I agree with Bear - the question is seriously flawed.
1. ANY number of Threads can be executing the same block of code "at the same time" without a problem. It is the variables that you have to think about, not the code. This is true in any language, not just Java.
2. ONLY the request Thread which calls service can modify the request and response variables - no other request Thread can see them.
3. ONLY the request Thread can modify variables created and reference locally inside the service method. NOTE the important word "locally" - if the only reference to an object exists in the stack of the calling Thread, it is completely isolated.