Java: check symbolic link file existence

Question

We talk about java 1.6 here. Since symoblic link is not yet supported, how can examine the existence of them.

1: tell wheather the link file itself exists (return true even if the link is broken)

2: follow the link and tell wheather underlying file exists.

Is there really no way to realize this except JNI?

Winou

A better solution copied from Apache FileUtils can be found here: stackoverflow.com/questions/813710/… – Sidwohl

Answer 1

looks that way for now... unless you go with openjdk http://openjdk.java.net/projects/nio/javadoc/java/nio/file/attribute/BasicFileAttributes.html#isSymbolicLink()

Gastrin

Just in case anyone would wonder, the static method you could use is java.nio.file.Files.isSymbolicLink(Path path). – Venusberg

Now link is returning 404, this why you should take extra care before adding any link to your answer. – Evocator

Answer 2

It is slow but you could compare getAbsolutePath() and getCanonicalPath() of a File. So use only outside a performance critical code path.

Reconvert

This doesn't work for Windows links, only *nix symbolic links. – Flem

Answer 3

The following should give you a start:

if (file.exists() && !file.isDirectory() && !file.isFile()) {
    // it is a symbolic link (or a named pipe/socket, or maybe some other things)
}

if (file.exists()) {
    try {
        file.getCanonicalFile();
    } catch (FileNotFoundException ex) {
        // it is a broken symbolic link
    }
}

EDIT : The above don't work as I thought because file.isDirectory() and file.isFile() resolve a symbolic link. (We live and learn!)

If you want an accurate determination, you will need to use JNI to make the relevant native OS-specific library calls.

Deuteragonist

Thanks, but file.exists() will return false for symbolic-link I think. – Winou

It doesn't on Linux. But unfortunately, file.isFile() and file.isDirectory() resolve a symbolic link and return a result based on the type of the resolved object. – Deuteragonist

It return false on Linux. You can also refer the evaluation part of bugs.sun.com/bugdatabase/view_bug.do?bug_id=4956115. – Winou

@Winou - it returned true on Linux when I tested it for a symbolic link to an existing file. The point is that it tests both whether the link AND the target of the link exists. – Deuteragonist

Answer 4

hmm..not yet supported..will it ever be supported is the question that comes to my mind looking at this question...symbolic links are platform specific and Java is supposed to b platform independent. i doubt if anything of the sort is possible with java as such..you may have to resort to native code for this portion and use JNI to bridge it with the rest of your program in java.

Supranational

Java1.7 is going to implement symbolic-link interface, so what's their consideration for platform independent? – Winou

hmm..thats interesting..you may want to take a look at this : stackoverflow.com/questions/813710/… – Supranational

Answer 5

#2 is easy: just use File.isFile etc., which will traverse the link. The hard part is #1.

So if you cannot use java.nio.file, and want to avoid JNI etc., the only thing I found which works:

static boolean exists(File link) {
    File[] kids = link.getParentFile().listFiles();
    return kids != null && Arrays.asList(kids).contains(link);
}

Not terribly efficient but straightforward.

Gadolinium