Getting FileSystemNotFoundException from ZipFileSystemProvider when creating a path to a resource

I have a Maven project and inside a method I want to create a path for a directory in my resources folder. This is done like this:

try {
    final URI uri = getClass().getResource("/my-folder").toURI();
    Path myFolderPath = Paths.get(uri);
} catch (final URISyntaxException e) {
    ...
}

The generated URI looks like jar:file:/C:/path/to/my/project.jar!/my-folder.

The stacktrace is as following:

Exception in thread "pool-4-thread-1" java.nio.file.FileSystemNotFoundException
    at com.sun.nio.zipfs.ZipFileSystemProvider.getFileSystem(ZipFileSystemProvider.java:171)
    at com.sun.nio.zipfs.ZipFileSystemProvider.getPath(ZipFileSystemProvider.java:157)
    at java.nio.file.Paths.get(Paths.java:143)

The URI seems to be valid. The part before ! points to the generated jar-file and the part after it to my-folder in the root of the archive. I have used this instructions before to create paths to my resources. Why am I getting an exception now?

Solution 1:

You need to create the file system before you can access the path within the zip like

final URI uri = getClass().getResource("/my-folder").toURI();
Map<String, String> env = new HashMap<>(); 
env.put("create", "true");
FileSystem zipfs = FileSystems.newFileSystem(uri, env);
Path myFolderPath = Paths.get(uri);

This is not done automatically.

See http://docs.oracle.com/javase/7/docs/technotes/guides/io/fsp/zipfilesystemprovider.html

Solution 2:

If you intend to read the resource file, you can directly use getClass.getResourceAsStream. This will set up the file system implictly. The function returns null if your resource could not be found, otherwise you directly have an input stream to parse your resource.

Solution 3:

Expanding on @Uwe Allner 's excellent answer, a failsafe method to use is

private FileSystem initFileSystem(URI uri) throws IOException
{
    try
    {
        return FileSystems.getFileSystem(uri);
    }
    catch( FileSystemNotFoundException e )
    {
        Map<String, String> env = new HashMap<>();
        env.put("create", "true");
        return FileSystems.newFileSystem(uri, env);
    }
}

Calling this with the URI you are about to load will ensure the filesystem is in working condition. I always call FileSystem.close() after using it:

FileSystem zipfs = initFileSystem(fileURI);
filePath = Paths.get(fileURI);
// Do whatever you need and then close the filesystem
zipfs.close();