Load a file in Resources with FileInputStream

Question

I know the safe way to open a file in the resources is:

  InputStream is = this.getClass().getResourceAsStream("/path/in/jar/file.name");  

now the problem is that my file is a model for a decider in the Weka Wrapper package and the Decider class has only a method:

  public void load(File file) throws Exception 

load takes the file and opens it as a FileInputStream. Do you see a workaround? I really would like to ship the model putting it in the resources. I was thinking to create a temporary file, write the content of the model in the temp file and then pass the temporary file to Weka, but it is so dirty.. other options?

Answer 1

I see 2 solutions:

Solution 1

Read the classpath ressource to a temp file and delete it after you called load(File)

InputStream cpResource = this.getClass().getClassLoader().getResourceAsStream("file.name");
File tmpFile = File.createTempFile("file", "temp");
FileUtils.copyInputStreamToFile(cpResource, tmpFile); // FileUtils from apache-io
try {
    decider.load(tmpFile);
} finally {
    tmpFile.delete();
}

Solution 2

If the ClassLoader that loads the resource is a URLClassLoader you can try to find the absolute file name. But this only works if the resource you want exists as a file on the filesystem. It doesn't work if the file is contained in a jar.

ClassLoader classLoader = this.getClass().getClassLoader();
if(classLoader instanceof URLClassLoader){
    URLClassLoader urlClassLoader = URLClassLoader.class.cast(classLoader);
    URL resourceUrl = urlClassLoader.findResource("file.name");

    if("file".equals(resourceUrl.getProtocol())){
        URI uri = resourceUrl.toURI();
        File file = new File(uri);
        decider.load(file);
    }
}

I would suggest to write a utility class that tries to find the absolute file through the class loader or if it can't get it this way uses the temp file approach as fallback.

Or in a more object-oriented way:

    public class FileResourceTest {

    public static void main(String[] args) throws IOException {
        File resourceAsFile = getResourceAsFile("file.name");
        System.out.println(resourceAsFile);
    }

    private static File getResourceAsFile(String resource) throws IOException {
        ClassLoader cl = FileResourceTest.class.getClassLoader();
        File file = null;
        FileResource fileResource = new URLClassLoaderFileResource(cl, resource);
        try {
            file = fileResource.getFile();
        } catch (IOException e) {
            fileResource = new ClasspathResourceFileResource(cl, resource);
            file = fileResource.getFile();
        }
        return file;
    }

    public static interface FileResource {

        public File getFile() throws IOException;

    }

    public static class ClasspathResourceFileResource implements FileResource {

        private ClassLoader cl;
        private String resource;

        public ClasspathResourceFileResource(ClassLoader cl, String resource) {
            this.cl = cl;
            this.resource = resource;
        }

        public File getFile() throws IOException {
            InputStream cpResource = cl.getResourceAsStream(resource);
            File tmpFile = File.createTempFile("file", "temp");
            FileUtils.copyInputStreamToFile(cpResource, tmpFile);
            tmpFile.deleteOnExit();
            return tmpFile;
        }

    }

    public static class URLClassLoaderFileResource implements FileResource {

        private ClassLoader cl;
        private String resource;

        public URLClassLoaderFileResource(ClassLoader cl, String resourcePath) {
            this.cl = cl;
            this.resource = resourcePath;
        }

        public File getFile() throws IOException {
            File resourceFile = null;
            if (cl instanceof URLClassLoader) {
                URLClassLoader urlClassLoader = URLClassLoader.class.cast(cl);
                URL resourceUrl = urlClassLoader.findResource(resource);
                if ("file".equals(resourceUrl.getProtocol())) {
                    try {

                        URI uri = resourceUrl.toURI();
                        resourceFile = new File(uri);
                    } catch (URISyntaxException e) {
                        IOException ioException = new IOException(
                                "Unable to get file through class loader: "
                                        + cl);
                        ioException.initCause(e);
                        throw ioException;
                    }

                }
            }
            if (resourceFile == null) {
                throw new IOException(
                        "Unable to get file through class loader: " + cl);
            }
            return resourceFile;
        }

    }
}

You can also use a thrid party library like commons-vfs that allows you to reference a file within a jar. E.g. jar:// arch-file-uri[! absolute-path]. Since commons-vfs specifies an own FileObject that represents a file you must still copy the content to a local java.io.File to adapt to the Decider.load(File) API.

EDIT

This is very helpful! Is there anything in newer versions of java that already supports this requirement?

Even if I haven't take a look at every class of every newer version I would say no. Because a classpath resource is not always a file. E.g. it can be a file within a jar or even a remote resource. Think about the applets that java programmers used a long time ago. Thus the concept of a classpath and it's resources is not bound to a local filsystem. This is obviously a good thing, because you can load classes from almost every URL and this makes it more flexible. But this flexibility also means that you must read the resource and create a copy if you need a File.

But maybe some kind of utility code like the one I showed above will make it into the JRE. Maybe it is already there. If so please comment and let us all know.