JAXB Unmarshalling complexity

Question

Hello all,
I need help on JAXB
the situation is as below...
i have to write a program that communicates with a server. the communication is request-response kind of communication which is to be carried out through XML.
so everytime i have to send any request i need to create an XML and then send it to the server. i get the response in the form of XML. then i have to parse the XML to find out whats the response.
previously i used DOM and SAX for creating XML and parsing XML respectively.
now every response i get, i have to validate it against a DTD.
now i'm trying out JAXB. so i first converted all the DTDs to XSD (xml schema docs). because JAXB doesn't support DTDs.
now with JAXB, i'm creating the XML requests and sending it to the server and getting the response.
but here is one problem.....
as with JAXB, of i have to unmarshall the xml response i have to first create a JAXB context. and unmarshall it to that xml object that i'm expecting. but suppose i get some other response then how am i supposed to know that.
i'll explain the above with a small example.
suppose i have the following DTDs
SearchRequest.dtd..............for search requests
SearchResponse.dtd..............for search responses
ErrorResponse.dtd.............for if the server sends any error.
now first i convert this DTDs to XSDs and so i get
SearchRequest.xsd, SearchResponse.xsd, ErrorResponse.xsd
and then using the JAXB compiler i create the JAXB derived classes in the following package
com.xyz.SearchRequest
com.xyz.SearchResponse
com.xyz.ErrorResponse
now suppose i created the xml request and sent it to the server and i got the response. and i have to unmarshall it.
(the server will send me SearchResponse.xml if everything if fine but ErrorResponse.xml it theres an error).
now heres the problem. the server will send the SearchResponse or ErrorResponse. and ill have to determine what it is before i start to unmarshall.
i hope i have communicated my problem....
thankyou
java-jones

Answer 1

Not sure if this will help you or if this is what you are looking for, but from the JAXB Users' Guide:
More About Unmarshalling
The Unmarshaller class in the javax.xml.bind package provides the client application the ability to convert XML data into a tree of Java content objects. The unmarshal method for a schema (within a namespace) allows for any global XML element declared in the schema to be unmarshalled as the root of an instance document. The JAXBContext object allows the merging of global elements across a set of schemas (listed in the contextPath). Since each schema in the schema set can belong to distinct namespaces, the unification of schemas to an unmarshalling context should be namespace-independent. This means that a client application is able to unmarshal XML documents that are instances of any of the schemas listed in the contextPath; for example:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo:com.acme.bar" );
Unmarshaller u = jc.createUnmarshaller();
FooObject fooObj = (FooObject)u.unmarshal( new File( "foo.xml" ) ); // ok
BarObject barObj = (BarObject)u.unmarshal( new File( "bar.xml" ) ); // ok
BazObject bazObj = (BazObject)u.unmarshal( new File( "baz.xml" ) ); // error