This question sounds similar to How to create a directory in memory? pseudo file system / virtual directory. Basically, my suggestion is to use a more general solution- an in-memory virtual filesystem (and I don't mean on OS level, like Linux' ramfs/tmpfs).
One example is to use the Java 7 NIO APIs, which now provide an SPI for implementing a file system via FileSystemProvider. It seems that the ShrinkWrap filesystem implements this SPI.
A more accessible option would be to use Apache Commons VFS' ram filesystem: it requires only Java 5. If you need to be compatible with Java 5 and 6, this is probably your best bet.
I first remember reading about in-memory filesystems in Java from this article, which apart from pointing out solutions like Commons VFS and JBoss Microcontainer, gives a nice example use case for the NetBeans IDE.
While an in-memory virtual filesystem is a nice general solution of avoiding the OS-level filesystem (with the associated performance benefits), it probably suffers from other disadvantages, which more specialized solutions could address. For instance, I am not sure how using this filesystem would behave when used concurrently from multiple threads. It might work fine as long as you don't access the same files, or you might need to create separate filesystems (which might be prohibitive in terms of resource usage).
