オンメモリのファイルシステムJimfs ref: http://qiita.com/takaki@github/items/a9b270993719699c415a

file0.java

package example.misc;

import com.google.common.jimfs.Configuration;
import com.google.common.jimfs.Jimfs;

import java.nio.charset.StandardCharsets;
import java.nio.file.FileSystem;
import java.nio.file.Files;
import java.nio.file.Path;

public class JimFsDemo {

    public static void main(final String[] args) throws Exception {
        try (final FileSystem fs = Jimfs.newFileSystem(Configuration.unix())) {
            final Path tmp = fs.getPath("/tmp");
            Files.createDirectories(tmp);
            final Path tmpFile = Files.createTempFile(tmp, "jimfs", "tmp");
            Files.write(tmpFile, "abcdef".getBytes(StandardCharsets.UTF_8));

            // System.out.println(tmpFile.toFile().getAbsolutePath()); // toFileで例外が出るのでダメ
            System.out.println(tmpFile.getParent().getFileName());
            System.out.println(tmpFile.getFileName());

            final byte[] read = Files.readAllBytes(tmpFile);
            System.out.println(new String(read, StandardCharsets.UTF_8));
        }
    }

}