A Rule that eases working with files during test execution in OSGi environments. See http://eclipsesource.com/blogs/?p=22765

FileResource

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.text.MessageFormat;

import org.apache.commons.io.IOUtils;
import org.jdom.input.DOMBuilder;
import org.junit.rules.ExternalResource;
import org.junit.rules.TemporaryFolder;
import org.w3c.dom.Document;
import org.xml.sax.SAXException;

/**
 * Class FileResource helps setting up input test files for unit tests. The motivation and usage
 * hints are given at http://eclipsesource.com/blogs/?p=22765 Feel free to use and adjust this class
 * to your needs, but keep the author information in its place.
 *
 * @author Matthias Kempka (mkempka@eclipsesource.com)
 */
@SuppressWarnings("nls")
public class FileResource extends ExternalResource {
    public static class ExternalFolder extends TemporaryFolder {
        private boolean initialized;

        public void teardown() {
            if (initialized) {
                delete();
                super.after();
                initialized = false;
            }
        }

        public synchronized void setup() throws Throwable {
            if (!initialized) {
                super.before();
                initialized = true;
            }
        }
    }
    private final String resourceName;
    private File file;
    private final ClassLoader classLoader;
    private final ExternalFolder folder;

    /**
     * @param loader the classloader of the source file
     * @param resourceName the resource name of the source file. Must be in the root directory of
     *            the class loader.
     */
    public FileResource(final ClassLoader loader, final String resourceName) {
        this(new ExternalFolder(), loader, resourceName);
    }

    /**
     * The use of an external folder allows to copy multiple files next to each other. Intended use:
     *
     * <pre>
     * private ExternalFolder folder = new ExternalFolder();
     * &#064;Rule
     * public FileResource xml = new FileResource(folder, getClass().getClassLoader(), &quot;myfile.xml&quot;);
     * &#064;Rule
     * public FileResource dtd = new FileResource(folder, getClass().getClassLoader(), &quot;mydtd.dtd&quot;);
     * </pre>
     *
     * @param folder an instance of {@link ExternalFolder} that defines the parent folder of a file
     * @param loader the classloader of the source file
     * @param resourceName the resource name of the source file. Must be in the root directory of
     *            the class loader.
     */
    public FileResource(ExternalFolder folder, final ClassLoader loader, final String resourceName) {
        this.classLoader = loader;
        this.resourceName = resourceName;
        this.folder = folder;
    }

    public String getContent() {
        try {
            return getContent("UTF-8");
        } catch (IOException e) {
            throw new IllegalStateException("Couldn't get test content from file: " + file + "");
        }
    }

    public String getContent(final String charSet) throws IOException {
        FileInputStream input = new FileInputStream(file);
        try {
            return IOUtils.toString(input, charSet);
        } finally {
            IOUtils.closeQuietly(input);
        }
    }

    public File getFile() {
        return file;
    }

    public String getName() {
        return file.getName();
    }

    @Override
    protected void before() throws Throwable {
        super.before();
        folder.setup();
        InputStream stream = classLoader.getResourceAsStream(resourceName);
        if (stream == null) {
            throw new IllegalStateException(MessageFormat.format("Can''t open ''{0}''", resourceName));
        }
        file = folder.newFile(resourceName);
        FileOutputStream outputStream = new FileOutputStream(file);
        IOUtils.copy(stream, outputStream);
        IOUtils.closeQuietly(stream);
        IOUtils.closeQuietly(outputStream);
    }

    @Override
    protected void after() {
        file.delete();
        folder.teardown();
        super.after();
    }

    public Document getContentAsW3CDocument() {
        javax.xml.parsers.DocumentBuilderFactory factory = javax.xml.parsers.DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(true);
        javax.xml.parsers.DocumentBuilder builder = null;
        try {
            builder = factory.newDocumentBuilder();
        } catch (javax.xml.parsers.ParserConfigurationException ex) {
        }
        FileInputStream is = null;
        try {
            is = new FileInputStream(file);
            org.w3c.dom.Document doc = builder.parse(is);
            return doc;
        } catch (FileNotFoundException e) {
            throw new IllegalArgumentException("Can't create input stream from resource.", e);
        } catch (SAXException e) {
            throw new IllegalArgumentException("Can't parse input stream from resource.", e);
        } catch (IOException e) {
            throw new IllegalArgumentException("Can't create input stream from resource.", e);
        } finally {
            IOUtils.closeQuietly(is);
        }
    }

    public byte[] getBytes() {
        try {
            return IOUtils.toByteArray(new FileInputStream(getFile()));
        } catch (FileNotFoundException e) {
            throw new IllegalArgumentException("Can't create input stream from resource.", e);
        } catch (IOException e) {
            throw new IllegalArgumentException("Can't create input stream from resource.", e);
        }
    }

    public org.jdom.Document getContentAsJDomDocument() {
        return new DOMBuilder().build(getContentAsW3CDocument());
    }

}

Thanks for sharing the code!

If you use @RunWith(PowerMockRunner.class) annotation in your test class and if you have more than one FileResource (even using the same instance of ExternalFolder with private ExternalFolder folder = new ExternalFolder();) you'll get an exception for the first FileResource declared.

I'll need to check later but the tearDown() method from the ExternalFolder class is being called earlier when using PowerMockRunner.