How to convert javax.xml.transform.Source into an InputStream?

First try to downcast to javax.xml.transform.stream.StreamSource. If that succeeds you have access to the underlying InputStream or Reader through getters. This would be the easiest way.

If downcasting fails, you can try using a javax.xml.transform.Transformer to transform it into a javax.xml.transform.stream.StreamResult that has been setup with a java.io.ByteArrayOutputStream. Then you return a java.io.ByteArrayInputStream. Something like:

Transformer t = // getTransformer();
ByteArrayOutputStream os = new ByteArrayOutputStream();
Result result = new StreamResult(os);
t.transform(inputSource, result);
return new ByteArrayInputStream(os.getByteArray());

Of course, if the StreamSource can be a large document, this is not advisable. In that case, you could use a temporary file and java.io.FileOutputStream/java.io.FileInputStram. Another option would be to spawn a transformer thread and communicate through java.io.PipedOutputStream/java.io.PipedInputStream, but this is more complex:

PipedInputStream is = new PipedInputStream();
PipedOutputStream os = new PipedOutputStream(is);
Result result = new StreamResult(os);
// This creates and starts a thread that creates a transformer
// and applies it to the method parameters.
spawnTransformerThread(inputSource, result);
return is;