

Once you have created your PathMatcher instance, you are ready to match files against it. The PathMatcher interface has a single method, matches, that takes a Path argument and returns a boolean — it either matches the pattern or it does not. The following code snippet looks for files that end in .java or .class and prints those files to standard out:

PathMatcher matcher = FileSystems.getDefault().getPathMatcher("glob:*.{java,class}");

Path filename = ...;
if (matcher.matches(filename)) {
    System.out.println(filename);
}

Recursive Pattern Matching

Searching for files that match a particular pattern goes hand-in-hand with walking a file tree. How many times do you know a file is somewhere on the file system, but where? Or perhaps you need to find all files in a file tree that have a particular file extension.

The Find example does precisely that — Find is similar to the Unix find utility, but pared down functionally. It would be easy to extend this example to include other functionality. For example, the find utility supports the -prune flag to exclude an entire subtree from the search — you could implement that functionality by returning SKIP_SUBTREE in the preVisitDirectory method. To implement the -L option which follows symbolic links, you could use the four-argument walkFileTree method and pass in the FOLLOW_LINKS option. (If you do this, make sure you test for circular links in the visitFile method.)

To run this application, use the following format:

% java Find <path> -name "<glob_pattern>"

The pattern is placed inside quotes so any wild cards are not interpreted by the shell. For example:

% java Find . -name "*.html"

Here is the source code for the Find example:

/**
 * Sample code that finds files that match the specified glob pattern.
 * For more information on what constitutes a glob pattern, see
 * http://java.sun.com/docs/books/tutorial/essential/io/fileOps.html#glob
 *
 * The file or directories that match the pattern are printed to
 * standard out.  The number of matches is also printed.
 *
 * When executing this application, you must put the glob pattern
 * in quotes, so the shell will not expand any wild cards:
 *              java Find . -name "*.java"
 */

import java.io.*;
import java.nio.file.*;
import java.nio.file.attribute.*;
import static java.nio.file.FileVisitResult.*;
import static java.nio.file.FileVisitOption.*;
import java.util.*;


public class Find {

    public static class Finder extends SimpleFileVisitor<Path> {
        private final PathMatcher matcher;
        private int numMatches = 0;

        Finder(String pattern) {
            matcher = FileSystems.getDefault().getPathMatcher("glob:" + pattern);
        }

        //Compares the glob pattern against the file or directory name.
        void find(Path file) {
            Path name = file.getName();
            if (name != null && matcher.matches(name)) {
                numMatches++;
                System.out.println(file);
            }
        }

        //Prints the total number of matches to standard out.
        void done() {
            System.out.println("Matched: " + numMatches);
        }

        //Invoke the pattern matching method on each file.
        @Override
        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
            find(file);
            return CONTINUE;
        }

        //Invoke the pattern matching method on each directory.
        @Override
        public FileVisitResult preVisitDirectory(Path dir) {
            find(dir);
            return CONTINUE;
        }

        //If there is some error accessing the file system, let the
        //user know the precise error.
        @Override
        public FileVisitResult preVisitDirectoryFailed(Path dir, IOException exc) {
            if (exc instanceof AccessDeniedException) {
                System.err.println(dir + ": cannot access directory");
            } else {
                System.err.println(exc);
            }
            return CONTINUE;
        }

        @Override
        public FileVisitResult visitFileFailed(Path file, IOException exc) {
            System.err.println(exc);
            return CONTINUE;
        }
    }

    static void usage() {
        System.err.println("java Find <path> -name \"<glob_pattern>\"");
        System.exit(-1);
    }

    public static void main(String[] args) throws IOException {

        if (args.length < 3 || !args[1].equals("-name"))
            usage();

        Path startingDir = Paths.get(args[0]);
        String pattern = args[2];

        Finder finder = new Finder(pattern);
        Files.walkFileTree(startingDir, finder);
        finder.done();
    }
}

Recursively walking a file tree is covered in Walking the File Tree.