Validating XML as it is being received over a Java TCP server socket

Hi all,
I should start off by saying that I am transitioning to Java from other languages (C/C++, C#) so I am not familiar with much of the existing functionality that is inherent to the Java language, or the many libraries that have been written to supplement functionality that is not built into the core code.  

I have a Java ServerSocket which will be receiving XML messages.  The fundamental question I have is - how do I know if and when I have received the whole XML message?  

If there were a way to ensure that in every case the XML message would arrive intact without timeouts or in transit errors then the task would be as simple as looking for the closing root tag, but this is, obviously not the real world.

So the different possible problems that I can see arising are:

* The client stalls and stops sending data.  This is easily handled by setting a timeout on the socket.
* The client sends an invalid XML message.
* The client sends a valid XML message, but does not conform to an XML message that the application does not know how to process (i.e. doesn't conform to a DTD or XSD)

So the though I have as to how to handle these scenarios is to write the incoming data to a stream which can validate the XML as it arrives for immediate problems, will recognize when the XML message is complete, and will validate it against a DTD or XSD.

Does such an animal already exist, or is there a ready way to develop such a thing?