What is the best way to handle transforming a file to itself?

Currently I do:
File source = new File(servlet_context.getRealPath(_xml));
transformer.transform(new StreamSource(source), new StreamResult(source));

Opening a file both for reading and writing at the same
time is always risky. Either write to a buffer, and write
the buffer to disk after the transformation has finished,
or write to a temporary file and rename to the original
file afterwards.
Something like;
 ByteArrayOutputStream baos=new B...;
 File source = new File(servlet_context.getRealPath(_xml));
 transformer.transform(new StreamSource(source), new StreamResult(baos));
 byte buf[]=baos.toByteArray();
 new FileOutputStream(source).write(buf);

BTW doing this in a servlet context appears to be
rather strange...