How do I create a directory within the current working directory, in Java

filesystems, java

Solution

The java.io package does not have a `Directory` class, but you can use the `mkdir()` method on the `File` class instead:

(new File("Foo")).mkdir()

Note that `mkdir()` has two separate failure modes:

- "If a security manager exists and its `checkWrite()` method does not permit the named directory to be created" then a `SecurityException` will be thrown.

- If the operation fails for another reason, `mkdir()` will return false. (More specifically, it will return true if and only if the directory was created.)

Point 1 is ok — if you don't have permission, throw. Point 2 is a little sub-optimal for three reasons:

- You need to inspect the boolean result from this method. If you ignore the result the operation could silently fail.

- If you get a false return you have no idea why the operation failed, which makes it difficult to recover, or formulate a meaningful error message.

- The strict "if and only if" wording of the contract also means that the method returns false if the directory already exists.

Aside: Contrast Point 3 with the behaviour of the .NET `Directory.CreateDirectory()` which does nothing if the directory exists. This kind of makes sense — "create a directory"; "ok, the directory is created". Does it matter if it was created now or earlier; by this process or another? If you really cared about that wouldn't you be asking a different question: "Does this directory exist?"

The next caveat is that `mkdir()` will not create more than one directory at a time. For my simple example of a directory named "Foo" this is fine; however, if you wanted to create a directory called Bar within the directory Foo (i.e. to create the directory "Foo/Bar") you must remember to use the `mkdirs()` method instead.

So to work around all of these caveats, you can employ a helper method such as the following:

public static File createDirectory(String directoryPath) throws IOException {
    File dir = new File(directoryPath);
    if (dir.exists()) {
        return dir;
    }
    if (dir.mkdirs()) {
        return dir;
    }
    throw new IOException("Failed to create directory '" + dir.getAbsolutePath() + "' for an unknown reason.");
}

Problem

Original source