Is there a way for a Java app to gain root permissions?

Question

When running Files.walk(Paths.get(\"/var/\")).count() as an unprivileged user, the execution might throw an exception as there are folders inside /var/

Answer 1

If what you want is actually skipping the paths where you have no access, you have two approaches:

Streams

In the answer to this question it is explained how to obtain the stream of all files of a subtree you can access.

But this example can be expanded to other use cases too.

FileVisitor

Using a FileVisitor adds a lot of code, but grants you much more flexibility when walking directory trees. To solve the same problem you can replace Files.walk() with:

Files.walkFileTree(Path start, FileVisitor visitor);

extending SimpleFileVisitor (to count the files) and overriding some methods.

You can:

1. Override the visitFileFailed method, to handle the case you cannot access a file for some reasons; (Lukasz_Plawny's advice)
2. (optional) Override the preVisitDirectory method, checking for permissions before accessing the directory: if you can't access it, you can simply skip its subtree (keep in mind that you may be able to access a directory, but not all its files);

e.g. 1

@Override
public FileVisitResult visitFileFailed(Path file, IOException exc) {
    // you can log the exception 'exc'
    return FileVisitResult.SKIP_SUBTREE;
}

e.g. 2

@Override
public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {

    if(!Files.isReadable(dir))
        return FileVisitResult.SKIP_SUBTREE;

    return FileVisitResult.CONTINUE;

}

FileVisitor docs

FileVisitor tutorial

Hope it helps.