Why you want to override service method?
Generally it is not adviced to override service method. whatever code one wants to write should be written in dGet/doPost methods.
Answer to your question:-
Is depends upon how you want to use class A and B.
If the need is such that each request should have their own copy of A and B objects then you have to write the code of object creation in doGet/doPost method otherwise if the A and B objects can be shared by different threads then you can use sessions or context.