Yes.

Or, if the referenced objects are threadsafe (String objects are), then
you can use AtomicReference.

No. Though I understand how 'yes' is tempting. Because
it would surely be easier that way.

class Subtractor {

   private int subtrahend;
   private int minuend;

   public synchronized int getSubtrahend() {
       return subtrahend;
   }

   public synchronized void setSubtrahend(int s) {
       this.subtrahend = s;
   }

   public synchronized int getMinuend() {
       return this.minuend;
   }

   public synchronized void setMinuend(int m) {
       this.minuend = m;
   }

   public synchronized int difference() {
       return minuend - subtrahend;
  }
}

Let's say you have multiple threads
accessing the same Subtractor object.
Let's call them Thread 1 and Thread 2, because
I'm writing this at 3:30 AM.

This is a possible flow:

Thread 1: setMinuend(5)
Thread 1: setSubtrahend(2)
Thread 1: difference() == 3
Thread 2: setMinuend(10)
Thread 2: setSubtrahend(8)
Thread 2: difference() == 2

Hooray!

Unfortunately, this is also a possible
outcome:

Thread 1: setMinuend(5)
Thread 1: setSubtrahend(2)
Thread 2: setMinuend(10)
Thread 2: setSubtrahend(8)
Thread 1: difference() == 2
Thread 2: difference() == 2

Ooops.

Why didn't the synchronized voodoo help? It did it's job--
no two threads were allowed to access a getter or setter
at the same time. Unfortunately, that's completely pointless.
We only get the right answer if setMinuend(), setSubtrahend,
and difference() aren't all called by the same thread without
any interruption.

Go forth and sin no more.

No. You /cannot/ use instance variables of the Servlet dynamically in
serving requests. At all. Ever. You can use them to hold configuration
data. You can also create separate per-service objects in which to pass
around the context of the request (after all, this is essentially what
HttpServletRequest and HttpServletResponse are, and there's nothing to
prevent you creating a class of your own); and you can use the instance
variables of these per-service objects if you want to.

But you cannot use instance variables of the Servlet, because you don't
know how different threads will interleave their actions.