Why do I see NullPointerException when I search files by mask under the root folder ?(windows)

Question

I investigate java nio2 possibilities.

I knew that I can search files using FileVisitor interface. To achieve this functionality I use glob pattern.

code of my example:

visitor interface realization:

class MyFileFindVisitor extends SimpleFileVisitor<Path> {
    private PathMatcher matcher;
    public MyFileFindVisitor(String pattern){
        try {
            matcher = FileSystems.getDefault().getPathMatcher(pattern);
        } catch(IllegalArgumentException iae) {
            System.err.println("Invalid pattern; did you forget to prefix \"glob:\"? (as in glob:*.java)");
            System.exit(1);
        }
    }
    public FileVisitResult visitFile(Path path, BasicFileAttributes fileAttributes){
        find(path);
        return FileVisitResult.CONTINUE;
    }
    private void find(Path path) {
        Path name = path.getFileName();
        if(matcher.matches(name))
            System.out.println("Matching file:" + path.getFileName());
    }
    public FileVisitResult preVisitDirectory(Path path, BasicFileAttributes fileAttributes){
        find(path);
        return FileVisitResult.CONTINUE;
    }
}

main method:

public static void main(String[] args) {
        Path startPath = Paths.get("E:\\folder");
        String pattern = "glob:*";
        try {
            Files.walkFileTree(startPath, new MyFileFindVisitor(pattern));
            System.out.println("File search completed!");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

This variant of main method works properly but if I change:

Path startPath = Paths.get("E:\\folder");

with

Path startPath = Paths.get("E:\\");

I see following stacktrace:

Exception in thread "main" java.lang.NullPointerException
    at sun.nio.fs.WindowsFileSystem$2.matches(WindowsFileSystem.java:312)
    at io.nio.MyFileFindVisitor.find(FileTreeWalkFind.java:29)
    at io.nio.MyFileFindVisitor.preVisitDirectory(FileTreeWalkFind.java:33)
    at io.nio.MyFileFindVisitor.preVisitDirectory(FileTreeWalkFind.java:13)
    at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:192)
    at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:69)
    at java.nio.file.Files.walkFileTree(Files.java:2600)
    at java.nio.file.Files.walkFileTree(Files.java:2633)
    at io.nio.FileTreeWalkFind.main(FileTreeWalkFind.java:42)

I don't the cause of this problem.

How to solve it?

David · Accepted Answer · 2014-08-12T00:08:02.920

The reason you are getting a null pointer exception is because when your visitor tests the very first path (E:\), there is no actual file name to test - this is a volume root directory. From the JDK Docs:

Path getFileName()

Returns the name of the file or directory denoted by this path as a Path object. The file name is the farthest element from the root in the directory hierarchy.

Returns:
    a path representing the name of the file or directory, or null if this path has zero elements

"Elements", in this case, means directory elements in the name. 'E:\' has no directory elements, as it is the root for the volume.

You should not assume that filename is always going to be not null.

private void find(Path path) {          
    Path name = path.getFileName();
    if (name != null) {
        if(matcher.matches(name)) {
            System.out.println("Matching file:" + path.getFileName());
        }
    }
}

Other things that you might need to watch for when walking Windows file systems include:

System protected directories, such as the recycle bins, that your walker may not be able to descend into
NTFS Junction points that might result in recursive directories that loop back on themselves, resulting in your code looping until you run out of heap or stack

Why do I see NullPointerException when I search files by mask under the root folder ?(windows)

1 Answers1