Java: How to find out if path is absolute regardless of the OS

Question

Is there anyway in Java to find out if the given path is absolute or not regardless of the platform the program is currently running. So, what I want is probably something like the following example:

On Linux:

new File("/home/").isAbsolute() // Should return true.
new File("C:/My Documents").isAbsolute() // Should *also* return true.

On Windows:

new File("C:/Documents").isAbsolute() // Should return true.
new File("/home/").isAbsolute() // Should *also* return true.

I can probably code something to get around with this, but I just wanted to find out if anyone knew a built-in class provided in Java to solve this problem. Or has anyone ever come this problem? And how did you solve it?

Thanks!

on linux, a path like "c:/my documents" is probably invalid, and it is certainly not absolute. In systems implementing the posix standard, all paths exist below a single 'root', and thus an absolute path starts at that root. Thus an absolute path always begins with "/". anything else is taken to be relative to the process's current working directory. On linux, the "C:/..." is taken to mean the directory inside the current working directory that is named "C:" which is most likely nonsense. — SingleNegationElimination, Jun 22 '09 at 03:07
Your question makes little sense. Imagine a hypothetical operating system where all paths are absolute. Let's say there is java on this operating system. So, to meet your requirement, you need a isFileAbsolute() method that always returns true, regardless of the platform the program is currently running. Easy to implement, but meaningless. — Erich Kitzmueller, Jun 22 '09 at 09:41
I have a situation where user configures a working directory on a remote computer. The remote computer could be running Unix or Windows (though the most common scenario is local machine is windows and remote machine is unix). The user-specified path has to be validated, and it is required to determine if a path is absolute. — Santosh Tiwari, Jan 13 '12 at 18:06
see [http://stackoverflow.com/questions/7627049/how-to-check-whether-the-path-is-relative-or-absolute-in-java][1] file.isAbsolute() [1]: http://stackoverflow.com/questions/7627049/how-to-check-whether-the-path-is-relative-or-absolute-in-java — Vlad Patryshev, Sep 07 '12 at 04:38

score 10 · Accepted Answer · answered Jun 22 '09 at 03:00

10

Nope.

There are some underlying FileSystem classes (that's Java 7, but they exist prior to it as well) that expose isAbsolute(), but they're not public - so you shouldn't use them, and even if you did your code would be full of reflection junk - and only the "correct" OS ones are included in the JRE, so you'd have to code around them anyway.

Here are the Java 7 implementations of isAbsolute(...) to get you started. Note that File.getPrefixLength() is package-private.

Win32FileSystem:

public boolean isAbsolute(File f) 
{
        int pl = f.getPrefixLength();
        return (((pl == 2) && (f.getPath().charAt(0) == slash))
                || (pl == 3));
}

UnixFileSystem:

public boolean isAbsolute(File f) 
{
        return (f.getPrefixLength() != 0);
}

answered Jun 22 '09 at 03:00

Kevin Montrose

22,191
9
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12

Note that after six years, this answer is outdated. Nowadays, the File class contains an "[isAbsolute()](http://docs.oracle.com/javase/7/docs/api/java/io/File.html#isAbsolute)" method that does exactly what you'd expect: _"Tests whether this abstract pathname is absolute. The definition of absolute pathname is system dependent. On UNIX systems, a pathname is absolute if its prefix is "/". On Microsoft Windows systems, a pathname is absolute if its prefix is a drive specifier followed by "\\", or if its prefix is "\\\\"_ – Guus Aug 24 '15 at 10:15
1

No this answer is not outdated and even with a jre 11 the File#isAbsolute method is still os dependent. On Unix os new File("C:\\Temp").isAbsolute() will say false and say true on windows. – mlapeyre May 25 '21 at 15:23

score 7 · Answer 2 · answered Apr 23 '15 at 21:50

7

In Java 7:

new File(path).isAbsolute()

answered Apr 23 '15 at 21:50

Val Blant

1,664
2
24
34

This is OS dependent and is not what OP needs. https://docs.oracle.com/javase/7/docs/api/java/io/File.html#isAbsolute() – marcioggs Mar 14 '22 at 09:30

score 4 · Answer 3 · answered Jul 30 '14 at 00:15

4

My crack at this using Apache FilenameUtil -

   public static boolean isAbsolute(final String path) {
      return FilenameUtils.getPrefixLength(path) != 0;
   }

Technically this is returning !relative. Which is fine for my purposes.

answered Jul 30 '14 at 00:15

remery

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1
4
8

stuhpa · Answer 4 · 2013-09-25T09:03:23.310

-1

I ended up using this (in Java 6):

private static boolean testPath(String path) {
    int prefixLen = FilenameUtils.getPrefixLength(path);
    if (testPathWin(path, prefixLen) || testPathLinux(prefixLen))
        return true;
    else
        return false;
}

private static boolean testPathWin(String path, int prefixLen) {
    if (prefixLen == 3)
        return true;
    File f = new File(path);
    if ((prefixLen == 2) && (f.getPath().charAt(0) == '/'))
        return true;
    return false;
}

private static boolean testPathLinux(int prefixLen) {
    return (prefixLen != 0);
}

edited Sep 25 '13 at 09:03

answered Sep 11 '13 at 11:09

stuhpa

306
3
13

You copied Java 7 implementation public boolean isAbsolute(File f) for windows but you are using apache commons FilenameUtils.getPrefixLength() method. Java 7 implementation and Apache implementation of getPrefixLength() are not the same. For UNC paths apache will return "\\foo\poo" -> "\\foo\" as prefix and length will be 6 – acheron55 Mar 14 '14 at 19:46
@Acheron55: I don't understand which "isAbsolute()" do you refer to? Where is it exactly in my source snippet? – stuhpa Mar 16 '14 at 22:55
check the source code of FilenameUtils.getPrefixLength(path). It may return any number of length for UNC paths. But you only check for length 2 or 3 in your testPathWin method. So it is wrong. It would be true if you could use java implementation of it. Because it only returns 0,1,2,3 values. Also this line: `if (testPathWin(path, prefixLen) || testPathLinux(prefixLen))` linux check will render your windows method useless since it has a much broader range. Let's say your length is 1 for a windows path, but your testPath method will still return true, which is false – acheron55 Mar 17 '14 at 16:06

Java: How to find out if path is absolute regardless of the OS

4 Answers4