Castor and sockets

Question

I'm new to Castor and data binding in general. I'm working on an application that, in part, needs to take data off of a socket and unmarshall the data to make POJOs. Now, I've got the socket stuff down, and I've even generated and compiled java files thanks to Ant and Castor.

Here's the problem: the data stream that I'll receive could be one of about 9 different objects. That is, I receive a stream of text (XML) that represents an object with stuff that I'll operate on; again, depending on the object type. If it were just one object, it'd be easy: call the unmarshall commands on it and go on my merry way. But, since it could be one of many kinds of objects, who do I know what to unmarshall? I read up on mapping, but either I didn't get it, or it seems like a static mapping, not a dynamic mapping.

Any help out there?

Answer 1

You are right, Castor expects a static mapping. But you can work with that. You can write some code that will modify the incoming xml so that, on your side, Castor can use one schema, and on your clients' side they don't have to change their schemas.

Change the schema that Castor expects to get to something with a common root-element, with under that your nine different alternatives for your different objects (I think you can restrict it so the schema will allow only one of the nine, if that doesn't work out you could just make all the sub-elements optional).

Then you can write code that modifies the incoming xml to wrap your incoming xml with that common root-element, then feeds the wrapped xml into a stream that gets read by the Castor unmarshaller.

There are at least 3 different ways to implement the xml-wrapping part: SAX, XSLT, and XML libraries (like JDOM, DOM4J, and XOM--I prefer XOM but any of them will work).

The SAX way is probably best if you're already familiar with SAX or if one of the other ways has worked but come up short on performance. If I had to implement that then I would create an XMLFilter that takes in xml and writes xml out, stacking that on top of another piece that writes xml to an OutputStream, and writing a wrapper method around the unmarshalling stuff to feed the incoming stream to the xmlreader, copy the OutputStream to another InputStream (an easy way is to use commons-io), and feed the new InputStream to the Castor unmarshaller.

With XSLT there is no fooling with SAX, although XSLT has a reputation for pain sometimes, it seems to me like this might be a relatively straightforward transformation, but I haven't taken a stab at it either. It is a long time since I used XSLT for anything. I am not sure about performance either, though I wouldn't write it off out of hand.

Using XOM or JDOM or DOM4J to wrap the XML is also possible, and the learning curve is a lot lower than for SAX or XSLT. The downside is the whole XML document tends to get sucked into memory at once so if you deal with big enough documents you could run out of memory.

Answer 2

I have a similar thing in Jibx where all of the incoming message objects implement a base interface which has a field denoting the message type.

The text/xml is serialized into the base interface and I then used the command pattern to call the respective business logic depending upon the message type which is defined in the base interface.

Not sure if this is possible using castor but take a look at Jibx as the performance is fantastic.

http://jibx.sourceforge.net/

Answer 3

I appreciate your insights. You both have given me some good information to go on and new knowledge that I didn't have. In the end, I got the process to work via a hack. I grab the text stream, parse out the root tag of the message, and then switch on it to determine the right object to create. I'm unmarshalling all of my objects independently and everyone is happy on our end.